Author Archives: TMartin

Into the Dragon’s Lair: Alligator Burrows as Traces

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American alligators (Alligator mississippiensis) tend to provoke strong feelings in people, but the one I encounter the most often is awe, followed closely by fear. Both emotions are certainly justifiable, considering how alligators are not only the largest reptiles living on the Georgia barrier islands, but also are the top predators in both freshwater and salt-water ecosystems in and around those islands. I’ve even encountered them often enough in maritime forests of the islands to regard them as imposing predators in those ecosystems, too.

Time for a relaxing stroll through the maritime forest to revel in its majestic live oaks, languid Spanish moss, and ever-so-green saw palmettos. Say, does that log over there look a little odd to you? (Photo by Anthony Martin, taken on St. Catherines Island.)

But what many people may not know about these Georgia alligators is that they burrow. I’m still a little murky on exactly how they burrow, but they do, and the tunnels of alligators, large and small, are woven throughout the interiors of many Georgia barrier islands. Earlier this week, I was on one of those islands – St. Catherines – having started a survey of alligator burrow locations, sizes, and ecological settings.

Entrance to an alligator burrow in a former freshwater marsh, now dry, yet the burrow is filled with water. How did water get into the burrow, and how does such traces help alligators to survive and thrive? Please read on. (Photograph by Anthony Martin and taken on St. Catherines Island, Georgia.)

In this project, I’m working cooperatively (as opposed to antagonistically) with a colleague of mine at Emory University, Michael Page, as well as Sheldon Skaggs and Robert (Kelly) Vance of Georgia Southern University. As loyal readers may recall, Sheldon and Kelly worked with me on a study of gopher tortoise burrows, also done on St. Catherines Island, in which we combined field descriptions of the burrows with imaging provided by ground-penetrating radar (also known by its acronym, GPR). Hence this project represents “Phase 2” in our study of large reptile burrows there, which we expect will result in at least two peer-reviewed papers and several presentations at professional meetings later this year.

Why is a paleontologist (that would be me) looking at alligator burrows? Well, I’m very interested in how these modern burrows might help us to recognize and properly interpret similar fossil burrows. Considering that alligators and tortoises have lineages that stretch back into the Mesozoic Era, it’s exciting to think that through observations we make of their descendants, we could be witnessing evolutionary echoes of those legacies today.

Indeed, for many people, alligators evoke thoughts of those most famous of Mesozoic denizens – dinosaurs – an allusion that is not so farfetched, and not just because alligators are huge, scaly, and carnivorous. Alligators are also crocodilians, and crocodilians and dinosaurs (including birds) are archosaurs, having shared a common ancestor early in the Mesozoic. However, alligators are an evolutionarily distinct group of crocodilians that likely split from other crocodilians in the Late Jurassic or Early Cretaceous Period, an interpretation based on both fossils and calculated rates of molecular change in their lineages.

Archosaur relatives, reunited on the Georgia coast: great egrets (Ardea alba), which are modern dinosaurs, nesting above American alligators (Alligator mississippiensis), which only remind us of dinosaurs, but shared a common ancestor with them in the Mesozoic Era. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Along these lines, I was a coauthor on a paper that documented the only known burrowing dinosaur – Oryctodromeus cubicularis – from mid-Cretaceous rocks in Montana. In this discovery, we had bones of an adult and two half-grown juveniles in a burrow-like structure that matched the size of the adult. I also interpreted similar structures in Cretaceous rocks of Victoria, Australia as the oldest known dinosaur burrows. Sadly, these structures contained no bones, which of course make their interpretation as trace fossils more contentious. Nonetheless, I otherwise pointed out why such burrows would have been likely for small dinosaurs, especially in Australia, which was near the South Pole during the Cretaceous. At least a few of these reasons I gave in the published paper about these structures were inspired by what was known about alligator burrows.

Natural sandstone cast of the burrow of the small ornithopod dinosaur, Oryctodromeus cubicularis, found in Late Cretaceous rocks of western Montana; scale = 15 cm (6 in). (Photograph by Anthony Martin, taken in Montana, USA.)

Enigmatic structure in Early Cretaceous rocks of Victoria, Australia, interpreted as a small dinosaur burrow. It was nearly identical in size (about 2 meters long) and form (gently dipping and spiraling tunnel) to the Montana dinosaur burrow. (Photograph by Anthony Martin, taken in Victoria, Australia.)

What are the purposes of modern alligator burrows? Here are four to think about:

• Dens for Raising Young Alligators – Many of these burrows, like the burrow interpreted for the dinosaur Oryctodromeus, serve as dens for raising young. In such instances, these burrows are occupied by big momma ‘gators, who use them for keeping their newly hatched (and potentially vulnerable) offspring safe from other predators.

Two days ago, Michael and I experienced this behavioral trait in a memorable way while we documented burrow locations. As we walked along the edge of an old canal cutting through the forest, baby alligators, alarmed by our presence, began emitting high-pitched grunts. This then provoked a large alligator – their presumed mother – to enter the water. Her reaction effectively discouraged us from approaching the babies; indeed, we promptly increased our distance from them. (Our mommas didn’t raise no dumb kids.) So although we were hampered in finding out the exact location of this mother’s den, it was likely very close to where we first heard the grunting babies. I have also seen mother alligators on St. Catherines Island usher their little ones through a submerged den entrance, quickly followed by the mother turning around in the burrow and standing guard at the front door.

Oh, what an adorable little baby alligator! What’s that? You say your mother is a little over-protective? Oh. I see. I think I’ll be leaving now… (Photograph by Anthony Martin, taken on St. Catherines Island.)

• Temperature Regulation – Sometimes large male alligators live by themselves in these burrows, like some sort of saurian bachelor pad. For male alligators on their own, these structures are important for maintaining equitable temperatures for these animals. Alligators, like other poikilothermic (“cold-blooded”) vertebrates, depend on their surrounding environments for controlling their body temperatures. Even south Georgia undergoes freezing conditions during the winter, and of course summers there can get brutally hot. Burrows neatly solve both problems, as these “indoor” environments, like caves, provide comfortable year-round living in a space that is neither too cold nor too hot, but just right. The burrowing ability of alligators thus makes them better adapted to colder climates than other crocodilians, such as the American crocodile (Crocodylus acutus), which does not make dwelling burrows and is restricted in the U.S. to the southern part of Florida.

• Protection against Fires – Burrows protect their occupants against a common environmental hazard in the southeastern U.S., fire. This is an advantage of alligator burrows that I did not appreciate until only a few days ago while in the field on St. Catherines. Yesterday, the island manager (and long-time resident) of St. Catherines, Royce Hayes, took us to a spot where last July a fire raged through a mixed maritime forest-freshwater wetland that also has numerous alligator burrows. The day after the fire ended, he saw two pairs of alligator tracks in the ash, meaning that these animals survived the fire by seeking shelter, and further reported that at least one of these trackways led from a burrow. The idea that these burrows can keep alligators safe from fires makes sense, similar to how gopher tortoises can live long lives in fire-dominated long-leaf pine ecosystems.

An area in the southern part of St. Catherines Island, scorched by a fire last July, that is also a freshwater wetland inhabited by alligators with burrows. The burrow entrances are all under water right now, which would work out fine for their alligator occupants if another fire went through there tomorrow. (Photograph by Anthony Martin, taken on St. Catherines Island.)

• Protection against Droughts – Burrows also probably help alligators keep their skins moist during droughts. Because these burrows often intersect the local water table, alligators might continue to use them as homes even when the accompany surface-water body has dried up. We saw several examples of such burrows during the past few days, some of which were occupied by alligators, even though their adjacent water bodies were nearly dry.

For example, yesterday Michael and I, while scouting a few low-lying areas for either occupied or abandoned dens, saw a small alligator – only about a meter (3.3 ft) long – in a dry ditch cutting through the middle of a pine forest. Curious about where alligator’s burrow might be, we approached it to see where it would go. It ran into a partially buried drainage pipe under a sandy road, a handy temporary refuge from potentially threatening bipeds. Seeing no other opening on that side of the road, we then checked the other side of the road, and were pleasantly surprised to find a burrow entrance with standing water in it. This small alligator had made the best of its perilously dry conditions by digging down to water below the ground surface.

Alligator burrow (right) on the edge of a former water body. Notice how water is pooling in the front of the burrow, showing how it intersects the local water table. The entrance also had fresh alligator tracks and tail dragmarks at this entrance, showing that it was still occupied despite the lack of water outside of it. (Photograph by Anthony Martin, taken on Cumberland Island, Georgia.)

Alligator burrows (left foreground and middle background) in a maritime forest, also not associated with a wetland but marking the former location of one. Although the one to the left was unoccupied when we looked at it, it had standing water just below its entrance. This meant an alligator could have hung out in this burrow for a while after the wetland dried up, and it may have just recently departed. Also, once these burrows are high and dry, bones strewn about in front of them also add a delicious sense of dread. Here, Michael Page points at a deer pelvis, minus the rest of the deer. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

What is especially interesting about the American alligator is how the only other species of modern alligator, A. sinensis in China, is also a fabulous burrower, digging long tunnels there too, which they use for similar purposes. This behavioral trait in two closely related but now geographically distant species implies a shared evolutionary heritage, in which burrowing provided an adaptive advantage for their ancestors.

Thus like many research problems in science, we won’t really know much more about alligator burrows until we gather information about them, test some of the questions and other ideas that emerge from our study, and otherwise do more in-depth (pun intended) research. Nonetheless, our all-too-short trip to St. Catherines Island this week gave us a good start in our ambitions to apply a comprehensive approach to studying alligator burrows. Through a combination of ground-penetrating radar, geographic information systems, geology, and old-fashioned (but time-tested) field observations, we are confident that by the end of our study, we will have a better understanding of how burrows have helped alligators adapt to their environments since the Mesozoic.

Juvenile alligators just outside two over-sized burrows, made and used by previous generations of older and much larger alligators. How might such burrows get preserved in the fossil record? How might we know whether these burrows were reused by younger members of the same species? Or, would we even recognize these as fossil burrows in the first place? All good questions, and all hopefully answerable by studying modern alligator burrows on the Georgia barrier islands. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Further Reading

Erickson, G.M., et al. 2012. Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation. PLoS One, 7(3): doi:10.1371/journal.pone.0031781.

Martin, A.J. 2009. Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia and their relation to Cretaceous polar environments. Cretaceous Research, 30: 1223-1237.

Martin, A.J., Skaggs, S., Vance, R.K., and Greco, V. 2011. Ground-penetrating radar investigation of gopher-tortoise burrows: refining the characterization of modern vertebrate burrows and associated commensal traces. Geological Society of America Abstracts with Programs, 43(5): 381.

St. John, J.A., et al., 2012. Sequencing three crocodilian genomes to illuminate the evolution of archosaurs and amniotes. Genome Biology, 13: 415.

Varricchio, D.J., Martin, A. J., and Katsura, Y. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society of London B, 274: 1361-1368.

Waters, D.G. 2008. Crocodlians. In Jensen, J.B., Camp, C.D., Gibbons, W., and Elliott, M.J. (editors), Amphibians and Reptiles of Georgia. University of Georgia Press, Athens, Georgia: 271-274.

Acknowledgements: Much appreciation is extended to the St. Catherines Island Foundation, which supported our use of their facilities and vehicles on St. Catherines this week, and Royce Hayes, who enthusiastically shared his extensive knowledge of alligator burrows. I also would like to thank my present colleagues and future co-authors – Michael Page, Sheldon Skaggs, and Kelly Vance – for their valued contributions to this ongoing research: we make a great team. Lastly, I’m grateful to my wife Ruth Schowalter for her assistance both in the field and at home. She’s stared down many an alligator burrow with me on multiple islands of the Georgia coast, which says something about her spousal support for this ongoing research.

Coquina Clams, Listening to and Riding the Waves

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A little more than a week ago, I co-led a class field trip to Cumberland Island, Georgia and the nearby Okefenokee Swamp for a course titled Ecosystems of the Southeastern U.S. Although I had been to both places more than a few times, none of the students – and a few of my colleagues – had never visited either, potentially casting these already special places in a more exciting light for them.

Nonetheless, as is typical with any field trip to a Georgia barrier island, I also noticed new phenomena while on Cumberland, once again demonstrating how field trips with students ideally also cause the instructors to be filled with wide-eyed wonder. Even better, a few seemingly lowly small bivalves – coquina clams (Donax variabilis) – provided the intellectual highlight for me while we were on Cumberland Island. This is saying something for an island bearing charismatic livestock as a touristic draw.

Resting traces (or are they escape traces?) of coquina clams (Donax variabilis) in the upper intertidal zone of a beach on Cumberland Island, Georgia. These clams are buried just underneath each bump of sand, but some others are much deeper and safer. How do I know that? You’ll find out. (Photograph by Anthony Martin.)

We first noticed the clams as a death assemblage just above the uppermost part of the surf zone on the beach. Their shells, some evident as single valves and others as pairs still hinged together, had been deposited by waves following a high tide, then moved slightly by the wind. These finely ribbed and polished shells readily showed why the specific name of coquina clams (D. variabilis) is applied to them, as they display a gorgeous variety of colors: yellow, orange, beige, blue, pink, and other schemes that surely would inspire interior decorators seeking paisley themes.

Coquina clams with both valves intact or apart, some partially covered by windblown sand, and variably colored. (Photo by Anthony Martin, taken on Cumberland Island, Georgia.)

