Teaching about Traces as Evidence

With the start of a new academic year, many university professors might be deliberating on what they’ll be teaching, and many students similarly (and hopefully) might be wondering what they will be taught. For me this academic year, my plan is not to put so much emphasis on the “what,” but more on the “how,” and put it in the form of a basic question: How could I be wrong?

In my experience, this is a question we professors and other educators we often ask, regardless of whether we are in the natural sciences, social sciences, humanities, or some blend of those educational realms. Now, this is not to say that we should continuously live our lives in doubt of our hard-earned skills and knowledge, succumbing to imposter syndrome. So what I will suggest is that we use it in our teaching, leading by example for our students. For instance, when my students see me question an initial interpretation of mine, correct that wrong interpretation, and show delight when this happens, then they will feel more comfortable asking themselves the same question, too.

So how do I apply this method to my research disciplines of paleontology and ichnology? If I am observing a natural phenomenon in the field, museum, or other settings, and I find myself jumping to a conclusion too rapidly, I take a moment to pause, back up, and try to disprove that hasty conclusion. Sometimes it turns out that, yes indeed, I was an idiot. But if this debunking process fails to find anything terribly wrong with my original explanation, or I modify it accordingly in the face of newly acquired evidence, then I’ll think this: So far, so good.

Eight-Legged-Otter-TracksWhoa, check out the tracks made by this eight-legged river otter! This eight-legged otter must have been the result of some freak mutation, or genetic engineering, or joined twin otters, or a robot spider with otter feet…What? Was it something I said? (Scale in centimeters; Photo by Anthony Martin.)

Moreover, because so much of paleontology and ichnology involves interpreting the products of non-witnessed lives, behaviors, and environments, such as bones, shells, leaves, tracks, and burrows, careful documentation of this evidence is key for making reasonable interpretations. Because we can’t prove ourselves wrong by watching a video of whatever happened in the pre-human past, we also have to ensure that the evidence can be shared and evaluated by other paleontologists and ichnologists.

In the following video, I explain these two basic scientific principles – how could I be wrong, and so far, so good – by using a few examples from a forested area next to the Emory University campus in Atlanta, Georgia. This is the place where I often teach first-year (freshman) students in a small-class seminar how to track the animals on and around our campus. Because most of these animals are nocturnal, most remain “invisible” to the students’ during their four years on campus. So my students really do learn how to use trace evidence to make reasonable hypotheses about animal presence and behaviors, and by the end of the semester, they get pretty good at it.

This sort of educational fruition is what made for the most fun part about doing this video, which was having a former student of mine who took the class four years ago play the role of my willing and eager “student.” In this, we demonstrated how the two basic principles – how could I be wrong, and so far, so good – are applied when in the field. It actually wasn’t much of a stretch for my former student, as Dorothy (Dottie) Stearns (Emory College ’16) was one of my best students in the class when she took it, and she really enjoys getting outside and tracking, so her enthusiasm is genuine.

The video is part of a series that Emory is producing on the theme of Evidence at Emory, with professors from a wide variety of disciplines explaining how they incorporate evidence-based reasoning in their courses. First-year students at Emory are the specific target of the videos so they are exposed to different disciplines and how scholars evaluate evidence in those disciplines. But there’s also hope that students will retain these discernment skills in life after college. Nonetheless, I think anyone who likes observing and thinking about what they observed can benefit from watching them. I could be wrong on that, but if not, I’m fine with that, too: for now.

Eight-Legged-Otter-TracksWait a minute, you’re saying these tracks could have been made by two otters, with one following closely behind the other? Huh, hadn’t thought of that. But that doesn’t mean eight-legged otters aren’t out there somewhere. Or freak mutated otters. Or genetically engineered otters. Or a robot spider with otter feet. What? Was it something I said?

Acknowledgements: Thanks to the Quality Enchancement Plan of Emory University for encouraging me to more overtly incorporate evidence as a main theme in my class, to Dottie Stearns for being such an awesome student/actor, and to the Center for Digital Scholarship, also of Emory University, for their fine work on the video production.

Tracking That Is Otterly Delightful

Writing about a place, its environments, and the plants and animals of those environments is challenging enough in itself. Yet to write about that place and what lives there, but without actually being there, seems almost like a type of fraud. Sure, given a specific place, I could read everything ever published about it, watch documentaries or other videos about it, carefully study 3-D computer-rendered images of its landscapes, interview people who have spent much time there, and otherwise gather information vicariously, all without experiencing it directly. But then is my writing just about the shadows on the wall of the cave?

