Tag Archives: sandy beaches

Deer on a Beach

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In the southeastern U.S., the most common large herbivorous mammal native to this region is the white-tailed deer (Odocoileus virginianus). Accordingly, deer traces, such as their tracks, trails, scat, and chew sign are abundant, easy to identify, and interpret. Some of these traces I discuss in my upcoming book, which has, like, you know, the same title as this blog. (Oh, all right, here’s the link.) But since writing the book, I’ve encountered many more examples of deer traces that surprise me, with implications for better understanding the behavioral flexibility of these mammals.

Yours Truly taking a break from biking to look at some deer tracks on a beach. Yes, that’s right: deer on a beach. Which I’ll take any day over, say, snakes on a plane. (Photograph by Ruth Schowalter, taken on Jekyll Island, Georgia.)

The ecology and ichnology of deer is a big subject, and I began writing a much longer post addressing just that, explored in exquisite detail, with stunningly brilliant insights and witty bon mots sprinkled throughout. Fortunately for all of us, I realized I was being a typical perfectionist (and pedantic) academic, instead of just getting to the point of this post. Thus the gentle reader will be spared such a tome for now, and instead I’ll talk about the cool deer traces my wife Ruth and I encountered while on Jekyll Island (Georgia) last week.

For the past four years, Ruth and I have traveled to Jekyll during our Thanksgiving break for a much–needed escape from teaching, grading, and urban environments of Atlanta, trading these in for wide beaches, beautiful salt marshes, fresh air, and exercise. Like previous years, we took our bicycles with us and spent several days there riding on its plentiful bike trails, or on the beaches at low tide.

Jekyll, unlike most other Georgia barrier islands, is partially developed, with about a thousand residents, and is amenable to tourists staying on the island. This made it convenient for us to pull up on Thursday, check into a hotel, saddle up, and start riding. Of course, we don’t just ride our bikes, but we also look for traces and other interesting tidbits of natural history while speeding along Jekyll’s beaches. For example, last year while riding there, we discovered interesting interactions happening between small burrowing clams, whelks, and shorebirds (links to those here and here), a phenomenon we had never noticed before on other Georgia barrier islands.

This year, on a gorgeous Friday morning on the south beach of Jekyll, we breezed past thousands of human and dog tracks, but grew bored with the ichnological homogeneity wrought by these two tracemakers. But then, something different popped out in the midst of these ordinary, domestically produced ones, prompting us to stop and look more closely. These were deer tracks, and from two deer walking together in the intertidal zone of the beach, where a dropping tide had cleaned the beach surface.

A broad expanse of sandy beach on the south end of Jekyll Island, exposed at low tide, and with two sets of deer tracks pointing downslope and then parallel to the shoreline. Note how these trackways are more-or-less equally spaced from one another, implying that the deer were next to one another and maintained their respective “personal spaces” at this point. (Photograph by Anthony Martin.)

We had seen deer tracks on Georgia barrier-island beaches before, but these are typically in the upper parts of Georgia beaches, closer to the dunes and above the high tide mark. Hence these trackways were unusual for us, showing an unexpected foray into a habitat that was not life-sustaining at all for these deer: no food, no cover, no bedding material, or other creature comforts provided by the forests and back-dune meadows. Just open beach.

Still, there they were, so we enjoyed this opportunity to figure out what they were doing while there. For one, we wondered exactly when they were on the beach. Fortunately, this was relatively easy to answer, as one of the nicer aspects of tracking animals in intertidal zones of beaches (other than being on a beach, of course) is that their tracks can be aged accurately in accordance with the tides. In this instance, high tide was in the early morning, at 3:43 a.m., and the low tide was at 10:18 a.m. We spotted the tracks at about 11:30 a.m., so it was still low tide then, but rising. The furthest down-beach extent of the deer tracks was in the middle of the intertidal zone. This implied that about three hours had elapsed after the high tide receded sufficiently to allow the deer to travel this far down the beach slope: so at 6:45-7:00 a.m. Dawn that morning was at 7:00 a.m., so their presence in this area just before dawn also synched well with the well-known crepuscular movements of deer.

Two sets of deer tracks, showing them moving downslope from above the high-tide mark (look at the rackline in the bottom third of the photo), and heading toward a runnel before turning to the left and paralleling the surf zone. You may have also noticed where their trackways cross over further down the beach. Say, looks like there’s some differences in their trackway patterns. I wonder why? (Photograph by Anthony Martin, taken on Jekyll Island.)

Further evidence of the freshness of these tracks was the moistness of the fine-grained sand, still holding their shape. The morning sunlight had dried them slightly along the edges, and especially the plates or ridges (pressure-release structures) outside of the tracks. The ocean breeze coming out of the east, though, was too gentle to have eroded the tracks, so they looked as if they had been made only a few hours before. Which they had.

Tracking deer doesn’t get much easier than this, folks. Fine-grained and well-packed sand, still moist enough to hold the shape of the tracks and pressure-release structures, gentle wind, and fresh tracks, only about four hours old. (Photograph by Anthony Martin, taken on Jekyll Island.)

We backtracked the deer to their entry point on the beach, which was from the eroded scarp of the primary dunes. One deer must have been following the other, as their tracks came together at this point. The lead deer made the decision to step down onto the beach, a drop of a little more than a meter (3.3 ft), and then the second one followed it down.