Just a little bit lower on the beach and in the freshly scoured intertidal zone, we then noticed many small bumps of sand. Underneath these bumps were living clams that buried themselves, which helps to avoid drying out between tides or predation by ravenous shorebirds. With regard to the latter, these bivalves still would have been easy targets for shorebirds intent on acquiring some fresh clam snacks: think of a person ducking under a blanket to avoid being eaten by a lion and how well that might work as a tactic. (Please, just think about it and don’t actually test this idea.)

However, instead of simply writing off all coquina clams as inept burrowers who deserve to die at the beaks of their avian overlords – a similar fate experienced by dwarf surf clams (Mulinia lateralis) – we should look well below the surface, and I mean the sand surface. Look again at the photo first shown above. See all of those tiny, paired holes in between the bumps? Those are the traces of siphons from more deeply buried coquina clams, which are much more likely to escape from bivalve-munching birds while also keeping moist until the next high tide.

Here’s the same photo as above, but zoomed in so you can see the details. Did you notice all of the little holes in between shallowly buried clams? If so, bravo. If not, oh well. (Despite the cropping, turning, and otherwise shuffling electrons, this photograph is still by Anthony Martin, and was still taken on Cumberland Island.)

Coquina clams are actually accomplished burrowers, a necessary adaptation for nearly any small animal living in the high-energy surf of a Georgia beach. In the event of a wave breaking on a beach and washing away the top layer of sand, thus exposing a coquina clam, it will open its valves only enough to stick out its foot, which it then vibrates rapidly. This movement loosens the wet sand underneath, and the clam’s smooth, streamlined shell does the rest of the job, allowing it to glide into its self-made local pit of quicksand and vanish from the surface.

Once under the sand, this clam remains in a vertical position and projects its siphons upwards, making paired holes visible at the sand surface. The resulting burrow is Y-shaped, with the clam body making the lower part of the “Y” and the two siphons leaving thinner traces above. On the Cumberland Island beach the given day we observed their traces, I suspect the more deeply buried clams had the benefit of wetter sand early on as the tide dropped, then the ones hiding under mere caps of sand did the best they could with less wet sand later.

Vertical sections of the Y-shaped burrows made by a coquina clam, dwarf surf clam, or similar small, burrowing bivalves on the Georgia coast, in which the clam body is removed and sediment filled in the empty spaces from above. Both views, taken at right angles from one another, assume the clam is not moving up or down in the sand, which actually isn’t very realistic. (Illustration by Anthony Martin.)

How do we apply this knowledge to the fossil record? Paleontologists have found similar small, Y-shaped burrows in fine-grained sedimentary rocks, trace fossils named Polykladichnus. Some of these burrows are interpreted as the works of suspension-feeding bivalves, although polychaete worms or small arthropods are possible tracemakers, too. Where the lower parts of bivalves rested in sediment and left impressions of their lower halves, which were later filled by overlying sediments, are trace fossils called Lockeia. As a result, paleontologists can reliably identify a former presence of bivalves in rocks that might not have any of their shells. (By the way, please remember that trace fossil names are not the names of the bivalves that made the traces, but of the trace fossil itself. Yeah, I know, that’s confusing. But if you need more of an explanation, I have one for you here.)

So what else is significant about coquina clams? Well, for one, their shells were abundant enough to have formed the framework for a loosely cemented limestone common in Florida, coquina limestone. This is a rock encountered by nearly anyone who has enjoyed (or suffered through) an introductory geology lab exercise on sedimentary rock identification. Students of mine have often compared these rock samples to popcorn balls or similar sugar-cemented treats, although I’ve noticed that no one has been tempted to eat them, no matter how long I kept them in lab that day.

Coquina limestone from Florida. Notice that some of the pieces are from other species of bivalves with rougher, corrugated shells, so these rocks are not made entirely of coquina clams. (Photograph by Mark Wilson of Wooster College, and taken from Wikipedia Commons here.)

But I saved the best tidbits of information for last, and something I’ll bet most people don’t know about these clams that makes them, like, totally cool. These little bivalves respond to sound and migrate seasonally. Yes, that’s right: these clams have their own form of “listening” and they can move en masse once the seasons change on the Georgia coast. Here’s how they do both:

Stop, Look, Listen – Of course, clams lack ears (although rumors still persist that they have legs). Thus they do not hear in the sense we do, but instead respond to low-frequency vibrations caused by waves striking the shore. Once these vibrations are detected, they react by: popping out of the sand; jumping up into the water; body surfing on the wave; and quickly reburying themselves in the sand once dumped by that same wave. Like some aficionados of heavy metal or punk rock, louder is better, as higher-decibel waves cause more clams to jump up out of the sand and into the water, an aquatic version of a molluscan mosh pit.

Shelled Migration – Coquina clams, like caribou, wildebeest, and arctic terns, migrate. Unlike the vast distances covered by those animals, though, coquina clams simply move up and down the slope of a beach with the changes of the seasons. Using their wave-surfing and burrowing abilities, they move from the lower intertidal zone – which is where they live during the spring, summer, and fall – to the upper part of the beach, which becomes their winter homes.

So I hope that all of this pondering over a few shells, bumps, and holes on a Cumberland Island beach has helped lend an appreciation for the small wonders on any given Georgia barrier island. Who knows what little discovery the next field trip will bring, or whether some facsimile of what is seen might also be preserved in the fossil record? As the old saying goes, time will tell, whether that time is in the present or the geologic past.

Further Reading

Ellers, O. 1995a. Discrimination among wave-generated sounds by a swash-riding clam. Biological Bulletin, 189: 128-137.

Ellers, O. 1995b. Behavioral control of swash-riding in the clam Donax variabilis. Biological Bulletin, 189: 120-127.

Pemberton, S.G., and Jones, B. 1988. Ichnology of the Pleistocene Ironshore Formation, Grand Cayman Island, British West Indies. Journal of Paleontology, 62: 495-505.

Ruppert, E.E., and Fox, R.S. 1988. Seashore Animals of the Southeast. University of South Carolina Press, Columbia, South Carolina: 429 p.

Turner, H.J., Jr., and Belding, D.L. 1957. The tidal migrations of Donax variabilis Say. Limnology and Oceanography, 2: 120-124.

Going Hog Wild on the Georgia Barrier Islands

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(The following is the third part of a series about traces of invasive species of mammals on the Georgia barrier islands and the ecological effects of these traces. Here is an introduction to the topic, the first entry about the feral horses of Cumberland Island, and the second entry about the feral cattle of Sapleo Island.)

Anytime I hear someone refer to a Georgia barrier island as “pristine,” I wince a little bit, smile, and say, “Well, bless your heart.” The truth is, nearly every island on the Georgia coast, no matter how beautiful, is not in a pristine state, having been considerably altered by humans over the past 4,500 years, whether these were Native Americans, Europeans, or Americans. These varying degrees of change are sometimes subtle but nonetheless there, denoted by the loss of original habitats and native species or the addition of non-native species.

Still, one Georgia barrier island comes close to fulfilling this idealistic label: Wassaw Island, which during its 1,000-year geologic history somehow escaped commercial logging, agriculture, animal husbandry, and year-round settlements. Partially because of this legacy, Wassaw is designated as a National Wildlife Refuge, and is reserved especially for ground-nesting birds. One of the ways this island works well as a refuge for these birds is – as of this writing – its “hog free” status, a condition that can be tested with each visit by looking for the obvious traces of this invasive species.

The interior of Wassaw Island, with maritime forest surrounding a freshwater wetland created by alligators, the rightful owners of the island. On Wassaw, there are no tracks or signs of feral hogs, qualifying it as a “pristine” island. (Photograph by Anthony Martin.]

Contrast this with Cumberland Island National Seashore, where hogs run wild and freely. The huge pits here are in an intertidal zone of a beach on the northwest corner of the island. Naturalist Carol Ruckdeschel (background) for scale. (Photograph by Anthony Martin.)

Feral hogs (Sus scrofa) have a special place in the rogue’s gallery of invasive mammals on the Georgia barrier islands, and most people agree they are the worst of the lot. Hogs are on every large undeveloped island – Cumberland, Sapelo, St. Catherines, and Ossabaw – and they wreak ecological havoc wherever they roam. The widespread damage they cause is largely related to their voracious and omnivorous diet, in which they seek out and eat nearly any foodstuff, whether fungal, plant, or animal, live or dead. Their fine sense of smell is their greatest asset in this respect: every time I have tracked feral hogs, their tracks show head-down-nose-to-the-ground movement as the norm, punctuated by digging that uses a combination of their snouts and front hooves to tear up the ground in their quest for food. In other words, they generally act like, well, you know what.

Most importantly from the standpoint of native animals that try to live more than one generation beyond a single hog meal, feral hogs eat eggs. Hence ground-nesting birds and turtles are among their victims, and hogs are quite keen on eating sea turtle eggs. Mothers of all three species of sea turtles that nest on the Georgia coast – loggerhead (Caretta caretta), green (Chelonia mydas), and leatherback (Dermochelys coriacea) – dig subsurface nests filled with 100-150 eggs full of protein and other nutrients, making tempting targets for any free-ranging feral hogs. Similarly, hogs also threaten another salt-water turtle, the diamondback terrapin (Malaclemys terrapin); this turtle lays its eggs in shallow nests near the edges of salt marshes, which hogs manage to find. Conservation efforts to save diamondback terrapins from human predation have mostly succeeded (it used to be a tasty ingredient in soups), but hogs can’t read and don’t discriminate when it comes to eating eggs. Here is where feral hogs are particularly dangerous as an invasive species: unlike feral horses or cattle, which “merely” degrade parts of their ecosystems: feral hogs can contribute directly to the extinction of native species. As I often tell my students, if you want to cause a species to go extinct, stop it from reproducing.

Sea-turtle nest on Sapelo Island, marked by a stake and protected by plastic fencing to prevent feral hog and raccoon depredation of its eggs. An individual raccoon would only eat about 1/3 of the eggs in a sea-turtle nest, whereas pigs would just keep on eating. (Photograph by Anthony Martin.)

As an ichnologist, though, what astounds me the most about these hogs is the extremely wide ecological range of their traces. I have seen their tracks – often made by groups traveling together – in the deepest interiors of maritime forests, in freshwater wetlands, and crossing back-dune meadows, high salt marshes, coastal dunes, and beaches. If their traces became trace fossils, paleontologists would refer to them as a facies-crossing species, in which facies (think “face”) are the identifiable traits of a sedimentary environment preserved in the geologic record. Based on their tracks and sign, they are ubiquitous in terrestrial and marginal-marine environments. Oh, and did I mention they are also good swimmers? Swimming across a tidal channel at low tide is an easy feat for them, enabling hogs to spread from island to island, without the assistance of humans.

Run away, run away! Feral hogs in a St. Catherines Island salt marsh, consisting of two juveniles and an adult, do not stick around to see whether humans are going to shoot them; they just assume so. This sighting, along with their widespread tracks and other traces, show how feral hogs can occupy and affect nearly every environment on a Georgia barrier island. (Photograph by Anthony Martin.)

So to better understand why feral hogs are such successful invaders of the Georgia islands, it’s helpful to think about their evolutionary history. As expected, this history is complicated, just like that of any domesticated species in which selective breeding narrowed the genetic diversity we see today. About 15 subspecies of Sus scrofa have been identified, making its recent family tree look rather bushy. Based on genetic studies, divergence between wild species of Sus scrofa (so-called “wild boars”) and various subspecies may have happened as long ago as 500,000 years ago in Eurasia, although humans did not capture and start breeding them until about 9,000 years ago.

Depiction of a European wild boar from 1658, in The History of Four-Footed Beasts and Serpents by Edward Topsell. Original image from a woodcut, digital image in Wikipedia Commons here.

The closest extant relatives to these hogs native to North America are peccaries, which live in the southwestern U.S., Central America, and South America. However, peccaries are recent migrants to North America, and only one Pleistocene species (Mylohyus nasutus) is known from the fossil record of the eastern U.S. This means that the post-Pleistocene ecosystems of the eastern U.S., and especially those of the Georgia barrier islands, have never encountered anything like these animals. Also, unlike the feral horses of Cumberland Island and the feral cattle of Sapelo Island, the feral hogs of the Georgia barrier islands were likely introduced early in European colonization of the coast, and may have started with the Spanish in the 16th century.

Unfortunately, part of the selective breeding of Eurasian hogs was for early sexual maturity and large litter sizes. Female feral hogs can reach breeding age at 5 months, and litters typically have 4-8 piglets, but can be greater than 12; females also can produce three litters in just more than a year. Do the math, and that adds up to a lot of pigs in a short amount of time. Furthermore, on Georgia barrier islands with few year-round human residents, the only predation pressures young piglets face daily include raptors (no, not that kind of raptor) or alligators. This means young hogs reach sexual maturity soon enough to rapidly overrun a barrier island.

Feral hog trackway in a sandy intertidal zone of Cumberland Island, showing a typical gallop pattern (four tracks together –> space –> four tracks together), symbolizing how they are running roughshod over this and other islands. (Photograph by Anthony Martin.)