River-Otter-Tracks-Sapleo-Beach-1What do you see in this photo? I see fine quartz and heavy-mineral sand, originally parts of much larger rocks and forming parts of the Appalachian Mountains. I see the sand blowing down a long beach, but pausing to form ripples. I see a river otter galloping alongside the surf, slowing to a lope, then a trot, then back to a lope and a gallop. I see a brief rain shower, only about two hours after the otter has left the beach. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)

This pondering, of course, brings us to river otters. Yesterday, while on the third of a four-day writing retreat to Sapelo Island on the Georgia coast, my wife Ruth and I spent nearly an hour tracking a river otter along a long stretch of beach there. Had I read about river otters and their tracks before then? Yes. Had I watched video footage of river otters? Yes. Had I written about river otters and their tracks before then? Yes. Had I seen and identified their tracks before then? Yes. Had I seen river otters in the wild for myself? Yes, yes, and yes.

But still, this was different. When I first spotted the tracks on the south end of a long stretch of Cabretta Beach on Sapelo, I thought they would be ordinary. Granted, finding otter tracks is always a joy, especially when I’ve seen them on stream banks in the middle of Atlanta, Georgia. (Seriously, folks: river otters live in the middle of Atlanta. How cool is that?) And because Sapelo only has a few humans and is relatively undeveloped, your chances of coming across otter tracks on one of the beaches there isn’t like winning a lottery. But still.

Otter-Tracks-Lope-Pattern-SapeloRiver otter (Lutra canadenis) tracks in what I (and some other trackers) call a “1-2-1” pattern. For gait, that translates into a “lope,” which is typical for an otter. In this pattern, one of the rear feet exceeds the front foot on one side, but the other rear foot ends up beside that same front foot; one front foot is behind. If that second rear foot lags behind the front foot, then it’s a “trot,” but if it exceeds the front foot (both rear feet ahead of both front feet), then that’s a “gallop.” Also, check out the wind ripples beneath the tracks, and raindrop impressions on top of them. (Photo by Anthony Martin, taken on Sapelo Island, Georgia; photo scale in centimeters, with the long bar = 10 cm (4 inches))

What made these tracks different was that they went on, and on, and on. These otter tracks spoke for the otter, saying in no uncertain terms that walking, trotting, loping, and galloping on a beach was the only thing it had on its schedule that morning. For nearly a kilometer (0.6 miles), we followed its tracks in the sandy strip of land between the high-tide line on the right and low coastal dunes on the left.


Follow the river otter tracks for as far as you can in this photo. Then, when you can’t see them any more, decide where it went. Does that sound like a challenge? It probably would be if you’ve only written about tracking otters, but it can be tough for experienced trackers, too. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)

The tracks were only a few meters away from high tide, but sometimes turned that way, vanished, then reappeared further down the beach. This told us the otter was out close to  peak tide that morning (between 6-8 a.m.) and was mixing up its exercise regime by occasionally dipping into the surf. Raindrop impressions on top of the tracks confirmed this, as the tracks looked crisp and fresh except for having been pitted by rain. For us, rain started inland and south of there on the island around 10 a.m., but reached the tracks sooner than that. We were there about three hours after then, so the otter was likely long gone, on to another adventure. Nonetheless, we made sure to look up and ahead frequently, just in case the trackmaker decided to come back to the scene of his or her handiwork.

For those of you who are intrigued by animal tracks (and why would you not be?), I suggest you try following those made by one animal, and follow it for as long as you can. That way you can learn much more about it as an individual animal, rather than just its species name. In my experience, after tracking an animal for a long time, nuances of its behavior, decisions, and even its personality emerge.

For example, this otter was mostly loping (its normal gait), but once in a while slowed to a walk or trot, or sped up, when it galloped. In short, the tracks showed enough variations to say that the otter was likely reacting to stimuli in its surroundings, and in many different ways. What gave it a reason to slow down? What impelled it to move faster? Why did it jump into the surf when it did, and why did it come out? Or, do otters just want to have fun?

River-Otter-Gallop-Pattern-SapeloGallop pattern for a river otter, in which both of its rear feet exceeded the front feet, making a group of four tracks. In this instance, the group defines a “Z” pattern when drawing a line from one track to another, but gallops sometimes also produce “C” patterns. Notice also how the groupings are separated by a space with no tracks. This is also diagnostic of a gallop pattern: the longer the space, the longer the “air time” for the animal, when it was suspended above the ground between when its feet touched the ground. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)

Now I realize that discerning a “personality” and “moods” of a non-human animal based on a series of its tracks might sound like a little too “woo-woo” and “New Agey” for my skeptical scientist friends to accept, followed by jokes about my becoming a pet psychic. As a fellow skeptical scientist, I’m totally OK with that. In fact, I will join them in making fun of people who try to tell us that, say, they know what a Sasquatch was thinking as it strolled through a forest while successfully avoiding all cameras and other means of physical detection.

