Cumberland Island, Georgia: Not a Barrier to Education

When learning about the natural sciences, there comes a time when just reading and talking about your topics in the confines of a classroom just doesn’t cut it. This semester, we had reached that point in a class I’m teaching at Emory University (Barrier Islands), in which we all needed a serious reality check to boost our learning. So how about a week-long field trip, and to some of the most scientifically famous of all barrier islands, which are on the coast of Georgia?

Last Friday, March 8, our excursion officially began with a long drive from the Emory campus in Atlanta, Georgia to St. Marys, Georgia. Fortunately, Saturday morning was much easier, only requiring that we walk across the street, step onto a ferry, and ride for 45 minutes to Cumberland Island. Cumberland was our first island of the trip, and the southernmost of the Georgia barrier islands. I have written about other topics there, including the feral horses that leave their mark on island ecosystems, the tracks of wild turkeys, and those marvelous little bivalves, coquina clams.

So rather than my usual loquacious ramblings, punctuated by whimsical asides, this blog post and others later this week will be more photo-centered and accompanied by mercifully brief captions. This approach is not only a practical necessity for proper time management while teaching full-time through the week, but also is meant to give a sense of the daily discoveries that can happen through place-based learning on the Georgia coast. I hope you learn with us, however vicariously.

After a 45-minute ferry ride to Cumberland Island, the students received a different sort of lecture when naturalist extraordinaire Carol Ruckdeschel – who is writing a book about the natural history of Cumberland Island – met with them and gave them a brilliant overview of the island ecology. She mostly talked with the students about the effects of feral animals on the island, then spent another hour with us in the maritime forest and through the back-dune meadows. It was a real treat for the students and me, and a great way to start the field trip.

A leaf-cutter bee trace! Despite my writing about these and illustrating them in my book, these distinctive incisions were the first I can recall seeing on the Georgia barrier islands. These traces were abundantly represented in the leaves of a red bay tree we spotted along a trail through the maritime forest, making for a great impromptu natural history lesson for the students.

A freshly erupted ghost shrimp burrow on the beach at Cumberland, in which the students were lucky enough to witness the forceful ejection of muddy fecal pellets by the shrimp from the top of its burrow. I mean, really: explain to me how the life of an ichnologist-educator can get any better than that?

The fine tradition a field lunch, made even more fine by the addition of fine quart sand to our meals, freely delivered by a brisk sea breeze. Did the sand leave any microwear marks on our teeth? I certainly hope so.

A student is delighted to test my ichnologically based method for finding buried whelks underneath beach sands, and find out that it is indeed correct. (Was there any doubt?) Here she is proudly holding a live knobbed whelk, which I can assure you she promptly placed back into the water once its photo shoot was finished for the day.

Just to join in the fun, other students decided my “buried whelk prospecting” method required further testing. Let’s just say this student did not disprove the hypothesis, but rather seemed to confirm it, and doubly so. It’s almost as if ichnology is a real science! (Yes, these whelks went back into the water, too.)

OK, enough about marine predatory gastropods (for now). How about some of the largest horseshoe crabs (limulids) in the world? We found a large deposit of their carapaces above the high-tide mark, some of which were probably molts, but others recently dead. Sadly, though, we did not see any of their traces. Bodies only do so much for me.

Where do dunes come from? Well, a mother and father dune love each other very much… No wait, wrong story. What happens is that dead cordgrass from the salt marshes washes up onto the beach, where it starts slowing down wind-blown sand enough that it accumulates. Now it just needs some wind-blown seeds of sea oats and other plants to start colonizing it, and next thing you know, dune. Dude.

Ah, a geological tradition in action: comparing actual sand from a real outdoor environment to the sand categories on a handy grain-size chart, and using a hand lens. It’s enough to bring a tear to the eyes of this geo-educator. Or maybe that was just the wind-blown sand.

Finally, something that really matters, like ichnology! This is a three-for-one special, too: sanderling feces (left), tracks, and regurgitants (right), the last of these also known as cough pellets. Looks like it had coquina and dwarf surf clams for breakfast.

Wow, more shorebird traces! The tracks are from a loafing royal tern, and it clearly needed to get a load off its mind before moving on with the rest of its day.

Tired of shorebird traces? How about a modern terrestrial theropod? Wild turkey tracks in the back-dune meadows of Cumberland were a happy find, leading to my grilling the students with the seemingly simple question, “What bird made this?” They did not do well on this, but hey, it was the first day, and at least no one said “robin” or “ostrich.”

Did somebody say “doodlebug?” This long, meandering, and collapsed tunnel of an ant lion (a larval neuropteran, or lacwing) tells us that this insect was looking for prey in all the wrong places.

Behold, tracks that bespeak of great, thundering herds of sand-fiddler crabs that used to roam the sand flats above the salt marsh. Where have they gone, and will they ever come back? Who knows where the males might be waving their mighty claws? Do the female fiddler crabs suffer from big-claw envy, or are they enlightened enough to reject cheliped-based hierarchies imposed upon them by fiddler-crab society? All good questions, deserving answers, none of which make any sense.

Yes, that’s right, feral horses are really bad for salt marshes. Between overgrazing and trampling, they aren’t exactly what anyone could call “eco-friendly.” My students had heard me say this repeatedly throughout the semester, and Carol Ruckdeschel said the same thing earlier in the day. But then there’s seeing it for themselves, another type of learning altogether.

And the day ended with beautiful ripple marks, beckoning from the sandflat below the boardwalk on our trip back onto the ferry. Even this ichnologist can appreciate the aesthetic appeal of gorgeous physical sedimentary structures.

What’s the next island? Jekyll, which is just north of Cumberland along the Georgia coast, visited yesterday. Stay tuned, and look for those photos soon.

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.

Coquina Clams, Listening to and Riding the Waves

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.

Tracking the Wild Horses of Cumberland Island

(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.