Yet as we have learned in North America, and particularly on the Georgia barrier islands, feral hogs rapidly revert to their Pleistocene roots. Similar to the feral cattle of Sapelo Island, these hogs are rarely seen by people, especially on islands where humans regularly hunt them. Every time I have spotted them on Cumberland, Sapelo, St. Catherines, or Ossabaw, they instantly turn around, briefly flash their potential pork loins and ham hocks, and flee. As anyone who has raised hogs can tell you, pigs are smart and learn quickly. Hence I imagine that after only one or two shootings of their siblings or parents, they readily associate upright bipeds with imminent death, especially if these bipeds are carrying “boomsticks.” (Speaking of which, I know of at least one sea turtle researcher who does his part to decrease feral hog populations – while also feeding the local vultures – through his able use of such a baby-sea-turtle-protection device.)

Hence much of what we learn about these free-ranging pigs and their behaviors in the context of the Georgia barrier islands is from their traces. Among the most commonly encountered feral hog traces are:

• Tracks

• Rooting pits

• Wallows

• Feces

Feral hog tracks are potentially confused with deer tracks, as they both consist of paired hoofprints and overlap in their size ranges, which are about 2.5-6 cm (1-2.5 in) long. Nonetheless, feral hog tracks are less “pointed,” have nearly equal widths and lengths, rounded ends, and the two hoofs often splay. Two dew claws – vestigial toes – frequently register behind the hoofs, especially when hogs step into soft sand or mud or are running. Trackways normally show indirect register of the rear foot onto the front footprint in a diagonal walking pattern, but can also display a whole range from slow walk to full gallop patterns. With repeated use of pathways, trackways become trails, although I’m not sure if hogs are merely using and expanding previously existing whitetail deer trails, if they are blazing their own, or a combination of the two. (I suspect the last of these is the most likely.)

Feral hog tracks, showing nearly equal lengths and widths, rounded ends, and splaying of hooves, all three of which help to distinguish these from whitetail deer tracks. Scale in centimeters. (Photo by Anthony Martin, taken on Sapelo Island.)

Feral hog trackway on upper part of a sandy beach (moving parallel to shore), showing slow diagonal walking pattern, verified by hoof dragmarks between sets of tracks. Scale = 10 cm (4 in). (Photo by Anthony Martin, taken on St. Catherines Island.)

Rooting pits are broad but shallow depressions – as much as 5 m (16 ft) wide and 30 cm (1 ft) deep – that are the direct result of feral hogs digging for food. In most instances, I suspect they are going for fungi and plant roots, but they probably also eat insect larvae, lizards, small mammals, and any other animals that live in burrows. These pits are typically in maritime forests and back-dune meadows, but I have seen them in salt marshes and dunes, and, most surprisingly, in the intertidal areas of beaches. What are they seeking and eating in beach sands? I think anything dead and buried that might be giving off an odor. I have even seen their tracks associated with broken carapaces of horseshoe crabs (Limulus polyphemus), a menu item that never would have occurred to me if I had not seen these traces.

Rooting pit in back-dune meadow on St. Catherines Island. Former student, who answers to the parent-given appellation of “Andrew,” for scale. (Photograph by Anthony Martin.)

Evidence of feral hog feeding on a horseshoe crab (Limulus polyphemus). All I can say is, it must have been really hungry. (Photo by Anthony Martin, taken on St. Catherines Island.)

Wallows are similar in size and appearance to rooting pits, but have a different purpose, which is to provide hogs with relief from both the Georgia summer heat and biting insects that invariably go with this heat. These structures are often near freshwater wetlands in island interiors, but I’ve seen them next to salt marshes, too. If these wallows intersect the local water table, they also make for attractive little ponds for mosquitoes to breed, meaning these hog traces indirectly contribute to the potential spread of mosquito-borne diseases.

Wallow in maritime forest, Sapelo Island, with a standing pool of water indicating the local water table at the time. (Photo by Anthony Martin.)

Hog feces may look initially like deer pellets, but tend to aggregate in clusters. Most of the ones I have seen are filled with vegetation, but the extremely varied diets of feral hogs means you should expect nearly anything to show up in their scat.

Feral hog feces on Sapelo Island, which is more clumped than that of whitetail deer. Scale in centimeters. (Photo by Anthony Martin, taken on Sapelo Island.)

Which of these traces would make it into the fossil record? I would certainly bet on at least some of their tracks getting preserved, based on the sheer ubiquity of these traces in nearly every sedimentary environment of a Georgia barrier island. Other likely traces would be their pits and wallows, although their broad size and shallow depths would make them difficult to recognize unless directly associated with tracks. Feces would be the least likely to make it into the fossil record as coprolites, unless these contained a fair amount of bone or other mineralized stuff, which could happen with hogs.

What to do about these hogs, and how to decrease the impacts of their traces? Well, as most people know, pigs are wonderful, magical animals that were domesticated specifically for their versatile animal protein. So one solution is more active and year-round hunting of hogs, and using them to supplement breakfasts, lunches, and dinners of local residents on the Georgia coast, a neat blend of reducing a harmful feral species while encouraging a chic “locavore” label on such food.

However, the sheer numbers of hogs on some of the islands would likely require a more systematic slaughter to make a dent in their numbers, an approach that would probably deter any ecotourism unrelated to hog hunting. (Let’s just say that firearms and bird watching are an uneasy mix.) The introduction of native predators is another possible solution. For example, Cumberland Island has a population of bobcats (Lynx rufus) that was introduced primarily to control the whitetail deer population, but these cats probably also take a toll on the feral hogs (although how much is unknown). I have even heard suggestions of reintroducing red wolves (Canis rufus) to a few of the islands. These pack-hunting predators were native to the southeastern U.S. before their extirpation by fearful European settlers, and probably would reduce feral hog populations, but just how much of an impact they would have is hard to predict.

In summary, the feral horses, cattle, and hogs of the Georgia barrier islands have significant effects on the ecology and geology of the Georgia barrier islands, and will continue to do so until creative solutions are proposed and implemented to reduce and otherwise manage their numbers. In the meantime, though, these invasive species present opportunities for us to study their traces, learn more about their unseen behaviors, and compare these behaviors with their evolutionary histories. More science is always good, and in this respect, the Georgia barrier islands are the gifts that keep on giving.

Traces of feral mammals on Sapleo Island: feral hog tracks strolling past a piece of feral cattle scat in a sandy road next to a maritime forest. What is the fate of these invasive species on the Georgia barrier islands, and how will these environments continue to change because of their presence? (Photo by Anthony Martin, taken on Sapelo Island.)

Further Reading

Ditchkoff, S.S., and West, B.C. 2007. Ecology and management of feral hogs. Human-Wildlife Conflicts, 1: 149-151.

Giuffra, E., Kijas, J.M.H., Amarger, V., Carlborg, Ö., Jeon, J.-T., and Andersson, L. 2000. The origin of the domestic pig: independent domestication and subsequent introgression. Genetics, 154: 1785-1791.

Mayor, J.J., Jr., and Brisbin, I.L. 2008. Wild Pigs in the United States: Their History, Comparative Morphology, and Current Status. University of Georgia Press, Athens, Georgia: 336 p.

Taylor, R.B., Hellgren, E.C., Gabor, T.M., and Ilse, L.M. 1998. Reproduction of feral pigs in southern Texas. Journal of Mammalogy, 79: 1325-1331.

Wood, G.W., and Roark, D.N. 1980. Food habits of feral hogs in coastal South Carolina. The Journal of Wildlife Management, 44: 506-511.

Tracking the Wild Cattle of Sapelo Island

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(The following is part of a series about traces of key invasive species of mammals on the Georgia barrier islands and the ecological effects of these traces. Here is an introduction to the topic from last month, and the first entry was about the feral horses of Cumberland Island.)

If I were pressed to name my favorite Georgia barrier island, it would be a tough choice, but it would be Sapelo. Many reasons support this preference, both practical and emotional, which I will relate before getting to the topic featured in the title.

Trails made by feral cattle traveling far into a salt marsh on Sapelo Island, Georgia. But I thought cows only stayed in grassy fields and chewed their cuds? Please read on. (Photograph by Anthony Martin.)

As I mentioned in a previous entry, Sapelo is an excellent place to take university students for teaching basic coastal ecology, geology, ichnology, and taphonomy. Many ecologists consider it as the birthplace of modern ecology, which happened in the 1950s and ‘60s, and it hosted studies that established many basic principles of neoichnology (the study of modern traces) in the 1970s and ‘80s. For the latter, one of the key figures was Robert (Bob) Frey, who was my Ph.D. advisor when I attended the University of Georgia. Sapelo’s human history is also fascinating, dating back to more than 4,000 years ago – evidenced by a prominent Native-American shell ring – and continues through today with Hog Hammock, the only Gullah (“saltwater Geechee”) community left on the Georgia coast.

I have been to Sapelo dozens of times, with or without students, and each time there, I continue to be surprised and delighted by some new observation that reveals itself to those with open eyes and minds. Thus it has everything a field-oriented scientist could want, especially one who likes to learn something different with each visit.

All of these facts and feelings, though, may also lend to an impression that Sapelo is an idyllic and ecologically “pure” place, a true slice of what a Georgia barrier island should aspire to be. Alas, it is not, and like other Georgia barrier islands, Sapelo has been ecologically altered because of exotic plants and animals introduced there during colonial and post-colonial times. Among these species, the most noteworthy on Sapelo is Bos taurus, the only population of wild cattle on any Georgia barrier island and one of the few in the continental U.S.

Unlike the feral horses on Cumberland Island, nearly everyone agrees on the origin of the wild cattle on Sapelo: they are most likely descended from domestic cattle released on the island by millionaire R.J. Reynolds, Jr. (of carcinogenic fame). Although the details are sketchy as to exactly when and why he did this, Reynolds, who owned most of Sapelo from 1933 until his death in 1964, let loose his dairy cows and bulls in the first half of the 20th century. Many generations of these cattle have bred in the wild since, and still roam the island in sufficient numbers to warrant some attention from wildlife biologists, ecologists, and others interested in learning about their behavior and impacts on the local ecosystems.

In my experience, though, the words “wild” and “cattle” are rarely used in everyday conversations about these animals that, through our domestication of them, provide us with milk, cheese, and meat. Ask someone to describe a cow, for instance, and most people will be unflattering: “slow,” “docile,” and “stupid” are among the most common adjectives applied, which is sometimes followed by a giggling reference to the Midwestern U.S. tradition of cow-tipping.

Thinking of tipping this cow? Be my guest, and be sure to forward the resulting video to Animal Planet for others’ lurid entertainment. The “cow” is actually a feral bull, and it was standing its ground at the edge of a field on Sapelo Island, fully aware that we spindly little bipeds were staring at it, and seemingly daring us to get closer. The poor quality of this photo is because I had my camera on maximum digital zoom: my momma didn’t raise no dumb kid. (Photograph by Anthony Martin.)

Yet these cattle are descended from wild species, aurochs (Bos primigenius) that survived the end-Pleistocene mass extinctions. You know, the same extinctions event that wiped out mammoths, mastodons, giant ground sloths, wooly rhinoceroses, saber-toothed cats, dire wolves, and other formidable megafauna of the Pleistocene. Hence aurochs must have had adaptive advantages over their Pleistocene cohorts. This was perhaps was related to their preferred ecosystems of wetland forests and swamps: remember that point with reference to Sapelo. Following the mass extinction, though, people in Eurasia, Africa, and India domesticated aurochs about 8,000 years ago. Through selective breeding, people came up with the present-day varieties we see of Bos taurus, which is considered a subspecies of B. primigenius.

Painting titled The Aurochs, by Heinrich Harder (1858-1935), probably made in 1920. Image is in the public domain and I found it on this Web site, authored by Peter Maas. Contrast how the artist depicted an auroch fighting off a pack of wolves with current expectations of how domestic cattle should behave in the face of pack-hunting predators, and you’ll get a better sense of the actual behaviors shown by wild cattle on Sapelo Island.

I am reminded of this evolutionary heritage whenever I go to Sapelo, because the cattle there are cryptic creatures of the maritime forest. Yes, that’s right: cryptic and living in the forest. A casual day-trip visitor to Sapelo will almost never see one, let alone any of several small herds that roam the island. Whenever an individual bull or herd is encountered in more open, grassy areas, they seemingly revert to Pleistocene behavior and slip into the woods, quickly concealing themselves from the prying eyes of humans. In short, they are not slow, docile, or stupid, and would never allow a person to get close enough to make an short-lived and ill-fated attempt to tip any of them.

This is about all you’ll see of a recent presence of the feral cattle on Sapelo Island: tracks, and if you are lucky enough to sight one, it will leave a lot more tracks and sign for you to study than that all-too-brief glimpse. Scale is in centimeters, and look closely where the slightly smaller the rear-foot track (manus) registered directly on top of the fron-tfoot (pes) track. (Photograph by Anthony Martin.)

Hence any meaningful study of these cattle and their ecological effects on Sapelo requires the use of – you guessed it – ichnology. Consequently, I have tracked these cattle, sometimes with my students and sometimes by myself, during many visits there. Although these tracking forays have generated many anecdotal yarns of yore about these “wild cows of mystery” worth retelling, I will reluctantly restrict myself here to summarizing their traces and the effects of these traces on the landscapes of Sapelo.

Traces of feral cattle on Sapelo consist largely of their tracks, trails or otherwise trampled areas, feces, and chew marks. In my experience, the vast majority of their traces are on the northern half of the island, although herds or individual bulls will occasionally leave their marks in the southern half when they graze on grassy areas there.