But here’s what happens when you’ve tracked a lot (which I have) and made lots of mistakes while tracking, but later corrected them (ditto). Intuition kicks in, and it usually works. For instance, at one point in following this otter, I lost its tracks on a patch of hard-packed sand. (Granted, I should have gotten down on my hands and knees to look closer, but was being lazy. Hey, come on, I was on a writer’s retreat.) So I then asked myself, “Where would I (the otter) have gone?” and looked about 10 meters (30+ feet) ahead in what felt like the right place. There they were. This happened three more times, results that led me to conclude this was almost like some repeatable, testable, falsifiable science-like thing happening. So there.

River-Otter-Tracks-Sapelo-Beach-2-LabeledOK, remember when I asked you to follow the river otter tracks for as far as you could in this photo, and when you couldn’t see them any more, decide where it went? If not, go back and re-read it and look at the photo again. If you have, then look at the red arrow, backtrack to the footprints in the foreground of the photo, then go forward. Do you see how the tracks are staying in the subtly lower area, just left of the slightly higher sand piled on the plant debris? Keep picking out those low areas, and you’ll end up where the arrow is pointing. After all, if I were an otter, that’s where I would go. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)

Oh yeah, regarding my main topic sentence: What’s all this have to do with writing about a place? Well, because of that otter and its tracks, I now understand at least one otter much better than before, and feel like I can write with a little more authority about otters in general. You know, like what you just read.

Otter-Tracks-Lope-Pattern-Sapelo-2Do you understand this river otter and its place a little better now, thanks to it leaving so many tracks while it enjoyed a morning at the beach, and because I tracked it for such a long time, and then wrote about that experience in that same place? Please say “yes,” as I want to keep writing about stuff like this. P.S. Thanks to Sapelo Island, this river otter, and my wife Ruth for teaching me so much yesterday. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)

The Paleozoic Diet Plan

Given the truth that the Atlantic horseshoe crab (Limulus polyphemus) is more awesome than any mythical animal on the Georgia coast (with the possible exception of Altmaha-ha, or “Altie”), it’s no wonder that other animals try to steal its power by eating it, its eggs, or its offspring. For instance, horseshoe-crab (limulid) eggs and hatchlings provide so much sustenance for some species of shorebirds – such as red knots (Calidris canutus) and ruddy turnstones (Arenaria interpres) – that they have timed their migration routes to coincide with spawning season.

Ravaged-Limulid-SCISomething hunted down, flipped over, and ate this female horseshoe crab while it was still alive. Who did this, what clues did the killer leave, and how would we interpret a similar scenario from the fossil record? Gee, if only we knew some really cool science that involved the study of traces, such as, like, I don’t know, ichnology. (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, on May 4, 2013.)

Do land-dwelling birds mammals eat adult horseshoe crabs? Yes, and I’ve seen lots of evidence for this on Georgia beaches, but from only three species: feral hogs (Sus crofa) and vultures (Coragyps atratus and Cathartes aura: black vultures and turkey vultures, respectively). In all of these interactions, no horseshoe-crab tracks were next to their bodies, implying they were already dead when consumed; their bodies were probably moved by tides and waves after death, and later deposited on the beach. This supposition is backed up by vulture tracks. I’ve often seen their landing patterns near the horseshoe-crab bodies, which means they probably sniffed the stench of death while flying overhead, and came down to have an al fresco lunch on the beach.

Nonetheless, what I just described is ichnological evidence of scavenging, not predation. So I was shocked last month when Gale Bishop, while he was monitoring for sea-turtle nests on St. Catherines Island (Georgia), witnessed and thoroughly documented an incident in which a raccoon (Procyon lotor) successfully preyed on a live horseshoe crab. Yes, that’s right: that cute little bandit of the maritime forest, going down to a beach, and totally buying into some Paleozoic diet plan, a passing fad that requires eating animals with lineages extending into the Paleozoic Era.

Limulid-Death-Spiral-SCISo what’s the big deal here? Horseshoe crab comes up on beach, gets lost, spirals around while looking for the ocean, and dies in vain, a victim of its own ocean-finding ineptitude: the end. Nope, wrong ending. For one thing, those horseshoe crab tracks are really fresh, meaning their maker was still very much alive, then next thing it knows, its on its back. Seeing that horseshoe crabs are not well equipped to do back-flips or break dance, I wonder how that happened? (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, and you can see the date and time for yourself.)

Here is part of the field description Gale recorded, which he graciously shared with me (and now you):

“Female Horseshoe Crab at 31.63324; 81.13244 [latitude-longitude] observed Raccoon feeding on upside-down HSC [horseshoe crab] on south margin of McQueen Inlet NO pig tracks. Relatively fresh HSC track. Did this raccoon flip this HSC?”

