Into the Dragon’s Lair: Alligator Burrows as Traces

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading

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

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

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

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

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

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

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

Gopher Tortoises, Making Deep and Meaningful Burrows

As I wrote this post, I was flying from Atlanta, Georgia to Minneapolis, Minnesota to attend the annual meeting of the Geological Society of America (GSA), where I’ll be with about 7-8,000 geoscientists from across and outside of the U.S. Why am I not doing something else, such as field work on the Georgia coast? Well, other than to learn the latest of what’s happening in the world of geology, seeing old friends, and meeting new ones, I’m here to share new scientific knowledge coming out of the Georgia coast with my fellow geologists and paleontologists. The subject of the presentation I will give tomorrow – Tuesday, October 11 – is about the wondrous burrows of a humble-looking, slow-moving, and seemingly lethargic reptile that actually is an ichnological force of nature: the gopher tortoise (Gopherus polyphemus).

A gopher tortoise in captivity, but living a safe and happy life at the 4-H Tidelands Nature Center on Jekyll Island, Georgia. Although it may not look like a big deal, it is a very impressive tracemaker, deserving the rapt attention of geologists and paleontologists. (Photograph taken by Anthony Martin.)

So you’re probably wondering why geologists and paleontologists should hear about gopher tortoises from me. It’s a good question, because I’m not a biologist, and these animals are famous for their very important role in ecosystems. Specifically, they are well known as keystone species in the sandy soils of longleaf pine-wiregrass communities of the southeastern U.S. Just like the keystone to a building, once you remove gopher tortoises from their ecosystems, a lot of other species disappear with it. Surprisingly, their ecological worth all revolves around their burrows.

And oh, what marvelous and grandiose burrows they make! The lengthiest of their measured burrows approach 14 meters (45 feet) long and as much as 6 meters (20 feet) vertically below the ground surface. These burrows commonly twist to the right or left on their way down, which probably helps protect its tortoise occupant against predators, while maintaining a constant temperature and humidity in the burrow. With so much digging, of course, a lot of sand has to be excavated, so the locations of their burrows are easily spotted by looking for piles of sand in the middle of a grassy field or in a longleaf-pine forest. For female tortoises, these sand piles also serve as nesting sites, where they bury their eggs to incubate.

Satellite view of gopher-tortoise burrows on St. Catherines Island, Georgia. Nearly all of the white spots you see in the photo – indicated by the yellow arrows – are the sand piles (aprons) outside of their burrows. Look closely, and you can see some of the trails worn down by tortoises traveling between burrows. Yes, these are animal traces you can see from space! (Original image from the U.S. Geological Survey and Google Earth, taken in May 2008.)

Close-up view of a sand apron outside of a gopher-tortoise burrow entrance. The large amount of sand tells you that this must be a very deep burrow. Field notebook is about 15 cm (6 in) long. (Photograph taken by Anthony Martin on St. Catherines Island, Georgia.)

In cross-section, their burrows have flat bottoms and rounded tops, similar to a tortoise body. Burrow widths varies with the length of the tortoise, as it needs to be wide enough for the tortoise to turn around in the burrow. So this means a 30-cm (12 in) wide burrow can accommodate a tortoise of that length or less. The powerful front limbs of tortoises are specially adapted for digging, ending in flat, spade-like feet with stout claws. Burrow walls are compacted by the hard shell of the tortoise as it moves up and down the burrow. These burrows descend steeply, at angles of 20-40°, which means they have to be good climbers to get out of their deep burrows.

Down-tunnel view of a gopher-tortoise burrow, with the light at the end of that tunnel not  from an oncoming train, but reflected morning sunlight on the tunnel wall at one of its turns. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Now think about a tunnel that’s about 10 m (33 ft) long and 30 cm (12 in) wide, and how much space that represents underneath the ground, and you’ll see what I mean about the vital role of these burrows ecologically, geologically, and (most importantly) ichnologically. In terms of ecology, about 200-300 species of invertebrate and vertebrate animals cohabit these burrows (whether a gopher tortoise is in it or not), including the longest snake in North America, the eastern indigo snake (Drymarchon couperi), the secretive gopher frog (Rana capito), the Florida mouse (Podomys floridanus), and a bunch of different insects. At least a few of the insects and the Florida mice make their own burrows, thus adding their little homes to the main burrow, like small anterooms to a big mansion.

Idealized conceptual sketch showing a cut-away view through a gopher-tortoise burrow with many additional burrows made by other animal species. Note especially the short horizontal tunnels near the burrow top, which would have been made by hatchling tortoises, and the vertical shafts that connect to these, which would have been made by Florida mice. (Illustration by Anthony Martin.)