Tracks made by these feral cattle are unmistakable when compared to those of any other hoofed animal on Sapelo – such as white-tailed deer or feral hogs – which is a function of their greater size. Tracks are shaped like robust, upside-down Valentine’s hearts, with two bilaterally symmetrical hoof impressions rounded in the front and back. Tracks are normally about 9-14 cm (3.5-5.5 in) long, although I have seen newborn calf tracks as small as 5-6 cm (2-2.3 in) long; track widths are slightly less (by about 20%) than lengths. These cattle, like deer, spend much of their time walking slowly, so their rear-foot (pes) impressions often overlap behind their front-foot (manus) impressions, but can also overprint in direct register. Trackways typically show a diagonal-walking pattern, although these can be punctuated by frequent “T-stops,” in which tracks form a “T” pattern, with the top of the “T” made by the front feet whenever a trackmaker stopped.

Near-perfect direct register of smaller rear foot into front-foot tracks made by adult feral cow, recorded in exquisite detail in fine-grained sand. Scale in centimeters. (Photograph by Anthony Martin, taken on Sapelo Island.)

Because these cattle, for the most part, obey herding instincts, they habitually follow one another along the same narrow pathways through maritime forests and salt marshes, resulting in well-worn trails that wind between live oaks in forest interiors or cut straight across marshes. Nonetheless, the cattle also like to use the open freeways provided by the sandy roads that criss-cross much of the northern part of the island, which makes tracking them much easier, especially after a hard rain has “cleaned the slate.” When using a road, the cattle break single file and walk parallel or just behind one another, indicated by their overlapping and side-by-side trackways. On forest trails, they often drag their hooves across the tops of logs downed along trails, chipping and otherwise breaking down the wood.

Feral cattle tracks showing different sizes – and hence age structures – of the cattle, with some trackways overlapping (following one another) and some parallel, taking up the entire width of a sandy road on the north end of Sapelo Island. (Photographs by Anthony Martin, composite of three stitched together in Photoshop™.)

Log on feral-cattle trail, showing chipped wood on surface where hooves dragged across the top, possibly over generations of trail use. White-tailed deer do a similar behavior on their trails, but do not cause such obvious traces. (Photograph by Anthony Martin, taken on Sapelo Island.)

OK, here’s a reminder of something I just said and showed in a photo earlier: these cattle also form trails that wind deeply into the salt marshes. Why? Turns out that instead of restricting themselves to a terrestrial-only diet, they are eating smooth cordgrass (Spartina alterniflora), which grows abundantly in the marshes. This feeding results in their leaving many other traces, such as near-ground-level cropping of Spartina with clean tears, accompanied by considerable trampling of grazed areas. Although I was surprised to discover this for myself several years ago, people who raised cattle on the island in the 19th and early 20th centuries, perhaps through necessity, knew about this alternative foodstuff and fed it to cattle as a substitute for hay. Sure enough, historical references verify the use of smooth cordgrass as part of their diet (of the cattle, not the people, that is).

Evidence that feral cattle of Sapelo walk into salt marshes as a herd and eat the smooth cordgrass (Spartina alterniflora) there, based on trampling and overgrazing. Michael Bauman, who was an Emory undergraduate student at the time, for scale. (Photographs by Anthony Martin.)

Close-up of traces left on smooth cordgrass from feral cattle grazing, which are at various heights according to the level of their grazing activity. (Photograph by Anthony Martin, taken on Sapelo Island.)

Of course, among the most obvious traces these cattle leave in their wake are the end products of digestion (pun intended), feces. These “cow patties” vary in size depending on both the size of the tracemaker and liquid content of the scat. The bigger the tracemaker and the greater the water content to the plants, the wider the patties, which can exceed dinner-plate size. Similar to the situation on Cumberland Island with its feral horses and their feces, the native dung beetles must not be able to keep up with such a bounty, as I see many unrecycled, dried patties throughout the island, and have nearly stepped on freshly dropped pies that showed no signs of having been discovered by caring dung-beetle mothers.

Looks like cow scat. Smells like cow scat. Feels like cow scat. Tastes like cow scat. Good thing we didn’t step in it! But notice that the tracemaker did, leaving a bonus trace (track) on top of its impressive pile. (Photograph taken by Anthony Martin, taken on Sapelo Island.)

Given that the northern part of the island has extensive salt marshes flanking the maritime forest, and places with fresh-water sloughs containing more wetland plants, it makes sense that the cattle would stay mostly in that half of the island. The absence of humans on the north end of the island – other than occasional deer hunters, Department of Natural Resources personnel, or crazy ichnologists – is also a plus, as these cattle avoid people whenever possible.

But how does any of this relate to geology and paleontology? Well, because these feral cattle interact so much with Sapelo salt marshes, I actually included these animals as marginal-marine tracemakers in my upcoming book (Life Traces of the Georgia Coast, just in case you needed reminding). This places these bovines in the same category as feral horses – which negatively affect coastal dunes and salt marshes – and feral hogs, which even go into the intertidal zones of beaches for their foraging.

The biggest difference between the cattle and these other two hoofed species, though, is their impact on the marshes. In all of my years of noting cattle tracks and other sign on Sapelo, I have never seen evidence of their going to the beach, or even to the coastal dunes. Instead, they stay in the forests and wetlands, whether the latter are fresh-water or salt-water ones. This possibly reflects how the cattle, within just a few generations, switched back to auroch behaviors of the Pleistocene, preferring to live in wooded wetlands instead of in the terrestrial grasslands we modern humans keep forcing them to graze.

Thus any paleontologists looking into the fossil record of aurochs or their ancestral species – whether of body fossils or trace fossils – might use these present-day clues when prospecting for fossils. This serves as a great example of why I urge paleontologists to pay attention to invasion ecology and conservation biology, in which “ecologically impure” invasive species give us valuable insights on their evolutionary histories.

What else can we learn about these feral cattle and their ecological and geological impacts on Sapelo, especially through studies of their traces? For one, knowing the actual number of cattle on the island would be useful, as their quantity surely relates to how well the island ecosystems can handle present and future populations. But probably more important than this would be better defining their behaviors in the context of these non-native ecosystems. How to do this with a species that stays hidden so well, one that has apparently reverted to a Pleistocene way of life? Fortunately, behaviors can be defined through the ichnological methods I have outlined here. These methods can then easily augment others normally used by conservation biologists, such as trail cameras and direct observation.

Once this is done, we will know much more about these wild cattle than before, and will no longer have to rely on whispered legends of the mysterious bovines of Sapelo Island. Regardless, there is certainly still room for such stories, perhaps even artwork, operas, plays, movies, and music. Cattle have played such an integral role in the development of humanity, there is every reason to suppose that, as long as they continue to live on Sapelo, they and their traces will continue to intrigue us.

Further Reading

Ajmone-Marsan, P., Fernando Garcia, J., and Lenstra, J.A. 2010. On the origin of cattle: how aurochs became cattle and colonized the world. Evolutionary Anthropology, 19: 148-157.

Bailey, C., and Bledsoe, C. 2000. God, Dr. Buzzard, and the Bolito Man: A Saltwater Geechee Talks about Life. Doubleday, New York: 334 p.

McFeeley, W.S. 1995. Sapelo’s People: A Long Walk into Freedom. W.W. Norton, New York: 200 p.

Sullivan, B. 2000. Sapelo Island (GA): Images of America. Arcadia Publishing,  Mt. Pleasant, South Carolina: 128 p.

Teal, M., and Teal, J.M. 1964. Portrait of an Island. Atheneum, New York: 167 p. [reprinted by University of Georgia Press, Athens, in 1997: 184 p.]

Tracking the Wild Horses of Cumberland Island

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(The following post is one of a series about traces of important invasive species of mammals on the Georgia barrier islands and the ecological effects of these traces. An introduction to this topic from last week is here.)

Perhaps the most charismatic yet problematic of non-native animals on any of the Georgia barrier islands are the wild horses (Equus caballus) of Cumberland Island. These horses are the source of much controversy, which becomes even more apparent whenever anyone tries to apply some actual science to them. So I will talk about them here from my perspective as a paleontologist and geologist in the hope that this will add another dimension to what is often presented as a two-sided and emotional argument.

Ah, the wild horses of Cumberland Island, Georgia, roaming free since the time of the Spanish in a pristine, unspoiled landscape, grazing contently on the sea oats and strolling through the coastal dunes, in perfect harmony with nature. How much of the preceding sentence is wrong? Almost all of it. If you want to find out why, please read on. But if your mind is already made up about the feral horses of Cumberland and you don’t want to hear anything bad said about them, then you might like this site. (Photograph by Anthony Martin.)

Cumberland Island, much of which is part of the U.S. National Park system as a National Seashore, is the only Georgia barrier island with a population of feral horses. Nevertheless, despite their uniqueness and fame – the latter figuring as key attractions in advertisements about Cumberland and inspiring dreamy book titles – their origins remain murky. One of the recurring romanticized claims is that these horses descended from livestock brought there by Spanish expeditions in the 16th century. This idea is reassuring to the people who repeat it for two reasons:

(1) It establishes horses as living in the landscape for a long time (especially by American standards), meaning that their presence there now is considered “natural.”

(2) It lends itself to the comforting thought that the horses connect to a European cultural heritage, putting an Old World imprint on a New World place.

However, once said enough times, such just-so stories become faith-based and any evidence contradicting them is not tolerated. Thus even when genetic studies of the Cumberland horses show they are not appreciably different from populations of horses on other islands of the eastern U.S. (arguing against a purely Spanish origin), any questioning of the stated premise – in my experience – provokes angry responses from its defenders.

I suspect this virulent reaction is a direct result of challenging both the “naturalness” and “cultural heritage” of the horses on Cumberland. In reality, though, these are opposing values. After all, an admission that these feral horses came from European stock at any point during the past 500 years supports how they clearly do not belong on Cumberland Island, or anywhere else in the Western Hemisphere if we’re talking about the last 10,000 years or so. In other words, the point is moot whether the current horse population originated in the 16th, 17th, 18th, 19th, or 20th century, or is a mixture of older and newer stock. If only horses could talk, then we would know for sure. (A detailed history of the horses on Cumberland Island is provided here for anyone interested in learning more about this.)

Arguments of heritage aside, these horses are newcomers in a geological and ecological sense. The fossil record of the modern Georgia barrier islands backs this up, as some of the islands (including Cumberland) have sediments more than 40,000 years old, but none have body or trace fossils of horses, or anything like a horse. Although three species of horses were living on the mainland part of North America during the Pleistocene Epoch until their respective extinctions more than 10,000 years ago, none were known to have inhabited any of the barrier islands, Pleistocene or recent. The closest ancient analogue to horses on any of the Georgia barrier islands would have been bison (Bison bison), but their bones are rare. This scarcity leads paleontologists to wonder whether the islands ever had self-sustaining populations of large herbivores.

So with all of that human history and pre-history in mind, the traces made by the feral horses of Cumberland and their ecological effects are exceptional to it and every other Georgia barrier island, and hence worth our attention. Just to keep this simple, I will cover three primary types of traces made by these horses. What these traces all have in common (other than being made by a horse, of course) is the decidedly negative impacts these have on the native plants and animals of Cumberland, including keystone species in the oft-labeled “pristine” ecosystems of the island.

• Tracks and trails – These traces are the abundant and easily spotted on Cumberland, even to someone with little or no training in ichnology. Horses are unguligrade, which means they are walking on their toenails (unguals), and the ungual (more popularly called a hoof) is on a single digit. Hooves make circular to slightly oval compression shapes, but if preserved in the right substrate – like a firm mud or fine sand – they will show a “Pac-Man”-like form. Front-foot (manus) tracks are slightly larger than rear-foot (pes) tracks; manus impressions are 11-14 cm (4.3-5.5 in) long and 10-13 cm (4-5.1 in) wide, whereas pes impressions are 11-13 cm (4.3-5.1 in) long and 9-12 cm (3.5-4.7 in) wide, with variations in size depending on ages of the horses making the tracks.

Trackway of feral horse moving through the coastal dunes of Cumberland Island. Note the diagonal walking pattern and how front- and rear-foot impressions merge to make oblong compound traces.

An important point to keep in mind when tracking horses or any other hoofed animals is that their feet readily cut through sediments and vegetation, leaving much more sharply defined and deeper impressions than padded feet of an equivalent-sized animal. Because Georgia-coast sands contain whitish quartz and darker heavy minerals, these contrasting sand colors help to outline horse tracks on surfaces and in cross-section as deep and sharply defined structures that cut across the bedding.

When asked to think about horses in motion, it might be tempting to imagine them galloping, especially along a beach at sunset. Nonetheless, a horse would tire quickly if it galloped all day, especially for no valid reason. Instead, its normal gait is a slow walk, which causes the rear foot to register partially on top of the front-foot impression, but slightly behind; with a slightly faster walk, the rear foot will exceed the front-foot impression. The overall trackway pattern then is what many trackers call “diagonal-walking,” as the right-left-right alternation of steps can be linked with imaginary diagonal lines. Trackway width, also known as straddle, is about 20-40 cm (8-16 in) if a horse was just walking normally, but narrows noticeably once it starts picking up speed.

Feral-horse tracks on Cumberland Island, a close-up of the same trackway shown in the previous photo. This one was likely doing a slow walk, with indirect register of the rear foot just behind and onto the front-foot impression. The scale (my shoe) is a size 8½ mens. (Photograph by Anthony Martin.)