Raccoon-Tracks-Pee-Limulid-Eaten-SCIWell, well. Looks like we had a little commotion here. Lots of marks made from this horseshoe crab getting pushed against the beach sand, and by something other than itself. And that “something else” left two calling cards: a urination mark (left, middle) and just above that, two tracks. I can tell you the tracks are from a raccoon, and Gale swears the urination mark is not his. (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, and on May 4, 2013.)

I first saw these photos posted on a Facebook page maintained by Gale Bishop, the St. Catherines Island Sea Turtle Program (you can join it here). This was one of this comments Gale wrote to go with a photo:

GB: “This HSC must have been flipped by the Raccoon; that was NOT observed but the fresh crawlway indicates the HSC was crawling across the beach and then was flipped – only tracks are Rocky’s!”

[Editor’s note: “Rocky” is the nickname Gale gives to all raccoons, usually applied affectionately just before he prevents them from raiding a sea-turtle nest. And by prevent, I mean permanently.]

My reply to this:

AM: “VERY fresh tracks by the HSC, meaning this was predation by the raccoon, not scavenging.”

In our subsequent discussions on Facebook, Gale agreed with this assessment, said this was the first time he had ever seen a raccoon prey on a horseshoe crab, and I told him that it was the same for me. This was a big deal for us. He’s done more “sand time” on St. Catherines Island beaches than anyone I know (every summer for more than 20 years), and in all my wanderings of the Georgia barrier island beaches, I’ve never come across traces showing any such behavior.

(Yes, that’s right, I know you’re all in shock now, and it’s not that this was our first observance of this phenomenon. Instead, it is that we used Facebook for exchanging scientific information, hypotheses, and testing of those hypotheses. In other words it is not just used for political rants, pictures of cats and food, or political rants about photos of cat food. Which are very likely posted by cats.)

Now, here’s where ichnology is a pretty damned cool science. Gale was on the scene and actually saw the raccoon eating the horseshoe crab. He said it then ran away once it spotted him. (“Uh oh, there’s that upright biped with his boom stick who’s been taking out all of my cousins. Later, dudes!”) And even though I trust him completely as a keen observer, excellent scientist, and a very good ichnologist, I didn’t have to take his word for it. His photos of the traces on that Georgia beach laid out all of the evidence for what he saw, and even what happened before he got there and so rudely interrupted “Rocky” from noshing on horseshoe-crab eggs and innards.

Raccoon-Galloping-Limulid-Death-Spiral-Traces-SCIAnother view of the “death spiral” by the horseshoe crab, which we now know was actually a “life spiral” until a raccoon showed up and updated that status. Where’s the evidence of the raccoon? Look in the middle of the photos for whitish marks, grouped in fours, separated by gaps, and each forming a backwards “C” pattern. Those are raccoon tracks, and it was galloping away from the scene of the crime (toward the viewer).

Raccoon-Galloping-Pattern-SCISo you don’t believe me, and need a close-up of that raccoon gallop pattern? Here you go. Both rear feet are left, both front feet are right, and the direction of movement was to the left; when both rear feet exceed the front, that’s a gallop, folks. Notice the straddle (width of the trackway) is a lot narrower than a typical raccoon trackway, which is what happens when it picks up speed. When it’s waddling more like a little bear, its trackway is a lot wider than this. Conclusion: this raccoon was running for its life.

Although this is the only time Gale has documented a raccoon preying on a horseshoe crab – and it is the first time I’ve ever heard of it – we of course now wonder whether this was an exception, or if it is more common that we previously supposed. The horseshoe crab was a gravid female, and was likely on the beach to lay its eggs. Did the raccoon somehow know this, and sought out this limulid so that – like many shorebirds – it could feast on the eggs, too, along with some of the horseshoe crab itself? Or was it opportunistic, in that it was out looking for sea-turtle eggs, saw the horseshoe crab, and thought it’d try something a little different? In other words, had it learned this from experience, or was it a one-time experiment?

All good questions, but when our data set is actually a datum set (n = 1), there’s not much more we can say about this now. But given this new knowledge, set of search patterns, and altered expectations, we’re more likely to see it again. Oh, and now that you know about this, so can you, gentle reader. Let us know if you see any similar story told on the sands of a Georgia beach.

You want one more reason why this was a very cool discovery? It shows how evolutionary lineages and habitats can collide. Horseshoe crabs are marine arthropods descended from a 450-million-year-old lineage, and likely have been coming up on beaches to spawn all through that time. In contrast, raccoons are relative newcomers, coming from a lineage of land-dwelling mammals (Procyonidae) that, at best, only goes back to Oligocene Epoch, about 25 million years ago. When did a horseshoe crab first go onto land and encounter a land-dwelling raccoon ancestor? Trace fossils might tell us someday, especially now that we know what to look for.