The decision point, where one of two deer took the lead and stepped down from the primary dunes to the beach (indicated by tracks at top and bottom of the photo). Note the ghost-crab burrow in the middle-right part of the photo, just above the photo scale. (Photograph by Anthony Martin, taken on Jekyll Island.)

What was really interesting for me, as an ichnologist and just a plain ol’ tracker, was to see the differences in how they stepped down and moved once both deer were on the beach. Based on the trackway patterns, the lead deer simply took a big step down, landed with little drama, and began moving in a normal (baseline) gait for a deer, which is a diagonal pattern with indirect and direct register (rear-foot track on top of front-foot track on the same side). In contrast, the second deer leaped nearly two meters from the dune scarp to the beach, landed heavily, and broke into a gallop, denoted by a set of four tracks – both rear footprints ahead of both front footprints – followed by a space, then another set of four tracks.

Me taking a closer look at the tracks of the “jumper,” whose first tracks show up just behind me, whereas the other deer preceding it simply took a big step down. (Photograph by Ruth Schowalter, taken on Jekyll Island.)

A contrast in trackway patterns by deer on a beach: one that made a normal, diagonal-walking pattern with direct or indirect register (rear foot registering totally or partially on the front-foot impression), and the other galloping, in which front feet landed, then were exceeded by both rear feet, followed by a suspension phase. (Photograph by Anthony Martin, taken on Jekyll Island.)

A close-up of those tracks, in which Deer #1 (right) was strolling relaxedly, not kicking up so much sand, whereas Deer #2 (left) was taking sand with it as it forcefully punched through and extracted its feet from the sand while galloping. (Photograph by Anthony Martin, taken on Jekyll Island.)

This stark difference in their gait patterns led me to ask a simple question: why? This is where a bit of intuition came into play, in which I imagined the following scenario:

  • The first deer arrived at the dune scarp first, surveyed the scene, saw no threats in the immediate area, stepped down onto the beach, and walked normally.
  • The second deer, following behind the first, must have temporarily lost sight of the first deer once it stepped off the dune scarp. Not wanting to be left behind, it quickened its pace up to the scarp edge, spied its companion walking nonchalantly down the beach, and jumped.
  • The best way to catch up with its companion from there was to gallop, which it did.

With this hypothesis in mind – that maybe one deer was trying to catch up with the first one to join it – I had to be a good scientist and test it further. Looking down the beach, we saw how the tracks of the walking and the galloping deer eventually crossed one another, with the walking one crossing left, and the galloping one crossing right. Aha! I could use the old tried-and-true method used by generations of geologists, cross-cutting relations! This principle states that whatever cross-cuts another medium (say, a fault cross-cutting bedrock) is the younger of the two events. In this instance, I tracked the galloping deer to where it crossed and stepped on the tracks of the walking deer. Hence it came afterwards, but perhaps only a few minutes later, as the preservational quality of its tracks were identical to the first deer’s tracks. So it was very likely following and trying to catch up with its companion.

Close-up of the where Deer #2 stepped on the tracks of Deer #1 as it tried to catch up. This cross-over point is also where Deer #2 started going to the right of Deer #1, and was on the ocean side of it once they started traveling together, side-by-side. (Photograph by Anthony Martin, taken on Jekyll Island.)

Close-up of where Deer #2 stepped on the tracks of Deer #1 as it crossed its trackway, eventually traveling to the right of Deer #1. Scale in centimeters. (Photograph by Anthony Martin, taken on Jekyll Island.)

The tracks went down-slope for a distance further, and at some point turned to the left (north), showing where they walked next to one another, about 1.5 m (5 ft) apart and paralleling the surf zone. Where did they go from there? We don’t know, but I suspect they soon went back up into the dunes and back-dune meadows, just in time to avoid all of the humans and dogs who would be on the beach in the next few hours following sunrise. Still, the tracks conjured a beautiful image, of two white-tailed deer walking down the beach together, side-by-side, as the sun came up over the ocean to their right.

Not wanting to spend our entire morning tracking these two deer, we said, “OK, that was neat,” and got back on our bikes for more riding. Later, though, while reflecting on this lesson imparted by the deer tracks in a paleontological sense, I extended their range back into prehistory. How might such tracks from terrestrial mammals have been preserved in ancient beach sediments?  If they did get preserved, how would we would recognize them for what they were, or would we just assume they must be traces from some marine-dwelling animal (probably an invertebrate)? And even if we did realize these traces came from big terrestrial mammals, would we have the skills to interpret how two or more animals were affecting each others’ behaviors, which we did so easily with modern, fresh tracks directly in front of us, and knowledge of the daily tides and sunrise? This is the power of ichnology, in which these life traces motivate us to move mentally from the present, to the past, and back again.

As it was, we ended up not seeing a deer during the four days we spent on Jekyll. Nevertheless, we came away with a good story of at least two deer, knowing about their almost-secret trip to the beach, just a few hours before our own.

Further Reading

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

Halls, L.K. 1984. White-tailed Deer: Ecology and Management. Stackpole Books, Mechanicsburg, Pennsylvania: 864 p.

Hewitt, D.G. (editor). 2011. Biology and Management of White-tailed Deer. Taylor & Francis, Oxon, U.K.: 674 p.

Webb, S.L., et al. 2010. Measuring fine-scale white-tailed deer movements and environmental influences using GPS collars. International Journal of Ecology, Article ID 459610, doi:10.1155/2010/459610: 12 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.