Given enough back-and-forth movement along preferred paths, repeating and overlapping trackways result in trails, which can be picked out as linear bare patches of exposed sand or mud cutting through vegetation. Because horses are much larger than the native white-tailed deer (Odocoileus virginianus) on Cumberland, their trails are considerably wider.

Feral-horse trail along the edge of a low salt marsh where they have trampled and overgrazed the smooth cordgrass in that marsh (Spartina alterniflora). (Photograph by Anthony Martin, taken on Cumberland Island.)

• Chew marks – Horses are grazers and low-level browsers, and they eat a wide variety of vegetation on Cumberland. The most important plant species they eat through grazing are smooth cordgrass (Spartina alterniflora), sea oats (Uniola paniculata), and live oak (Quercus virginiana).  All three of these plants are keystone species in their respective ecosystems: smooth cordgrass predominates in the low salt marshes, sea oats are the mainstay plants of coastal dunes, and live oaks are the largest and most long-lived trees in the maritime forests. Their effects of horses consuming  smooth cordgrass and sea oats is straightforward, as these plants hold in sediments in place keep them from eroding, but how do horses affect live oaks? They eat the seedlings, which means that older oaks are being replaced by younger ones at a slower rate.

Grazing traces consist of clean cuts of vegetation within a vertical swath and over a broad area. Horses, unlike white-tailed deer, have teeth on both their upper and lower jaws, thus they shear plants on the branches, stems, or leaves. In contrast, deer leave more ragged marks, as they only have teeth on their lower jaws and hence have to pull on vegetation to break it off. Horses also can make a browse line, which is an abrupt horizontal line of decreased vegetation at a certain consistent height that more-or-less correlates with the average head height of the horses.

• Dung – During any given stroll on Cumberland, you cannot avoid seeing, smelling, and stepping in horse feces. This abundance of fecal material means that the feces are not being recycled quickly enough into the ecosystems, which implies that native populations of dung beetles are overwhelmed by such abundance. I have seen a few traces of dung beetles in fresh piles of feces, but no matter how hard I have looked, I have yet to witness great thundering herds of beetles rolling balls of dung across the Cumberland Island landscape.

An impressive collection of horse dung, which was probably dropped by a single horse. Note the small holes in the middle, which were likely made by dung beetles that tunneled into this rich supply of food for their offspring.

Close-up of those probable dung-beetle burrows, some with short trails attached. The white quartz sand sprinkled on top shows how it was pulled up by beetles from underneath the dung pile and onto the top surface, thus giving a minimum depth of the burrows. (Both photographs by Anthony Martin, taken on Cumberland Island.)

One of the more interesting ecological consequences of horse dung I have seen on Cumberland is how it influences the behavior of smaller animals as pellets or piles form a microtopography. For example, on some of the dunes near Lake Whitney on Cumberland – the largest body of fresh water on any of the Georgia barrier islands – I was surprised to see that small lizards – probably skinks – were moving around the dung piles or burrowing under them.

Horse droppings as a part of the landscape for small lizards. Here their tracks, accompanied by tail dragmarks, wind around partially buried feces in a sand dune. (Photograph by Anthony Martin, taken on Cumberland Island.)

Small lizard burrow entrance immediately below a horse pellet, showing its use as a sort of roof. This could probably inspire some clever statement on shingles and, well, you know, but I’ll refrain for now. (Photograph by Anthony Martin, taken on Cumberland Island.)

All three categories of traces – tracks, chew marks, and dung – can be found together in ecosystems wherever horses are trampling, grazing, and defecating, respectively.

So now let’s put on our paleontologist or geologist hats (not to be confused with archaeologist hats) and ask ourselves about the likelihood of such traces making it into the fossil record, and how we would recognize them if they did. Their likelihood of preservation, in order, would be tracks, feces, and chew marks. Tracks would be evident as large compression shapes in horizontal bedding planes or deep disruptions of bedding planes in vertical section. Feces, or their fossil versions called coprolites, might get preserved, although herbivore feces, filled with vegetative material, is less likely to make it into the fossil record compared to carnivore feces, which may have lots of bone material in it. The last of these – chew marks – would be nearly impossible to tell from normal tearing and other degradation of plant material before it became fossilized. Good luck on that.

But could the ecological damage caused by an invasive species, in which the introduction of a species serve as a sort of trace fossil in itself? In the case of horses or ecologically similar animals, subtle changes to the landscape over time might take place. This experiment actually has been done on Assateauge Island (North Carolina), which also has a feral horse population. In areas where horses were excluded by fences, the dunes were on average 0.6 meters (2 ft) feet higher than those of overgrazed and trampled dunes. Geologists conducted another study done on Shackleford Banks (North Carolina) in which they examined areas where fences had separated non-horse from horse-occupied parts of the island. These geologists similarly found that horses caused dunes to be less than 1.5 m (5 ft) high, whereas dunes without horses were as much as 3.5 m (11.5 ft) high. This meant that storms more easily penetrated the barriers provided by coastal dunes, more commonly resulting in storm-washover fans.

This change in the coastal geology of back-dune areas also means that ground-nesting shorebirds will become less common, as their nests and nestlings will be drowned or buried more frequently. Horses also are known to step on shorebird eggs and nests, or can scare away parents from nests, which increases the likelihood of egg or nest predators taking out the next generation of shorebirds.

If any horses made it to the Georgia barrier islands during the Pleistocene and established breeding populations, a geologic sequence following their arrival would look like this, from bottom to top: high dunes suffused with root traces (before horses); lower dunes corresponding with fewer root traces and deep disruptions of bedding (horse tracks); increased numbers of storm-washover fans; and a high salt-marsh. In short, a geologist would see an overall progression from a dune-dominated shoreline to a high salt marsh. Similarly, a paleontologist might see a decrease in root trace fossils and shorebird nests, eggshells, and tracks, possibly culminating in local extinctions of each.

This is your Georgia coast.

This is your Georgia coast with horses. Any questions?

Top panorama is of high-amplitude coastal dunes and well-vegetated back-dune meadows on Sapelo Island, whereas the bottom panorama is of low-amplitude dunes with no appreciable back-dune meadows on Cumberland Island. (Both panoramas based on photos taken by Anthony Martin.)

Based on what we know then, should the feral horses of Cumberland Island be removed? Yes. Will they be removed? Probably not. However, regardless of happens, I will keep teaching about the horses of Cumberland Island and their traces, both as an educator and a concerned citizen. Perhaps with enough awareness, circumstances will change for the better so that Cumberland Island can not only remain a beautiful place, but also will become more like what it was before the arrival of horses there.

(Next week in this series about invasive mammal species of the Georgia barrier islands and their traces, I’ll cover a less inflammatory but still intriguing topic: the feral cattle of Sapleo Island.)

Further Reading

Buynevich, I.V., Darrow, J.S., Grimes, T.A.Z., Seminack, C.T., and Griffis, N. 2011. Ungulate tracks in coastal sands: recognition and sedimentological significance. Journal of Coastal Research, Special Issue 64: 334-338.

De Stoppalaire, G.H., Gillespie, T.W., Brock, J.C., and Tobin, G.A. 2004. Use of remote sensing techniques to determine the effects of grazing on vegetation cover and dune elevation at Assateague Island National Seashore: impact of horses. Environmental Management, 34: 642-649.

Dilsaver, L.M. 2004. Cumberland Island National Seashore: A History of Conservation Conflict. University of Virginia Press, Charlottesville, Virginia: 304 p.

Elbroch, M. 2003. Mammal Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, Pennsylvania: 779 p.

Goodloe, R.B., Warren, R.J., Osborn, D.A., and Hall, C. 2000. Population characteristics of feral horses on Cumberland Island and their management implications. The Journal of Wildlife Management, 64: 114-121.

Sabine, J.B., Schweitzer, S.H., and Meyers, J.M. 2006. Nest Fate and Productivity of American Oystercatchers, Cumberland Island National Seashore, Georgia. Waterbirds, 29: 308-314.

Turner, M.G. 1987. Effects of grazing by feral horses, clipping, trampling, and burning on a Georgia salt marsh. Estuaries and Coasts, 10: 54-60.

Turner, M.G. 1988. Simulation and management implications of feral horse grazing on Cumberland Island, Georgia. Journal of Range Management, 41: 441-447.

 

 

 

Alien Invaders of the Georgia Coast

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(This is the first in a series of posts about invasive species on the Georgia barrier islands, their traces, the ecological impacts of these traces, and why people should be aware of both their traces and impacts.)

Paleontologists like me face a challenge whenever we study modern environments while trying to learn how parts of these environments might translate into the geologic record. Sure, we always have to take into account taphonomy (fossil preservation), through which we acknowledge that nearly none of the living and dead bodies we see in a given environment will become fossilized; relatively few of their tracks, trails, burrows, or other traces are likely to become trace fossils, either.

Because of this pessimistic (but realistic) outlook, paleontologists often rub a big eraser onto whatever we draw from a modern ecosystem, telling ourselves what will not be there millions of years from now. We then retroactively apply this concept – a part of actualism or, more polysyllabically, uniformitarianism – to what happened thousands or millions of years ago. When paleontologists do this, they assume that today’s processes are a small window through which we can peer, giving insights into processes of the pre-human past.

Feral horse (Equus caballus) tracks crossing coastal dunes on Cumberland Island, Georgia. During their evolutionary history, horses originated in North America and populations migrated to Asia, but populations in North America went extinct during the Pleistocene Epoch about 10,000 years ago. Using the perspective of geologic time, then, could someone argue that horses are actually “native,” and these feral populations are restoring a key part of a pre-human Pleistocene landscape? (Photograph by Anthony Martin.)

However, a huge complication in our quest for actualism is this reality: nearly every ecosystem we can visit on this planet is a hybrid of native and alien species, the latter introduced – intentionally or not – by us. Thus when we watch modern species behaving in the context of their environments, we always need to always ask ourselves how non-native species have cracked the window through which we squint, through the past darkly.

This theme is considered in Charles C. Mann’s most recent book, 1493: Uncovering the New World Columbus Created, in which he argues how nearly all terrestrial ecosystems occupied by people were permanently altered by the rapid introduction of exotic species worldwide following Columbus’s landfall in the Western Hemisphere. Going even further back, though, the introduction of wild dogs (dingoes) into mainland Australia by humans about 5,000 years ago irrevocably changed the environments of an entire continent. Examples like these show that European colonization and its aftermath in human history during the last 500 years was not the sole factor in the spread of non-native species, and hints at how species invasions have been an integral part of humanity and its movement throughout the world.

Something tells me we’re not in Georgia any more. A male-female pair of dingoes (Canis lupus dingo) pose for a picture in Kakadu National Park, Northern Territory, Australia. Although now considered “native,” dingoes are an example of an invasive species that had a huge impact once brought over by people from southeast Asia about 5,000 years ago. For one, its arrival is linked to the extinction of native carnivorous mammals in the mainland Australia, such as thylacines (Thylacinus cynocephalus) and Tasmanian devils (Sarcophilus harrisii). (Photograph by Anthony Martin.)

Well-meaning (but deluded) designations of “pristine,” “untouched,”and “unspoilt” aside, the Georgia barrier islands are no exception to alien invaders. Moreover, like many barrier-islands systems worldwide, they differ greatly from island to island in: which species of invaders are there; numbers of individuals of each species; and the degree of how these organisms impact island ecosystems and even their geological processes.

Feral cat tracks in back-dune meadows of Jekyll Island, Georgia. Jekyll is one of the few Georgia barrier islands with a significant human presence year-round, hence these cats are descended from domestic cats that were either purposefully or accidentally let loose by residents. What impact do these cats have on native species of animals and ecosystems, and are these effects comparable to those of other invasive species on other islands? Scale = 15 cm (6 in). (Photograph by Anthony Martin.)

This is one of the reasons why I devoted several pages of my upcoming book, Life Traces of the Georgia Coast, to the traces of invasive species – tracks, trails, burrows, and so on – despite their failing an “ecological purity test” for anyone who might prefer to focus on native species and their traces. With regard to invasive species, the genie is out of the bottle, so we might as well study what is there, rather than apply yet another metaphorical eraser to species that are drastically shaping modern ecosystems and affecting the behavior of native species, thus likewise altering their traces.

A large pit of disturbed sand in a back-dune meadow caused by feral hogs (Sus crofa) on St. Catherines Island, Georgia. Because feral hogs are wide-ranging omnivores with voracious appetites, they cause considerable alterations to island habitats, from maritime forests to intertidal beaches. How do these traces affect the behavior and ecology of other species, especially native ones, in such a broad range of environments on the Georgia barrier islands? Can their traces actually alter the geological character of the islands? (Photograph by Anthony Martin.)

What are some of these invasive species? What makes for an “invasive species” versus a mere “exotic species”? How do the traces of invasive species affect native species on the Georgia barrier islands, and the ecology and geology of the islands themselves? And how do paleontologists and geologists figure into the study of invasive species?

These are all questions that I hope to explore in upcoming weeks here, and for the sake of simplicity, I will showcase an invasive species of mammal and its traces each week. Some of the photos shown here serve as a visual teaser of the invasive species and their traces that will be covered: feral horses (Equus caballus), cattle (Bos taurus), hogs (Sus crofa), and cats (Felis domestica). Yes, I know, there are many others, but these four are among the most ecologically significant species, they consist of animals that nearly everyone knows, and – best of all – they make easily identifiable traces. So these fours species will provide a starting point in our learning how the Georgia barrier islands can be used as case studies in the traces and ecological effects of traces made by invasive species.

