The Ichnology of Pacific Rim

Last week I surrendered to geekdom peer pressure and went to see the new summer blockbuster Pacific Rim. Living up to my namesake, St. Anthony, I normally don't have a problem resisting such temptations, and just wait to see a movie like this in some other format: DVD, Netflix, or the way movies were originally intended to be seen, on a tiny screen on the back of an airplane seat. But what really pushed me to go was the following image, only glimpsed for a few seconds in one of the trailers:

Kaiju-Track-IntertidalOoo, look, a trace made in an intertidal sandflat! Perhaps it's from a ghost crab, moon snail, or shorebird. Hey, wait a minute, something doesn't quite look right. Are those people next to it? (Image from

Yes, that’s right: it's a gigantic footprint, and in what looks like an intertidal coastal environment, between the low tide mark and coastal dunes. That was all the incentive I needed, as I further wondered what other ichnological wonders would be included in the film. I was also encouraged to see where other scientifically inclined bloggers had fun with Pacific Rim by taking a look at its biology (here, here, and most recently, here) and physics (here here, and most recently, here). So given a $5 afternoon matinee and a spouse (Ruth) willing to indulge my sci-fi inner nerd (OK, so it’s not so “inner”), I had every reason to document the various traces and tracemaking activities in the film. You know, for science and science education.

The verdict? Well, I have to admit some mild disappointment with how the director – Guillermo del Toro - chose to focus on the conflicts between massive amphibious creatures (kaiju) constructed by interdimensional aliens and human-guided fighting machines (jaegers), rather than on their traces. Nonetheless, I managed to find some ichnological gems scattered throughout. For example, the footprint shown in the trailer did indeed look glorious on a big screen, and the human figures associated with it reminded me of Jason Isley’s whimsical underwater photos. But let’s take a closer look at what this footprint tells us about its maker.

Although viewed from an oblique angle, the track seems longer than wide, and has four clearly defined digits, although a probable fifth digit is visible on the side farthest from the viewer. All of the digits are forward-pointing and taper abruptly at their ends. The tracks also has an indentation on the “heel” (proximal) part of the foot, and is more-or less-bilaterally symmetrical. Pits inside of the track may represent additional anatomical traits, such as scales or other bumps on its skin, or could be sediment that underwent liquefaction or other soft-sediment deformation.

Kaiju-TrackInterpreted kaiju track, extrapolated from oblique view. Scale = 10 m (33 ft).

Using the people around the tracks as informal units of measurement, and assuming from the hiragana-katakana in the newscast image that this track - like many items - was made in Japan, we can estimate the dimensions of the track. Average heights for Japanese males and females are 1.71 m and 1.58 m, respectively, and the average of those is 1.64 m. Using one figure (boxed) as a unit that equals 1.64 m (5.4 ft), the footprint had about 18.4 Japanese-Person-Units (JPU) length and 10.1 JPU width, which converts to about 30 m (98 ft) long and 17 m (56 ft) wide. This results in a length:width ratio of about 1.8.

Kaiju-Track-MeasuredLength and width measurements for kaiju track, including figure used as 1.0 JPU = 1.64 m. Width measurement is assumed on basis of probable fifth digit impression on side of track furthest from the viewer.

Unlike in most articles published in high-impact journals, I'll actually admit potential sources of error in these measurements before I'm forced to retract this blog post under a cloud of scandal, followed by my accepting a high-paying position on Wall Street, where such inaccuracies are rewarded without penalty. For example, the width measurement, because it is being taken from an oblique angle (not so accurate) instead of from directly above (much more accurate) probably underestimates the actual width. So the actual width is probably closer to 20 m (67 ft), which reduces the length:width ratio to about 1.5. The length measurement would also benefit from more of an overhead view, and probably would best be studied using aerial high-resolution LiDAR scanning. So there.

To put this in ichnological perspective, when these dimensions are compared to typical sauropod dinosaur tracks from the Early Cretaceous of Texas - where everything is supposed to be bigger - the sauropod comes out looking pretty puny indeed. In this example, the rear track length is 87 cm (34 in) and width is 59 cm (23"), and although its length:width ratio comes out fairly close to my estimation for the kaiju track (1.47), it is only about 2% of its size. Some "thunder lizard."

Sauropod-Tracks-TexasSauropod tracks from the Early Cretaceous (about 120-million-years-old) Glen Rose Formation of central Texas.The larger track is from the left rear foot, and the smaller one in front of it is the left front foot; this sauropod was walking slowly with an "understep" gait, in which its rear foot stayed behind its front. Please read the preceding text for all of that measurement stuff, which ichnologists sometimes call "data." (Photograph taken by Anthony Martin in Dinosaur Valley State Park, near Glen Rose, Texas.)

Kaiju+Sauropod-Tracks copyTo-scale comparison between sauropod track (arrow, lower left) and kaiju track (right) to same scale. Looks like some cute little saurischian would be feeling a little inadequate. As Cowboy Curtis once said on Pee Wee's Playhouse, "You know what they say: Big feet, big boots!" Scale = 10 m.

Speaking of high impact, how about track depth and other features of this individual track that might tell us about behavior of the kaiju tracemaker? Oddly enough, the kaiju track looks too shallow to me, measuring only about 1.6 JPU, or about 2.5 m (8 ft) deep. It also lacks pressure-release structures, which are sedimentary structures caused by the tracemaker applying and releasing pressure against the wall of the track. Considering that kaiju were supposed to weigh tens of thousands of tons, this track should have a greater depth, along with major ridges and plates outside of the track outline that would have been imparted by any forward or lateral movement of its foot.