So now you can see why this ichnologist (that would be me) became rather enamored with these burrows. For one thing, they have great preservation potential in the fossil record. A  general rule in ichnology for the preservation of burrows is “deeper is better,” in that burrows that go to great depths are less likely to be eroded by surface weathering and erosion, and more likely to be fossilized. Secondly, we know that vertebrate animals in the geologic past also made big burrows, such as synapsids and even small dinosaurs. I’ve done research on the few dinosaur burrows interpreted from the geologic record, and am especially interested in how such large burrows might compare with similar burrows made by modern animals, such as gopher tortoises.

But how to study these burrows without digging them out and leaving the tortoises undisturbed? Fortunately, two colleagues of mine at Georgia Southern University – Sheldon Skaggs and Robert (Kelly) Vance – came up with an elegant solution, which was to use ground-penetrating radar, also known by its acronym of GPR. This method uses a portable unit to transmit microwaves underground (don’t worry, not these aren’t intense enough to cook the tortoises), which reflect off surfaces with different qualities, especially the curved, compacted surfaces of burrow walls. Computers then process and render these reflections into three-dimensional images that more-or-less represent the forms and geometries of the burrows.

Sure enough, we tried out this technique on gopher-tortoise burrows on St. Catherines Island of the Georgia coast in January and July this year. Although we can’t share all of our results just yet, we did successfully make three-dimensional images of the burrows, all without us having to burrow ourselves, or bother the tortoises by becoming homewreckers. Veronica Greco, a wildlife biologist on St. Catherines Island who has studied the behavior and breeding of the tortoises, also helped us to better understand the biology of these reptiles.

Although it looks like Sheldon (center) is mowing the lawn and I’m (right) just supervising, he’s actually pushing a portable ground-penetrating radar (GPR) unit over a field that has some gopher-tortoise burrows in it, while I walk alongside to look at the reflection profiles. Kelly (background) is no doubt monitoring our every move, but is also recording our location. (Photograph by Ruth Schowalter, taken on St. Catherines Island, Georgia.)

My talk at the GSA meeting will be about how we used GPR to study the burrows in a non-invasive way, and how our results might be applied to studying similar burrows in the fossil record. After the meeting is over, we plan to summarize our results in a research article, which we’ll submit to a journal later this year for peer review.

Unfortunately, gopher tortoises are endangered because of huge losses in acreage of longleaf-pine forests in the southeastern U.S. during the past 200 years or so. Knowing this makes our study of their burrows even more meaningful, for if these wonderful tracemakers go extinct in the near future, we will not have the chance to study them and their burrows. In this sense then, only geologists and paleontologists who know about their ichnology through studies like ours will be able to study their burrows, which would be a sad thing indeed. Let’s hope they survive and thrive, and we can continue to learn more about these superb burrowing animals and their traces.

(P.S. Many thanks to the St. Catherines Island Foundation for their support of our research!)

Further Reading

Aresco, M.J., 1999. Habitat structures associated with juvenile gopher tortoise burrows on pine plantations in Alabama. Chelonian Conservation and Biology, 3: 507-509.

Doonan, T.J., and Stout, I.J., 1994. Effects of gopher tortoise (Gopherus polyphemus) body size on burrow structure. American Midland Naturalist, 131: 273-280.

Epperson, D.M., and Heise, C.D., 2003. Nesting and hatchling ecology of gopher tortoises (Gopherus polyphemus) in southern Mississippi. Journal of Herpetology, 37: 315-324.

Guyer, C., and Hermann, S.M. 1997. Patterns of size and longevity for gopher tortoise burrows: implications for the longleaf pine-wiregrass ecosystem. Bulletin of the Ecological Society of America, 78: 254.

Jackson, D.R. and Milstrey, E.R. 1989. The fauna of gopher tortoise burrows. In Diemer, J.E. (editor), Proceedings of the Gopher Tortoise Relocation Symposium, State of Florida, Game and Freshwater Fish Commission, Tallahassee, Florida: 86-98.

Jones, C.A., and Franz, R. 1990. Use of gopher tortoise burrows by Florida mice (Podomys floridanus) in Putnam County, Florida. Florida Field Naturalist, 18: 45-68.

Lips, K.R. 1991. Vertebrates associated with tortoise (Gopherus polyphemus) burrows in four habitats in south central Florida. Journal of Herpetology, 25: 477-481.

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

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

Witz, B.W., and Wilson, D.S., and Palmer, M.D. 1991. Distribution of Gopherus polyphemus and its vertebrate symbionts in three burrow categories. American Midland Naturalist, 126: 152-158.