Trail made by feral cattle (Bos taurus) cutting through a salt marsh and extending to the horizon, providing a clue of how this forest-dwelling animal can travel deeply into and affect marginal-marine environments. How might such traces show up in the geologic record, and was there a species that might have made similar traces on the islands in the recent past? (Photograph by Anthony Martin.)

Shorebirds Helping Shorebirds, One Whelk at a Time

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How might the traces of animal behavior influence and lead to changes in the behavior of other animals, or even help other animals? The sands and the muds of the Georgia barrier islands answer this, offering lessons in how seemingly inert tracks, trails, burrows, and other traces can sway decisions, impinging on individual lives and entire ecosystems, and encourage seemingly unlikely partnerships in those ecosystems. Along those lines, we will learn about how the traces made by laughing gulls (Larus altricilla) and knobbed whelks (Busycon carica) aided sanderlings (Calidris alba) in their search for food in the sandy beaches of Jekyll Island.

A roughly triangular depression in a beach sand on Jekyll Island, Georgia, blurred by hundreds of tracks and beak-probe marks of many small shorebirds, all of which were sanderlings (Calidris alba). What is the depression, how was it made, and how did it attract the attention of the sanderlings? Scale = size 8 ½ (men’s), which is about 15 cm (6 in) wide. (Photograph by Anthony Martin.)

Last week, we learned how knobbed whelks (Busycon carica), merely through their making trails and burrows in the sandy beaches of Jekyll Island, unwittingly led to the deaths of dwarf surf clams (Mulinia lateralis), the latter eaten by voracious sanderlings. Just to summarize, the dwarf surf clams preferentially burrowed around areas where whelks had disturbed the beach sand because the burrowing was easier. Yet instead of avoiding sanderling predation, the clustering of these clams around the whelks made it easier for these shorebirds to eat more of them in one sitting. Even better, this scenario, which was pieced together through tracks, burrows, and trails, was later verified by: catching whelks in the act of burying themselves; seeing clams burrow into the wakes of whelk trails; and watching sanderlings stop to mine these whelk-created motherlodes of molluscan goodness.

Before and after photos, showing how the burrowing of a knobbed whelk caused dwarf surf clams to burrow in the same small area (top), which in turn provided a feast for sanderlings (bottom); the latter is evident from the numerous tracks, peak-probe marks, and clam-shaped holes marking where these hapless bivalves formerly resided. (Both photographs by Anthony Martin, taken on Jekyll Island, Georgia.)

Was this the only trace-enhanced form of predation taking place on that beach? By no means, and it wasn’t even the only one involving whelks and their traces, as well as sanderlings getting a good meal from someone else’s traces. This is where a new character – the laughing gull (Larus altricilla) – and a cast of thousands represented by the small crustaceans – mostly amphipods – enter the picture. How these all come together through the life habits and traces these animals leave behind is yet another example of how the Georgia coast offers lessons in how the products of behavior are just as important as the behavior itself.

Considering that knobbed whelks are among the largest marine gastropods in the eastern U.S., it only makes sense that some larger animal would want to eat one whenever it washes up onto a beach. For example, seagulls, which don’t need much encouragement to eat anything, have knobbed whelks on their lengthy menus.

So when a gull flying over a beach sees a whelk doing a poor job of playing “hide-and-seek” during low tide, it will land, walk up to the whelk, and pull it out of its resting spot. From there, the gull will either consume the whelk on the spot, fly away with it to eat elsewhere (“take-out”), or reject it, leaving it high and dry next to its resting trace. An additional trace caused by gull predation might be formed when gulls carry the whelk through the air, drop them onto hard surfaces – such as a firmly packed beach sand – which effectively cracks open their shells and reveals their yummy interiors.

Paired gull tracks in front of a knobbed whelk resting trace, with the whelk tracemaker at the bottom of the photo. Based on size and form, these tracks were made by laughing gulls (Larus altricilla). The one on the left is likely the one that plucked the whelk from its resting trace, as its feet were perfectly positioned to pick up the narrow end of the whelk with its beak. The second gull might have seen what the first was doing and arrived on the scene soon afterwards, hoping to steal this potential meal for itself. For some reason, though, neither one ate it; instead, they discarded their object of desire there on the sandflat. For those of you who wondered if I then just walked away after taking the photo, I assure you that I threw the whelk back into water. At the same time, though, I acknowledged that the same sort of predation and rejection might happen again to that whelk with the next tidal cycle. Other shorebird tracks in the photo are from willets and sanderlings. (Photograph by Anthony Martin, taken on Jekyll Island.)

Sure enough, on the same Jekyll Island beach where we saw the whelk-surf clam-sanderling interactions mentioned last week, and on the same day, my wife Ruth Schowalter and I noticed impressions where whelks had incompletely buried themselves at low tide, only to be pried out by laughing gulls. Although we did not actually witness gulls doing performing, we knew it had happened because their paired tracks were in front of triangular depressions, followed by more tracks with an occasional discarded (but still live) whelk bearing the same dimensions as the impression.

My wife Ruth aptly demonstrates how to document seagull and whelk traces (foreground) while on bicycle, no easy feat for anyone, but a cinch for her.  Labels are: GT = gull tracks; WRT = whelk resting trace; KW = knobbed whelk; SU = spousal unit; and LCEFV = low-carbon-emission field vehicle. (Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

With this search image of a whelk resting trace in mind, we then figured out what had happened in a few places when we saw much more vaguely defined triangular impressions. These were also whelk resting traces, but they were nearly obliterated by sanderling tracks and beak marks; there was no sign of gulls having been there, nor any whelk bodies. Hence these must have been instances of where the gulls flew away with their successfully acquired whelks to drop them and eat them somewhere else. But why did the sanderlings follow the gulls with the shorebird equivalent of having a big party in a small place?

Yeah, I did it: so what? A laughing gull, looking utterly guiltless, stands casually on a Jekyll Island beach, unaware of how its going after knobbed whelks also might be helping its little sanderling cousins find amphipods. (Photograph by Anthony Martin.)

Although many people may not know this, when they walk hand-in-hand along a sandy Georgia beach, a shorebird smorgasbord lies under their feet in the form of small bivalves and crustaceans. The latter are mostly amphipods (“sand fleas”), which through sheer number of individuals can compose nearly 95% of the animals living in Georgia beach sands. Amphipods normally spend their time burrowing through beach sands and eating algae between sand grains or on their surfaces.

Close-up view of the amphipod Acanthohaustorius millsi, one of about six species of amphipods and billions of individuals living in the beach sands of the Georgia barrier islands, all of which are practically begging small shorebirds to eat them. Photo from here, borrowed from NOAA (National Oceanic and Atmospheric Administration – a very good use of U.S. taxpayer money, thank you very much) and linked to a site about Gray’s Reef National Marine Sanctuary, which is about 30 km (18 mi) east of Sapelo Island, Georgia.

Because amphipods are exceedingly abundant and just below the beach surface, they represent a rich source of protein for small shorebirds. But if you really want to make it easier for these shorebirds to get at this food, just kick your feet as you walk down the beach. This will expose these crustaceans to see the light of day, and the shorebirds will snap them up as these little arthropods desperately try to burrow back into the sand. This, I think, is also what happened with the gulls pulling whelks off the beach surface. Through the seemingly simple, one-on-one predator-prey act of a gull picking up a whelk, it exposed enough amphipods to attract sanderlings, which then set off a predator-prey interaction between the sanderlings and amphipods, all centered on the resting trace of the whelk.

Two whelks near one another resulted in two resting traces, and now both are missing, which likely means they were taken by laughing gulls. Notice how all of the sanderling trampling and beak marks have erased any evidence of the gulls having been there. (Photograph by Anthony Martin, taken on Jekyll Island.)

So as a paleontologist, I always ask myself, how would this look if I found something similar in the fossil record, and how would I interpret it? What I might see would be a dense accumulation of small, overlapping three-toed tracks – with only a few clearly defined – and an otherwise irregular surface riddled by shallow holes. The triangular depression marking the former position by a large snail, obscured by hundreds of tracks and beak marks, might stay unnoticed, or if seen, could be disregarded as an errant scour mark. The large gull tracks would be gone, overprinted by the many tracks and beak marks of the smaller birds.

Take a look again at the scene shown in the first photograph, and imagine it fossilized. Could you piece together the entire story of what happened, even with what you now know from the modern examples? I’m sure that I couldn’t. Scale bar = 15 cm (6 in). (Photograph by Anthony Martin.)

Hence the role of the instigator for this chain of events, the gull or its paleontological doppelganger, as well as its large prey item, would remain both unknown and unknowable. It’s a humbling thought, and exemplary of how geologist or paleontologist should stop to wonder how much they are missing when they recreate ancient worlds from what evidence is there.

Cast (reproduction) of a dense accumulation of small shorebird-like tracks from Late Triassic-Early Jurassic rocks (about 210 million years old) of Patagonia, Argentina. These tracks are probably not from birds, but from small bird-like dinosaurs, and they were formed along a lake shoreline, rather than a seashore. Nonetheless, the tracemaker behaviors may have been similar to those of modern shorebirds. Why were these animals there, and what were they eating? Can we ever know for sure about what other animals preceded them on this small patch of land, what these predecessors eating, and how their traces might have influenced the behavior of the trackmakers? (Photograph by Anthony Martin; cast on display at Museo de Paleontológica, Trelew, Argentina.)

Another parting lesson that came out of these bits of ichnological musings is that all of the observations and ideas in this week’s and last week’s posts blossomed from one morning’s bicycle ride on a Georgia-coast beach. Even more noteworthy, these interpretations of natural history were made on an island that some scientists might write off as “too developed” to study, its biota and their ecological relationships somehow sullied or tainted by a constantly abundant and nearby human presence. So whenever you are on a Georgia barrier island, just take a look at the life traces around you, whether you are the only person on that island or one of thousands, and prepare to be awed.

Further Reading

Croker, R.A. 1968. Distribution and abundance of some intertidal sand beach amphipods accompanying the passage of two hurricanes. Chesapeake Science, 9: 157-162.

Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign of North America. Stackpole Books. Mechanicsburg, Pennsylvania: 456 p.

Grant, J. 1981. A bioenergetic model of shorebird predation on infaunal amphipods. Oikos, 37: 53-62.

Melchor, R. N., S. de Valais, and J. F. Genise. 2002. The oldest bird-like fossil footprints. Nature, 417:936–938.

Wilson, J. 2011. Common Birds of Coastal Georgia. University of Georgia Press, Athens, Georgia: 219 p.

Knobbed Whelks, Dwarf Clams, and Shorebirds: A Love Story, Told Through Traces

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For the last three Thanksgivings, my wife Ruth and I have fled the metropolitan Atlanta area and sought “nature therapy” through the environments of Jekyll Island on the Georgia coast. For this all-too-short vacation, we take our bicycles with us, stay in a hotel near the beach, and ride for hours on Jekyll’s plentiful bike paths or long beaches, taking in the fresh sea air and stopping to look at and document any animal traces that catch our interest. It is ichnology with a low carbon footprint, natural history that’s also eco-chic. Best of all, though, we have been to Jekyll enough times to know where the best traces are likely to be found. Because of this inside knowledge and enthusiasm for all things ichnological, we sometimes discover phenomena, that, as far as we know, were previously unnoticed on any of the undeveloped Georgia barrier islands.

This Thanksgiving break was one of those times. The cast of characters in our latest novel find includes: two molluscans, knobbed whelks (Busycon carica) and dwarf surf clams (Mulinia lateralis); and two species of shorebirds, sanderlings (Calidris alba) and laughing gulls (Larus altricilla). How these four animals and their traces related to one another made for a fascinating story, nearly all of it discerned through their traces left on that Jekyll Island beach.

A view of a sandy beach on Jekyll Island at low tide with clusters of shallowly buried dwarf surf clams (Mulinia lateralis). These bivalves and their burrows, combined with beak marks and tracks of one of their predators, sanderlings (Calidris alba), make for the dark patches on the sand. But do you also see the abundant knobbed whelks (Busycon carica) and their traces in this photo? If not, please read on. (Ruth Schowalter for scale, happily standing by her bicycle, and photograph by Anthony Martin.)

Jekyll is a developed island on the Georgia coast, its southern end about 30 kilometers (18 miles) north of the Georgia-Florida border, with sandy beaches, dunes, salt marshes, and maritime forests, all interrupted by residences, roads, golf courses, boutique shops, and other human-centered amenities. On the southeastern end of Jekyll, however, the beachside condominiums and hotels become fewer and the sandy natural areas correspondingly expand, holding bountiful traces of the local wildlife. With this geography in mind, we headed south on our bikes along the beach our first full day there. During this exhilarating outing, Ruth and I paused occasionally to figure out what animal activities might have taken place in the minutes or hours before our arrival, just after the high tide had turned and exposed broader areas of sandy beach.

We were not disappointed, as some traces immediately caught our attention. Low in the intertidal zone, we noticed upraised flaps of sand that marked the subsurface positions of variably sized knobbed whelks, which are among the largest marine snails in the eastern U.S. These whelks, brought in by the high tide and strong waves, had burrowed down into the sand as soon as the tide subsided. This behavioral mode has been positively reinforced by millions of years by natural selection, a tactic by the whelk that avoids both desiccation and predation.