So once again, these life traces provided us with a little more novelty, adding another piece to the natural history of the Georgia coast. Moreover, a raccoon preying on a horseshoe crab was another reminder that even experienced people – like Gale, me, and others who have spent much time on the Georgia barrier islands – still have a lot more to learn. Be humble, keep eyes open, and let the traces teach you something new.

(Acknowledgement: Special thanks to Dr. Gale Bishop for again spotting something ichnologically weird on St. Catherines Island, documenting it, and sharing what he has seen during his many forays there.)

Deer on a Beach

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.


Tracking Wild Turkeys on the Georgia Coast

Of the many traditions associated with the celebration of Thanksgiving in the U.S., the most commonly mentioned one is the ritual consumption of an avian theropod, Meleagris gallopavo, simply known by most people as “turkey.” The majority of turkeys that people will eat this Thursday, and for much of the week afterwards, are domestically raised. Yet these birds are all descended from wild turkeys native to North America. This is in contrast to chickens (Gallus gallus), which are descended from an Asian species, and various European mammals, such as cattle, pigs, sheep, and goats (Bos taurus, Sus scrofa, Ovis aries, and Capra aegagrus, respectively).

Trackway of a wild turkey (Meleagris gallopavo) crossing a coastal dune on Cumberland Island, Georgia. Notice how this one, which was likely a big male (“tom”), was meandering between clumps of vegetation and staying in slightly lower areas, its behavior influenced by the landscape. Scale = 20 cm (8 in). (Photograph by Anthony Martin.)

American schoolchildren are also sometimes taught that one of the founding fathers of the United States, Benjamin Franklin, even suggested that the wild turkey should be elevated to the status of the national bird, in favor of the bald eagle (Haliaeetus leucocephalus). With an admiring (although I suspect somewhat facetious) tone, he said:

He [the turkey] is besides, though a little vain & silly, a Bird of Courage, and would not hesitate to attack a Grenadier of the British Guards who should presume to invade his Farm Yard with a red Coat on.”

There are eight of us, and only one of you. Do you really want to mess with us? (Photograph by Anthony Martin, taken on Cumberland Island, Georgia.)

Unfortunately, because I live in the metropolitan Atlanta area, I never see turkeys other than the dead packaged ones in grocery stores. Nonetheless, one of the ways I experience turkeys as wild, living animals is to visit the Georgia barrier islands, and the best way for me to learn about wild turkey behavior is to track them. This is also great fun for me as a paleontologist, as their tracks remind me of those made by small theropod dinosaurs from the Mesozoic Era. And of course, as most schoolchildren can tell you, birds are dinosaurs, which they will state much more confidently than anything they might know about Benjamin Franklin.

Compared to most birds, turkeys are relatively easy to track. Their footprints are about 9.5-13 cm (3.7-5 in) long and slightly wider than long, with three long but thick, padded toes in front and one shorter one in the back, pointing rearward. In between these digits is a roundish impression, imparted by a metatarsal. This is a trait of an incumbent foot, in which a metatarsal registers behind digit III because the rear part of that toe is raised off the ground. The short toe is digit I, equivalent to our big toe, but not so big in this bird. Despite the reduction of this toe, its presence shows that turkeys probably descended from tree-dwelling species, as this toe was used for grasping branches. Clawmarks normally show on the ends of each toe impression, and when a turkey is walking slowly, it drags the claw on its middle toe (digit III), thus making a nicely defined linear groove.

Wild turkey tracks made while it was walking slowly up a gentle dune slope, dragging the claw on the middle digit of its right foot, making a long groove. Also notice the bounding tracks of a southern toad (traveling lower right –> upper left), cross-cutting the turkey tracks. (Photograph by Anthony Martin, taken on Cumberland Island.)

A normal walking pace (right foot –> left foot, left foot –> right foot) for a turkey is anywhere from 15-40 cm (6-16 in), and its stride (right foot –> right foot, left foot –> left foot) is about twice that, or 30-80 cm (12-32 in), depending on the age and size of the turkey. Their trackways show surprisingly narrow straddles for such wide-bodied birds, only 1.5 times more than track widths. This is because they walk almost as if on a tightrope, with angles between each step approaching 180°; so they still make a diagonal pattern, but nearly define a straight line. However, turkeys meander, stop, or change direction often enough to make things interesting when tracking them. Their flocking behavior also means their tracks commonly overlap with one another or cluster, making it tough to pick out the trackways of individual turkeys. However, in such flocks, the dominant male’s tracks are noticeably larger than those of the females or younger turkeys, so these can be picked out and help with sorting who’s who.

Turkey trackway in which it walked across the wind-rippled surface of a coastal dune on Cumberland Island, meandering while moseying. Same photo scale as before. (Photograph by Anthony Martin.)

An abrupt right turn recorded by a turkey’s tracks. Check out that beautiful metatarsal  impression in the second track from the right, and how the claw dragmark in the thrid track from the right points in the direction of the next track. (Photograph by Anthony Martin.)