Alternatively, this track may represent more of what I would call a “stamp,” which would have been made by placing a foot directly down onto a soft substrate and pulling it straight up, rather than from moving forward or laterally. Based on this evidence, the kaiju might have been attempting to squish pesky humans, rather thank performing its normal, forward-walking, city-destroying gait. Unfortunately, the preceding and next track are not shown in the photo, which would help to test this hypothesis.

Other than size, how does the form of this track compare to those of other known dinosaur tracks? The length: width ratio comes out close to that of a sauropod dinosaur, yet other qualitative traits of the track, such as thin digits that taper and end with sharp clawmarks, are more like that of a theropod. But I do want to point out a little coincidence. Have you ever seen the front-foot track of a typical raccoon? Hmmm...

Raccoon+Kaiju-TracksI give you you raccoon tracks, and I give you kaiju track. That is all. (Photo of raccoon tracks taken by Anthony Martin on Cumberland Island, Georgia.)

What’s really fun, though, is if you compare the kaiju track to known theropod tracks. Theropod tracks bearing four or more forward-pointing toes are quite rare, and the few identified probably belong to a group of theropods called therizinosaurs, which - by a strictly enforced paleo-nerd law - cannot be mentioned in a sentence without also using the descriptor "bizarre." Late Cretaceous dinosaur tracks recently reported from Alaska with four long, forward-pointing digits have been attributed to therizinosaurs. Were the creators of the kaiju track trying to compare it to that of a really strange theropod dinosaur? Maybe, maybe not.

Therizinosaur-Tamara-TrackArtistic rendition of Nothronychus mckinleyi, a therizinosaur from the mid-Cretaceous of North America (left) and a four-toed rear-foot track credited to a therizinosaur from Late Cretaceous rocks of Alaska (right). Therizinosaur artwork by paleoartist Nobu Tamara and available in Wikipedia Commons here; photo of track by David Tomeo and reproduced from Everything Dinosaur.

Although the Pacific Rim kaiju designers used a mix of invertebrate and vertebrate elements for anatomical details appearances of their monsters (detailed splendidly by Darren Naish here), I do wonder how they came up with the track, and which real-life animals - modern or extinct - were supposed to be evoked by this track's brief appearance onscreen. Hopefully the DVD and its Special Features will reveal all once that comes out.

(Incidentally, this attempt to divine the evolutionary relatedness of a science-fictional animal from a single track reminds me of a scene from the classic science-fiction film Forbidden Planet. At some point, an invisible monster comes aboard a spaceship on the aforementioned planet and kills its chief engineer. The ship scientist, Dr. Ostrow, then gave a fine interpretation of the monster based on a plaster cast made from one of its footprints, including how it traits seemed to go against all known evolutionary principles. It's such a fun scene, I've shown it in some of my classes as an example of "extraterrestrial ichnology.")

Other tracemaking in the movie, of course, included wholesale destruction of major population centers by the kaiju, clawmarks left on various city substrates, as well as kaiju scat. Unlike other fans of the movie, I've only seen it once so far, and cannot recall whether the following picture of its droppings was flashed on the movie screen or not.

What-a-load-of-kaiju-crapThe banner for this news clip says it all: kaiju excrement, and you can bet this much did indeed contaminate a portion of Manila, Philippines (or the "Phillipines," which may be a gated community just outside of Philadelphia.) On the flip side, I'll bet a certain sick Triceratops in the movie Jurassic Park is now a little less self-conscious about having its wastes piled higher and deeper on the big screen.

One line about their excrement – uttered by kaiju-organ harvester, Hannibal Chau (played by a hilarious Ron Perlman) - alludes to its commercial value based on its phosphorus content. This would accord with the economic importance given to bat or bird guano, which has been mined and sold as fertilizer, and even inspired wars. (I am not making that up.) Still, it would have been beyond awesome to have just one scene showing a deposit of its scat enveloping a large, recognizable monument to a politician in one of those cities.

Hannibal Chau (Ron Perlman), selling kaiju products for whatever might ail you. Alas, their scat is not mentioned in this ad, but he could easily do another one directed at Whole Foods. After all, it would be 100% organic and free-range fertilizer!

What about the jaegers? Their traces are much tougher to discuss, semantically speaking. Ichnologists classify tools themselves as traces of behavior, but most do not count marks made by tools (or machines) as traces. Nonetheless, because the jaegers are being controlled by humans, the marks they leave on the landscape, seascapes, and upside some kaiju’s head, might count as traces, too.

However, in one scene of the movie, in which a kaiju picked up a jaeger and threw it – inflicting much destruction of private and public property – these traces would be those of the kaiju, not the jaeger. I pointed out a similar situation with Jurassic Park. Toward the end of the movie, the poor, misunderstood protagonist of the film - the Tyrannosaurus rex - in an action tinged with self-loathing, hurled a Velociraptor at a mounted T. rex skeleton, no doubt expressing doubt about her place in a post-Mesozoic world. Existentialist angst aside, the destruction of the skeleton was a trace of the tyrannosaur's behavior, not that of the Velociraptor.

So next time you go to a movie featuring multi-ton monsters emerging from the deep sea and massive fighting machines, look for them to make traces, note the traces they make, how these traces may reflect some sort of evolutionary history for the tracemakers, and ask yourself what constitutes a trace. Then no matter how bad the movie, you'll still be guaranteed to enjoy it. Happy movie viewing and tracking!

Going Hog Wild on the Georgia Barrier Islands

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• Tracks

• Rooting pits

• Wallows

• Feces

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading

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

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

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

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

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

Tracking the Wild 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.




Alien Invaders of the Georgia Coast

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

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

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

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

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

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

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

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

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

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

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

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

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

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