Here’s how to spot a buried whelk. Look for a triangular interruption in an otherwise smooth surface, where a flap of sand is slightly raised. Sometimes this trace also has a small hole at one end of the triangle. Test your hypothesis by digging in gently with your fingers. If you’re wrong, then revise your search image for their traces until you get it right. The knobbed whelk pictured here is a small one, but check out the size of the one in the next picture. (Both photographs by Anthony Martin, taken on Jekyll Island.)

A whelk uses its muscular foot to bury itself, expanding and contracting it so that the foot probes into the still-saturated sand left by the high tide; once the foot anchors in the sand, it pulls the rest of the whelk sideways and down. This really isn’t so much “burrowing” as an intrusion, where the animal insinuates itself into the sand. Contrast this method with the active digging we normally associate with burrows made by most terrestrial animals with legs.

A robust specimen of a knobbed whelk (held by Ruth), showing off its well-developed foot, which it uses to bury itself. (Photograph by Anthony Martin, taken on Jekyll Island.)

A knobbed whelk caught in the act of burying itself, leaving a short trail behind and a mound of sand in front as it starts to get underneath the beach surface. (Photograph by Anthony Martin, taken on Jekyll Island.)

Once a whelk is buried, waves may wash over its trail, erasing all evidence of its preceding actions. Nonetheless, once emergent, seawater drains downward through the sand and tightens these grains around the whelk, denoting it as a triangular “trap door” that occasionally has a small hole at one end. This hole marks where the whelk expelled water through the bottom end of its shell.

Near these clear examples of whelk traces on this beach were clusters of dwarf surf clams. Similar to whelks, these clams were washed up by the hide tide and waves, and they instinctually burrowed once exposed on the surface. Although much smaller and more streamlined than knobbed whelks, they likewise use a muscular foot to intrude the sand, anchor, and pull in their shelled bodies. Under the right conditions, these clams will also leave a trail behind them before descending under the sand, although such traces are easily wiped clean by a single wave.

Cluster of dwarf surf clams that burrowed into the sand at low tide, some noticeable through little “sand caps” on top of them. Say, I wonder why there’s a triangular-shaped bare spot of sand toward one end of that cluster? (Swiss Army knife = 6 cm (2.4 in) long; photograph by Anthony Martin, taken on Jekyll Island.)

Although dwarf surf clams ideally orient themselves vertically and push two siphons through the sand – making a Y-shaped burrow – they sometimes only have enough strength to bury themselves on their sides, hidden by a mere cap of sand. This bivalve equivalent of hiding under a blanket makes them much more vulnerable to predation, especially from shorebirds that find these clams and make quick snacks of them, such as sanderlings.

Sanderling (Calidris alba), 50-100 g of pure avian fury, prowling the sandy tidal flat of Jekyll Island in search of prey. Moon snails, given their fierce predation on other molluscans, may be the “lions of the tidal flat,”  but as far as small crustaceans and clams are concerned, sanderlings are the “tyrannosaurs.” (Photograph by Anthony Martin, taken on Jekyll Island.)

Sanderlings eat many small crustaceans that live in the sand, but they are also fond of small bivalves, such as dwarf surf clams. Sure enough, wherever you find a cluster of these clams, you will also find abundant tracks and beak probe marks made by these birds. Both their tracks and the probe patterns made by their beaks are diagnostic of this species: when I see these traces on any Georgia beach, I don’t have to look at a bird-identification guide to know whether sanderlings, dunlins, plovers, or sandpipers were there. Their food choices are clarified even more when you see their tracks and beak-probe marks directly associated with almond-shaped holes, where they neatly extracted the clams from their burrows.
Sanderling tracks and beak-probe marks, with holes where clams were located by the sanderlings and  plucked out of their shallow burrows. (Swiss Army knife = 6 cm (2.4 in) long; photograph by Anthony Martin, taken on Jekyll Island.)

So how do these three species and their traces all relate to one another? (And what about the laughing gull?) Well, this is where it got even more interesting. Ruth and I soon started spotting triangular outlines within the clam clusters, bare spots on the sand devoid of both clams and beak marks. Underneath these were whelks. As we stood back and looked down the beach, we then saw how these clumps of clams were throughout the intertidal zone, and each was surrounding a whelk. Somehow the whelks had served as nucleation sites for clams, which had chosen to burrow in the sand around the whelks, instead of being randomly dispersed throughout the beach.

Remember this previous photo? There’s a whelk buried underneath that bare triangular patch.

Didn’t believe me? Well, there it is. It’s almost as if ichnology is a science, in which hypotheses, once confirmed by evidence-based reasoning, have predictive power.

Here are two more clusters of dwarf surf clams around buried whelks, hidden but still identifiable.

Quiz time: how many whelks are here? Thanks to ichnology, you don’t actually have to see them to dig them out for a census. (All four photographs by Anthony Martin and taken on Jekyll Island.)

Why were the clams burrowing around the whelks? Was this some sort of commensalism, in which the clams found more food around the whelks? No, because these clams are filter feeders, taking in water with suspended organic material for their sustenance, instead of ingesting the sand around them. How about protection? That didn’t seem likely either, because the whelk had no interest in defending the clams, and its body wasn’t even serving as a shield against shorebirds.

So I thought about how these clams burrow, and then it all made sense. Because dwarf surf clams are so small, sand grains are more like cobbles would be to you and me. Moving through these sediments thus takes considerable effort, especially as water drains from the sand and surface tension holds together the grains more tightly. This means the clams have to take advantage of sand that acts more like quicksand and less like concrete, and burrow when the sand has lots of water between the grains.

This is where the whelk became both the unwitting friend and enemy of the dwarf surf clams. As it burrowed, it fluidized the sand around it, shaking up the grains so that more space opened between them, which allowed in more water. This zone of disturbance and liquified sand was eagerly exploited by nearby clams, which easily burrowed into both the whelks’ trails and the immediate areas around their bodies.

Alas, this opportunity for safety provided by the whelk ultimately led to the sanderlings chowing down on the clams. What might have been a meticulous search for small clams sprinkled hither and tither throughout the broad Jekyll beach had now became a lot easier, thanks to both the whelks and the clams. All a sanderling had to do was find each motherlode of clams conveniently grouped around a buried whelk and start probing. It was an all-you-can-eat clam feast, and the traces clearly showed where some of these birds stopped and took their time gorging on the clams. Their tracks also showed where one stopped sanderling attracted the attention of others, which then rushed to the scene and joined in the buffet.

Wait, what have we here? A sanderling alters its course to investigate an obvious dense accumulation of dwarf surf clams. How did this population get so dense? Blame the knobbed whelk, which was just minding its own business by burrowing.

The carnage of sanderling plundering, in which about a third of buried dwarf surf clams were pulled from their burrows and the sand was trampled by thundering avian feet. This gruesome scene can all be laid at the feet, er, foot of the the whelk pictured here, which through its burrowing made it easier for the clams to burrow around it. (Both photographs by Anthony Martin and taken on Jekyll Island.)

But what about the laughing gull and its role in this story? Sorry, that will have to wait until next week’s post. In the meantime, in these days immediately following the Thanksgiving holiday in the U.S., let us all be thankful for the natural areas still preserved on Jekyll Island that allow for such wanderings of our bodies and minds, as well as the little personal discoveries of its life traces, infused with wonder, that can be shared with others.

Further Reading

Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign of North America. Stackpole Books. Mechanicsburg, Pennsylvania: 456 p.

Howard, J.D., and Dörjes, J., 1972. Animal-sediment relationships in two beach-related tidal flats: Sapelo Island, Georgia. Journal of Sedimentary Research, 42: 608-623.

MacLachlan, A., and Brown, A.C. 2006. The Ecology of Sandy Shorelines. Academic Press, New York: 373 p.

Powers, S.G., and Kittinger, J.N. 2002. Hydrodynamic mediation of predator–prey interactions: differential patterns of prey susceptibility and predator success explained by variation in water flow. Journal of Experimental Marine Biology and Ecology, 273: 171-187.

Wilson, J. 2011. Common Birds of Coastal Georgia. University of Georgia Press, Athens, Georgia: 219 p.

Fossils in Progress

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Despite whatever lamentations are made about the “incompleteness” of the fossil record, fossils are actually quite common. This truism is brought home even more so whenever trace fossils – tracks, burrows, and other evidence of organismal behavior – are included in a fossil checklist (as well they should be) when examining any given outcrop of sedimentary rocks formed in the past 550 million years or so.

For example, many a time I have visited an outcrop described previously as “lacking fossils,” and instead found it filled with trace fossils; hence what people meant was “lacking fossils” equals “no body fossils.” Normally these trace fossils are invertebrate burrows, which might be glibly identified as “worm burrows,” but tracks or other trace fossils may also reveal themselves to those who are looking for them. Indeed, this expectation of finding fossils is such that on occasions when geologists find a sedimentary rock layer devoid of either body or trace fossils, this is odd enough to cause geologists to scratch their heads and ask why.

But how do the former bodily remains of plants or animals, or traces of their behaviors, become preserved as fossils in the first place? This question other related ones are answered by the science of taphonomy. Coined by Russian paleontologist Ivan Yefremov, the etymology of this term stems from Greek, in which taphos ( = burial) and nomos (= law). In such a term, he was thus alluding to an expectation that natural processes that result in fossils becoming preserved are orderly and predictable.

An overview of taphonomy as a field of study would be far too lengthy to explore here, so instead I will use one example from the Georgia coast to show how it is supposed to work. This superb case in point is a relict marsh. It is what’s left of a salt marsh from about 500 years ago, and it has been revealing its nature to paleontologists, geologists, and students for the past few decades.

Overall view of relict marsh exposed on Cabretta Beach, Sapelo Island, Georgia. Me for scale, but photo taken 7 years ago, so the scale might now be slightly wider now. (Photograph taken by Ruth Schowalter.)

Just a little more than a week ago, my colleague Steve Henderson and I took a group of students from Emory University to Sapelo Island for a weekend field trip (detailed last week). One of our goals on this trip was to take them to a relict marsh on Cabretta Beach so that they could better appreciate how a sedimentary deposit makes a transition from living ecosystem to inert rock, yet filled with evidence of its formerly teeming life. Similar relict marshes are on St. Catherines Island and other Georgia-coast islands, but when it comes to teaching about taphonomy in the field, I prefer using the one on Sapelo.

Closer view of relict marsh on Sapelo Island, showing 500-year-old remains of smooth cordgrass (Spartina alterniflora), cross section of its muddy sediments, and quartz sand deposited on top by tides, waves, and wind. (Photograph by Anthony Martin.)

As mentioned in a previous entry, modern salt marsh on the Georgia coast have a few key components that make them among the most productive of all ecosystems: smooth cordgrass (Spartina alterniflora), marsh periwinkles (Littoraria irrorata), mud fiddler crabs (Uca pugnax), and ribbed mussels (Geukensia demissa). So if a Georgia salt marsh were to be buried quickly – say, by a storm that dumps a thick layer of sand on it – what would be preserved? The Cabretta relict marsh partially answers that question, showing us incipient trace and body fossils of these biota. They are not quite fossils, but on their way there, giving us a glimpse of the fossilization process well before it is completed.

For example, the tall, green or golden stalks of smooth cordgrass that we see today, adorned my millions of marsh periwinkles (Littoraria irrorata), are absent from the relict marsh. Only the lowermost ochre-colored stubs and extensive root systems remain, and traces made by the roots below what was the marsh surface.

Modern smooth cordgrass (Spartina alterniflora) and its constant companions, marsh periwinkles (Littoraria irrorata) on Sapelo Island, Georgia.

Cross-sectional view of relict marsh, what is left from a formerly magnificent marsh: stubs, roots, root traces, and not many periwinkles. (Both photographs by Anthony Martin.)

Once in a while, I also find old marsh periwinkle shells scattered on the surface of the relict marsh. These are made of calcium carbonate and will dissolve in slightly acidic waters, so these might not last for long once exposed. The real reason for why these tend to disappear quickly, though, is modern hermit crabs. Hermit crabs encounter these periwinkle shells on the relict marsh surface, say “Hey, free shells!”, then happily trot away with these, not caring that their “new” homes are actually 500 years old.

No mud-fiddler crab remains were apparent on the surface, nor have I seen them in 20-30 visits to this relict marsh. This is not surprising, as their exoskeletons are made of chitin and dissolve more quickly than molluscan shells. Nonetheless, their burrows are always abundantly evident on the surface as perfectly round holes, which are sometimes accompanied by new burrows made by modern fiddler crabs, as well as bivalves that will bore into this firmground.

Modern salt marsh surface on Sapelo Island with mud fiddler crabs (Uca pugnax) showing off a few of the behavioral traits they do best: eating, fighting, mating, and burrowing. Note that burrows, surface scrapings, and pellets are a few of the traces they make. Which of these traces get preserved?

Close-up of eroded relict marsh surface, showing cross-sections of old fiddler-crab burrows now being filled with modern beach sand. Think of how this will look in the fossil record. (Scale in centimeters).

Longitudinal view of former fiddler-crab burrows associated with smooth-cordgrass root traces. Fill the deeper parts of these burrows with sand, and they’re more likely to get preserved as trace fossils. Scale to right is 15 cm (6 in) long. (All photographs by Anthony Martin.)