One of the more remarkable points about these Georgia barrier-island turkeys, though, is how their tracks belie their stereotyped image as forest-only birds. Although they do spend much of their time in the forest, I’ve tracked turkeys through broad swaths of coastal dunes, and sometimes they will stop just short of primary dunes at the beach. So however difficult it might be to think about these birds as marginal-marine vertebrates, their tracks overlap the same places with ghost-crab burrows and shorebird tracks. Geologists and paleontologists take note: this exemplifies the considerable overlap between terrestrial and marginal-marine tracemakers that can happen in coastal environments. This also happened with dinosaurs that strolled onto tidal flats or otherwise passed through marginal-marine ecosystems.

Turkey tracks heading toward the beach, with the open ocean visible just beyond. Is this close enough to consider turkeys as marginal-marine tracemakers? (Photograph by Anthony Martin.)

Do these turkeys also have an impact on the dunes themselves? Yes, although these effects vary, from trackways disrupting wind ripples to more overt changes to the landscape. For instance, one of the most interesting effects I’ve seen is where they’ve caused small avalanches of sand downslope on dune faces. Interestingly, this same sort of phenomenon was also documented for Early Jurassic dinosaurs that walked across dry sand dunes, which caused grainflows that cascaded downhill with each step onto the sand.

Grainflow structure (arrow), a small avalanche caused by a turkey walking down a dune face. (Photograph by Anthony Martin.)

Close-up of grainflow structure (right) connected to turkey tracks, which become better defined once the turkey reached a more level surface. (Photograph by Anthony Martin, taken on Cumberland Island.)

What other traces do turkeys make? A lot, although I’ve only seen their tracks. Other traces include dust baths, feces, and nests. Dust baths, in which turkeys douse themselves with dry sediment to suffocate skin parasites, must be awesome structures. These are described as 50 cm (20 in) wide, 5-15 (1-3 in) deep, semi-circular depressions, and feather impressions show up in those made in finer-grained sediments. Although such structures would have poor preservation potential in the fossil record, I hold out hope that if paleontologists start looking more at modern examples, they are more likely to find a fossil dust bath, whether in Mesozoic or Cenozoic rocks.

Turkey feces, like most droppings from birds, have white caps on one end, but are unusual in that these can tell you the gender of their depositor. Male turkeys tend to make curled cylinders that are about 1 cm wide and as much as 8 cm long (0.4 X 3 in), whereas females make more globular (not gobbular) droppings that are about 1 cm (0.4 in) wide. These distinctive shapes are a result of their having different digestive systems. Turkeys are herbivores, so their scat normally includes plant material, but don’t be surprised to see insects parts in them, too. Still think about how exciting it would be to find a grouping of same-diameter cylindrical and rounded coprolites in the same Mesozoic deposit, yet filled with the same digested material, hinting at gender differences (sexual dimorphism) in the same species of dinosaur maker.

Turkeys normally make nests on the ground by scratching out slight depressions with their feet, but evidently this is a flexible behavior. On at least one of the Georgia barrier islands (Ossabaw), these birds have been documented as building nests in trees. Although this practice seems very odd for a large, ground-dwelling bird, it is an effective strategy against feral hogs, which tend to eat turkey eggs, as well as eggs of nearly every other species of bird or reptile, for that matter. Just to extend this idea to the geologic past, ground nests are documented for several species of dinosaurs, but tree nests are unknown, let alone whether species of ground-nesting dinosaurs were also capable of nesting in trees.

As everyone should know from their favorite WKRP episode, domestic turkeys can’t fly. But wild turkeys can, and use this ability to get into the branches of live oaks (arrow), high above their predators, or even curious ichnologists. (Photograph by Anthony Martin, taken on Cumberland Island.)

So whether or not you have tryptophan-fueled dreams while dozing later this week, keep in mind not just the evolutionary heritage of your dinosaurian meal, but also what their traces tell us about this history. Moreover, it is an understanding aided by these magnificent and behaviorally complex birds on the Georgia barrier islands. For this alone, we should be thankful.

Paleontologist Barbie, tracking wild turkeys on the Georgia coast to learn more about how these tracemakers can be used as modern analogs for dinosaur behavior and traces, and once again demonstrating why she is the honey badger of paleontologists. (Yes, photograph by me, and taken on Cumberland Island. P.S. Happy Thanksgiving!)

Further Reading

Dickson,J.G. (editor). 1992. Wild Turkeys: Biology and Management. Stackpole Books, Mechanicsburg, Pennsylvania: 463 p.

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

Fletcher, W.O., and Parker, W.A. 1994. Tree nesting by wild turkeys on Ossabaw Island, Georgia. The Wilson Bulletin, 106: 562.