Modern ribbed mussels are harder for us to see in the field because we would have to wade into soft, deep, sulfurous mud to get close to them, and however amusing that might be, we don’t have time to do our laundry before getting back into our rental vans for the ride home. So the students take our word for it that those mussels are indeed in the marsh, then we point to the old ones clumped on the relict-marsh surface that are still in life position.

Cluster of ribbed mussels (Guekensia demissa) directly associated with stubs of smooth cordgrass on relict marsh surface. Now that they’re exposed, how long will these shells last on the surface? (Photograph by Anthony Martin.)

Oysters (Crassostrea virginica) are less common in the relict marsh, but given the right exposure, these can be observed on some visits too. These clumps of oyster shells mark the edges of tidal creeks that wound through the marsh.

(Top) Modern salt marsh with tidal creek cutting through it and oyster bank exposed at low tide, Sapelo Island.

Former oyster bank peeking out of relict marsh, formerly buried for about 500 years, now revealed by erosion of the modern shoreline. (Both photographs by Anthony Martin.)

Because it was all too easy to spot the similarities between this relict marsh and a modern one less than 100 meters (330 feet) from where we stood, I then asked about other differences. For instance, take the fact that we were standing on the relict marsh while discussing its traits: could we do the same in the modern marsh nearby? No, was the universal answer, and I affirmed that they would likely be up to their waists in ribbed-mussel-produced mud. (I asked for volunteers to test this hypothesis, and they very smartly declined.)

This led to a discussion of why the relict marsh could be so firm, which introduced them to the concept of diagenesis: how a sedimentary deposit can change over time, an important consideration in taphonomy. Such alterations are especially apparent in muds, which lose considerable volume as these lose their water content, causing a “softground” to become a “firmground,” then eventually a “hardground.” The students were surprised when I told them that the relict marsh acting as the floor of our “classroom” was likely 2-3 times as thick as what was there now.

Would these students so blithely walk around on a modern salt marsh? I don’t think so, and please don’t experiment with this yourself. Nevertheless, a relict marsh, thanks to dehydration of its muds and compaction, is just fine for exploring on foot. (Photograph by Anthony Martin.)

We spent only about an hour at the relict marsh before regretfully walking back to our field vehicle, followed by a ferry ride to the mainland part of Georgia and a long drive home to Atlanta. Yet I felt assured that the lessons about taphonomy, ancient environments, ichnology, and diagenesis imparted by this relict marsh encompassed enough material to fill 4-5 class sessions in an indoor classroom. Moreover, if we had been all enclosed by four walls and a ceiling, and without a former marsh underfoot, there was no guarantee that these concepts would be understood or retained.

This is why we geoscientist-educators take our students outside, enriching our collective awareness of how environments change through time and how we piece together the clues left behind from ancient environments. It’s memorable, it’s fun, and it works. But don’t take my word for it. Whether you’re an educator or student, try it yourself sometime, whether on the Georgia coast or elsewhere, and see what happens.

Further Reading

Basan, P.B., and Frey, R.W. 1977. Actual-palaeontology and neoichnology of salt marshes near Sapelo Island, Georgia. In Crimes, T.P., and Harper, J.C. (editors), Trace Fossils 2. Liverpool, Seel House Press: 41-70.

Edwards, J.M. and Frey, R.W. 1977. Substrate characteristics within a Holocene salt marsh, Sapelo Island, Georgia. Senckenbergiana Maritima, 9: 215-259.

Frey, R.W. and P.B. Basan. 1981. Taphonomy of relict Holocene salt marsh deposits, Cabretta Island, Georgia. Senckenbergiana Maritima, 13: 111-155.

Frey, R.W., Basan, P.B. and Scott, R.M. 1973. Techniques for sampling salt marsh benthos and burrows. American Midland Naturalist, 89: 228-234.

Letzsch, W.S. and Frey, R.W. 1980. Deposition and erosion in a Holocene salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 50: 529-542.

Morris, R. W. and H. B. Rollins. 1977. Observations on intertidal organism associations on St. Catherines Island, Georgia. I. General description and paleoecological implications. Bulletin of the American Museum of Natural History, 159: 87-128.

Smith, J.M., and Frey, R.W. 1985. Biodeposition by the ribbed mussel Geukensia demissa in a salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 55: 817-825.

Using Traces to Teach about Traces

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This past weekend, my colleague Steve Henderson and I co-led a field trip to Sapelo Island, Georgia with 13 Emory University undergraduate students and our spouses. This trip is done biannually as a firm requirement for students taking a class of mine at Emory called Modern and Ancient Tropical Environments. This course, in turn, is a prerequisite for a 10-day field course we’ll do in December-January, ENVS 242, which appropriately has the same name as ENVS 241 except for the addition of “Field Course” at the end. That course, though, will take place on another island, albeit a very different one, San Salvador, one of the “Out Islands” of the Bahamas.

Why were we on Sapelo Island to prepare for a field course in the Bahamas? It was to fulfill several learning goals that will sound familiar to all science educators who take their students outside of a classroom for their learning. In no particular order, these are:

  • Get students to observe natural phenomena while in the field;
  • Ask good questions about what they’ve observed;
  • Learn how to properly record their observations;
  • Come up with explanations (hypotheses) for whatever questions were provoked by their field experiences; and
  • Staying safe while doing all of this, which included adjusting to whatever conditions we might encounter in the field.

Our spouses, Ruth Schowalter and Kitty Henderson, are also educators; Ruth teaches English as a Second Language (ESL) at Georgia Tech, and Kitty is a middle-school earth-science teacher in Covington, Georgia. Moreover, both have been to Sapelo Island many times, having gained a wealth of field-gained knowledge about its natural history. Hence our students were lucky to have all four of us there to introduce them to the island, and we likewise felt very fortunate to be there with such an eager group on a gorgeous fall weekend.

Environmental Studies students from Emory Univeristy with me (foreground) and Steve Henderson (right), looking at a 500-year-old relict salt marsh, exposed by erosion along Cabretta Beach on Sapelo Island, Georgia. Sure beats staying in a classroom to learn about modern and ancient environments. (Photograph by Ruth Schowalter.)

Of course, once on Sapelo or any other barrier island of the Georgia coast, I cannot help but use ichnology – the study of traces – as a uniting theme for my teaching. Steve, who did his Ph.D. research on Sapelo in the late 1970s, is more of a taphonomist, which is someone who studies how fossils are made, from death to burial to preservation. Nonetheless, ichnology and taphonomy overlap considerably, hence our respective approaches complement one another very well, a synergism aided by our having had the same Ph.D. advisor – Robert (Bob) Frey – at the University of Georgia. Once in the field, every track, burrow, feces, and body part of a dead animal we found – and the occasionally sighted live animal – became a dynamic learning opportunity for us, in which we could apply basic scientific methods that were all accented by a sense of wonder.

A dead blue crab (Callinectes sapidus) found in the middle of Sapelo Island, at least 2 kilometers (1.2 miles) from the ocean. How did it get there, and what happened to it? Our students went through the possibilities based on the evidence – main body nearly entire, no toothmarks on it, but bleached white and missing most legs. We finally concluded that it had been dropped by a large predatory bird, such as a great blue heron (Ardea herodias) or great egret (Ardea alba), which probably had shaken off most of the crab’s legs before attempting to eat it. A nice little lesson in taphonomy, for sure. (Photograph by Anthony Martin.)

But perhaps my favorite teaching techniques to use while on Sapelo or any other Georgia barrier island is to use the completely low-tech and ancient method of drawing in the sand. Through my own traces, then, I can teach my students about ichnology and its applications to understanding geologic processes. For example, one of the beaches on Sapelo – Cabretta Beach – is undergoing rapid erosion from a combination of longshore drift and sea-level rise. At this place, downed pines and oaks laid prone in the surf, a former forest now a beach. This was the perfect place to introduce the students to Walther’s Law, which states that laterally adjacent environments will succeed one another vertically in the geologic record. This principle then can be applied to figuring out how a given sequence of strata might reflect a rising or lowering of sea level in the past.

No PowerPoint? No projector? No computer? No problem. Teaching in the field is easy when you have such a nice canvas to work with. (Photograph by Ruth Schowalter.)

So with the sea behind me, a sandy beach wiped clean by the receding tide, and a handy stick, I scratched out a typical sequence of sedimentary strata and their diagnostic traces that would result from sea level going up (a transgression) on the Georgia coast. (Ruth and I were also inspired to create artwork on this theme, discussed in a previous entry.) Terrestrial environments with tree-root and insect traces were at the base of the sequence, succeeded vertically by sandy dune deposits with ghost-crab and insect burrows, then sandy beach deposits with ghost-shrimp burrows, topped off by offshore sandy muds and sands burrowed by fully marine echinoderms, such as heart urchins, sea stars, and brittle stars. I then asked the students to look around them and point to each of the laterally adjacent environments represented in my sand drawing, which they dutifully did. Finally, just to make sure our students got it, we inquired about what sequence should result if sea level dropped, and they correctly surmised that the place would revert back to terrestrial conditions, with the marine sediments buried below.

My applying the final touches on a sand-sketch masterpiece of a transgressive-regressive sequence of strata and its traces, as my students watch. Would you like to see it? Sorry, the tide came in just a few hours after I drew it, and we didn’t get a photo of it. So you’ll just have to draw your own, and preferably on a beautiful beach. (Photograph by Ruth Schowalter.)

As we all stood back to look at the transgressive-regressive sequence of strata, the formerly abstract concept of Walther’s Law became far more real for our students. The dead trees on either side of our group, an eroded dune and maritime forest behind us, and the sea in front of us, all reinforced this lesson, bolstered by our presence in a place with those environments being actively affected by geological and biological processes.

Another instance of using traces in the sand to teach about traces was with ghost-shrimp burrows. At low tide on the previous day of the field trip, the students found many small, volcano-like mounds on the intertidal beach surface some with neat piles of tiny mud-filled cylinders that looked like “chocolate sprinkles” sometimes seen on cupcakes. What were these?

I informed them that we were looking at the tops of ghost-shrimp burrows and their fecal pellets; earlier, we had seen the knobby, pelleted walls of these same ghost-shrimp burrows, which were the deeper parts. What does an entire ghost-shrimp burrow system look like in cross-section? Time for another sand drawing. This one introduced the students to what had been only disembodied words memorized for an exam – ghost shrimp, pellets, walls, vertical shafts, branching – that now could be supplemented by actual traces next to the drawing. You can’t beat these sorts of visual aids, a huge bonus from our being in the right places to see them.

Using a “clean slate” of a beach wiped smooth by the tide for sketching a cross-section of a typical ghost-shrimp burrow, many of which also happened to be underneath our feet. (Photograph by Ruth Schowalter.)

The final sketch of a ghost-shrimp burrow, showing its volcano-like top, narrow “chimney” leading down to the main shaft of the shrimp’s living chamber, some of the pellets lining its burrow walls, and the geometry of the burrow network below. (Photograph by Anthony Martin.)

Was my teaching technique new and innovative, worth presenting at an educational conference as an assessment-friendly pedagogy that would maximize outcome-based education? In short, no. Sand drawing as a tool for education has a very long tradition in indigenous cultures, especially those that have their own forms of ichnology (such as tracking) at their cores. For example, in central Australia, Ruth and I had seen a creation story etched in the ground that had been done some by the Arrente people who live near Uluru. This story likewise used animal traces (emu tracks) as a key feature, a sort of iterative use of traces for inspiration and teaching.

Creation story of the Arrente people drawn in the soil near Uluru in Northern Territory, Australia. The figure at the bottom is an emu, and its tracks are shown leading away from it. (Photograph by Anthony Martin.)

At the same place, we also watched an Arrente elder demonstrate how to make animal tracks using only his fingers and palms, which was also described in books we had read about

Did you know you can use your hands to make animal tracks? In this photo, I use the fine-grained dune sands of Sapelo Island to create a reasonable depiction of kangaroo tracks. Yes, I know, kangaroo tracks on the Georgia barrier islands are not very likely, but you get the idea. Next time I’ll do raccoon tracks instead.

Some of us educators are old enough to remember using a technological succession of blackboards and chalk, overhead projectors with pens, whiteboards with dry-erase pens, and now presentation software (Keynote, PowerPoint, and so on) for imparting lessons. So it gives me great comfort to know that, with a generation of students who have never known a world without computers with a concomitantly reduced connection to the outdoors, we can still switch back to using the ground beneath our feet, our eyes, hands, and imaginations to teach and learn about the life traces around us.

Further Reading

Bingham, J. 2005. Aboriginal Art and Culture. Raintree, Chicago, Illinois: 57 p.

Hoyt, J.H., and Hails, J.R. 1967. Pleistocene shoreline sediments in coastal Georgia: deposition and modification. Science, 155: 1541-1543.

Hoyt, J.H., Weimer, R.J., and Henry, V.J., Jr. 1964. Late Pleistocene and recent sedimentation on the central Georgia coast, U.S.A. In van Straaten, L.M.J.U. (editor), Deltaic and Shallow Marine Deposits, Developments in Sedimentology I. Elsevier, Amsterdam: 170-176.

Louv, R. 2005. Last Child in the Woods: Saving Our Children from Nature-Deficit Disorder. Algonquin Books, Chapel Hill, North Carolina: 390 p.

Middleton, G.V. 1973. Johannes Walther’s Law of the Correlation of Facies. GSA Bulletin, 84: 979-988.

Weimer, R.J., and Hoyt, J.H. 1964. Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments. Journal of Paleontology, 38: 761-767.