Loope, D.B. 2006. Dry-season tracks in dinosaur-triggered grainflows. Palaios, 21: 132-142.

How to Track a Vampire (Bat)

The arrival of Halloween reminds us to celebrate mythical creatures that frighten yet also intrigue us, although recent popular crazes have made this less of an annual event and more year-round. Along those lines, probably the top three of such imaginary beings are zombies, werewolves, and vampires. All of these can be classified as changelings of a sort, with two of them dead, but not really. Here in Georgia, public fascination with zombies has even provided employment opportunities, as many people compete for coveted slots as shuffling extras on the TV series The Walking Dead.

Among these inspirations for Halloween costumes, short stories, novels, musicals, TV shows, and movies, which would be the toughest for an aspiring Van Hesling to track down using ichnological methods? Zombies would be far too easy, considering their slow-moving, foot dragging, bipedal locomotion; their trackways would also commonly intersect as they bump into one another in their search for cranial sustenance. In other words, zombie trackway patterns would closely match those of people texting.

As a result, we have many modern analogs for zombie traces, which would also make their recognition in the fossil record that much easier. Traces made by the zombie-like characters portrayed in 28 Days Later, however, would be far different, showing greater distances between tracks and reflecting more rapid movement. (And all kidding aside, we actually do have trace fossil evidence of zombie ants from about 50 million years ago, an example of reality trumping fiction.)

Similarly, tracking werewolves would be straightforward, in that trackway patterns should show normal human bipedal locomotion followed by abrupt changes to quadrupedal patterns that would range from a trot to full gallop, gaits that are comparatively rare in humans. Anatomical details of tracks would also include a transition from five-toed plantigrade tracks to four-toed digitigrade ones, and metatarsal impressions would be replaced by heel-pad impressions. Additional traces to expect from a werewolf would be the direct effects of successful predation, such as blood spatters, scattering of prey body parts, toothmarks, and so on. (Don’t ask me about werewolf scat, though. I don’t even want to think about some of the things that would show up in that, especially if they started consuming suburbanites.)

Mixed assemblage of wolf and human tracks, which no doubt proves the existence of werewolves. Or not. Your choice. (Photograph by Anthony Martin, taken in Yellowstone National Park, Wyoming: scale = 10 cm (4 in).)

A closer look at those supposed “wolf” tracks. Yes, I know, they’re in the same area of Yellowstone National Park where a successful wolf-release program took place. But my doubt means you have to consider the impossible as equally valid.

A gorgeous “wolf” track with evidence of skidding to a halt and turning to the right. Could this have been made immediately after a human transformed into a wolf? My Magic 8-ball says, “Ask again later.”

Scene from some movie I’ll never see, in which one of the characters undergoes a mid-air transformation from a human to a werewolf (Canis lupus hormonensis), abruptly changing his tracks from a more plantigrade bipedal running to digitigrade quadrupedal movement. Sorry, I don’t know if any evidence of teen angst would preserve in such a trackway, nor do I care.

In contrast to zombies and werewolves, vampires would be the most challenging to track, considering their occasional aerial phases of movement, as depicted in Bram Stoker’s novel Dracula (1897) and various popular adaptations. Traces made during a pre-transformation phase – while still in human form – would be indistinguishable from those of a non-undead human, texting or not, and once in the air, no evidence of its movement would be recorded.

A large bat (megachiropteran) in flight, leaving no traces of its passing when traveling in a substrate of air.

So just to leave vampires for a moment, let’s talk about bats, which are real and do leave traces of their activities. Knowing that bats are among the most diverse and abundant of mammals (more than 1,200 species), I made sure to discuss their traces in my upcoming book, Life Traces of the Georgia Coast. Although I personally have not yet seen any of their traces on the Georgia barrier islands, these are predictable and identifiable, so I hold out hope that I or someone else will find them some day.

Probably the most likely traces made by bats that one could encounter on the Georgia barrier islands are their feces, which in other places, through the right geology (think caves) and collective action, can form economic resources (more on that later). About 75% of bat species are insectivores, and because they catch their meals on the fly, their scat will mostly contain winged insect parts. However, the geology of the Georgia barrier islands lacks limestone, and thus precludes the formation of caves or other environments serving as roosting spots for bat colonies. Thus bat feces, such as those dropped by the common brown bat (Myotis lucifugus), will be hard to find unless you look in the right place, such as below a favorite roosting spot. If you are lucky enough to notice these, though, these traces are dark 2-3 mm (0.1 in) wide and 5-15 mm (0.2-0.6 in) long cylinders and filled with parts of flying insects.

Two small samples of bat poop for you. You’re welcome. (Image from Internet Center for Wildlife Damage Management.)

Most other bats are fruit-eaters; this means these bats, like many birds, are also important seed dispersers through their excreting indigestible seeds covered in fertilizer. Speaking of fertilizers, massive deposits of bat feces (guano) also accumulate in caves and other places where millions of bats have roosted. These nitrogen- and phosphorous-rich deposits have been mined for fertilizers used in agriculture, an example of feeding traces helping to feed people.

Do bats come to the ground and leave tracks? Yes, once in a while they do, where they might forage and walk on all fours. When they do this, they make diagonal walking patterns, contacting with the thumbs on the tips of their wings – which are skin membranes connected to their other, elongated fingers – and their rear feet.

OK, now back to vampires, or rather, vampire bats. There are only three species of parasitic bats, all of which subsist on the blood of other mammals. For feeding, they slice skin with their sharp teeth, which leaves a small (several centimeters long, millimeters thin) incision. They then lap up whatever blood comes out, and the victim often isn’t aware of its blood loss. These wounds also heal, but leave visible scars.

What about other traces left by vampire bats? Surprisingly, scientists have actually asked themselves, “Hey, I wonder how vampire bats get around on the ground?”, and conducted experiments on terrestrial movement of the common vampire-bat (Desmodus rotundus), as well as the short-tailed bat of New Zealand (Mystacina tuberculata).

Just in case you needed another reason why science is cool, these scientists constructed bat-sized treadmills and placed these bats on them. This experiment confirmed that bats, including the common vampire bat, perform an alternating-walking movement in which the rear foot (pes) registers just behind the thumb, which also bears a claw. (This claw comes in handy as a sort of grappling hook at they climb onto their blood sources.)

Walking on Wings from Science News on Vimeo.

Based on this video, here is what I would hypothesize as the trackway pattern of a walking vampire bat. Note that the rear foot has five digits, nearly equal in length, and that the feet point away from the midline of the trackway.

But then they found out something most people didn’t expect. When they increased treadmill speeds, the bats bound and almost gallop, in which their rear feet nearly move past their wings. While bounding, these bats land on one of the digits on their wings, then push off with their rear feet, causing a suspension phase, reaching maximum speeds of 1.2 m/s. (Which, incidentally, is about the same speed as most people walking while texting, or slow zombies.) The resulting trackway patterns would be in sets of four – rear feet paired behind thumb impressions – separated from one another by about a body length. Based on my viewing of the videos, the trackways would show both half-bound and full-bound patterns, in which the rear feet are either offset or parallel, respectively.

Vampire Running from Science News on Vimeo.

And here is the hypothesized trackway pattern for a running vampire bat, which is almost like a gallop pattern, but more like a half-bound or full-bound. The feet actually should point a little more inward than during walking, and depending on the substrate, deformation structures might be associated with track exteriors.

Just to insert a little paleontology into this consideration of bat traces: has anyone found a trackway, feces, or other traces made by bast in the fossil record? No, unless you count old guano deposits as trace fossils (which I would if they exceed 10,000 years old). The body fossil record for bats extends back to the Eocene Epoch, about 50 million years ago, but such fossils are rare, too. Far more impressive than a bat body fossil, though, would be a fossil bat trackway would be the discovery of a lifetime, almost as noteworthy as finding an actual vampire. And if you found a fossil bat trackway where it was running? Time to start playing the lottery.

More readily available in ancient strata, though, are pterosaur tracks, whose makers likely walked in a manner similar to bats when on land. Hence bats, although not directly related to these flying reptiles, may provide analogues for how some small pterosaurs moved about when on the ground. Despite their long study and many pterosaur fossils, though, a few people are still arguing about how pterosaurs moved on the ground. So hopefully more studies of bat locomotion will help us to better understand the earthbound behaviors of pterosaurs.

The take-home message of the preceding is that even though zombies, werewolves, and vampires still garner plenty of attention from the public, the truth is that real animals of the past and present – like bats and pterosaurs – are actually more fantastic than we sometimes know. Sure, let’s continue to have fun with our mythical creatures, but in the meantime, also keep an eye out for traces left by the marvelous animals of today and yesteryear.

Further Reading

Elbroch, M. 2003. Mammal Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, Pennsylvania: 778 p.

Mazin, J.-M., Billon-Bruyat, J.-P., and Padian, K. 2009. First record of a pterosaur landing trackway. Proceedings of the Royal Society of London, B, 276: 3881-3886.

Padian, K., and Fallon, B. 2012. Meta-analysis of reported pterosaur trackways: testing the corrspondence between skeletal and footprint records. Journal of Vertebrate Paleontology, 32 [Supplement to 3]: 153.

Riskin, D.K. et al. 2006. Terrestrial locomotion of the New Zealand short-tailed bat Mystacina tuberculata and the common vampire bat Desmodus rotundus. Journal of Experimental Biology, 209: 1725-1736.