Ben StegengaViews:
If you’re familiar at all with the Gopher Tortoise, you’re probably already aware that the burrows these tortoises construct provide important habitat for hundreds of species. More than 350 other animal species have been documented using Gopher Tortoise burrows in one way or another, making them a keystone species. These animals range from small seldom noticed invertebrates like the Gopher Tortoise Hister Beetle and the Gopher Tortoise Tick to charismatic vertebrates like the Gopher Frog, Burrowing Owl, and even the occasional American Alligator! While many species only use tortoise burrows opportunistically, others are found nowhere else on earth, and their persistence as species depends wholly upon healthy tortoise populations. These organisms are known as obligate commensals. Obligate tortoise commensals may require tortoise burrows for their entire lifespans, or only a specific part of their natural history. Some are dietary specialists, like the Gopher Tortoise Noctuid Moth, whose caterpillars feed upon tortoise dung within the shelter of their burrows or the Gopher Tortoise Shell Moth whose caterpillars feed only on the keratin from the shells of dead tortoises. And yet others primarily use burrows as means of escaping harsh weather or fire.
One of the Gopher Tortoise’s most famous obligate commensals is the Eastern Indigo Snake. Throughout the northern portion of their range, Eastern Indigo Snakes depend heavily on tortoise burrows for shelter during the late fall and winter months. During the rest of the year, indigo snakes spread out across the landscape, often following river corridors where water and shade is easily accessible and prey is abundant. But as fall arrives indigo snakes must return to Gopher Tortoise sandhills to shelter from winter temperatures. This shift in habitat brings snakes into much closer proximity to each other, and marks the beginning of their breeding season. Not only do the tortoise-rich sandhills set the stage for indigo courtship and male-male combat, but the burrows themselves seem to play an important role as nest sites. From the few observations we have of indigo snake eggs in the wild, most of them were found inside or associated with tortoise burrows (link to Nov 2024 Fieldnotes video here). Their entire reproductive biology is linked to these sandhills, so that makes the winter months the opportune time to conduct research.
During most winters, we carry out standard occupancy surveys across a rotating selection of sites. These surveys require us to hike habitat and visually search for snakes or any evidence they might have left behind (shed skin, tracks, etc.). We also use a burrow scope to detect snakes that are inside tortoise burrows. This is a fairly effective survey method, and it allows us to mark, measure, weigh, and test snakes for disease. However, last winter we wanted to test a new survey method. This was a large collaborative effort to see if eDNA (DNA left in the environment by an organism) would be a viable way of detecting indigo snakes. In order to achieve this, we tested soil for snake DNA and compared its effectiveness against visual surveys and camera trap surveys which were the two already established methods for surveying for indigo snakes. (We are currently in the process of publishing our results, so stay tuned for that!)
I was pretty familiar with the survey method, but this was actually the first time I had run game cameras on tortoise burrows. The sites we were working on had well-managed habitat and a lot of herp diversity, so I was pretty excited about what we might see. The plan was to mount 10 cameras on stakes pointed at tortoise burrow entrances to catch any snakes moving in or out. We also had 10 cameras mounted on drift fences to detect snakes moving across the site. Cameras were set to be motion triggered in addition to a 30 second time lapse, so we accumulated over 1.4 million photos by the end of the project! Of course, many of these photos were of nothing, but we also captured a lot of wildlife activity.
After going through several hundred thousand photos, it became apparent there was a lot going on at burrow openings, and today I want to highlight some of the more interesting observations. Now we must remember that these are just snapshots of animal behavior, and we only have the limited context provided in these still frames. In many instances there are 30 second gaps between images and we can only see what’s present in that square meter or so right around the burrow opening. So admittedly, there will be some speculation on my part, but I am interpreting the scene as best I can with what information is provided. Regardless of interpretation, I think the big takeaway here is just how interconnected everything is in these longleaf ecosystems. These animals are being drawn to these spots for a reason. The photos clearly demonstrate that something as simple as a hole in the ground can have wide impacts on the ecology of entire communities, and likely in ways we have yet to discover. So I hope you find these observations and my ecological musings interesting and maybe even a little thought-provoking.
Foraging Songbirds
Throughout the survey season we documented at least 10 species of songbirds visiting tortoise burrow aprons, many of which actually ventured inside the burrow opening for a little bit. It’s hard to say what each of these birds was doing, but I got the impression that many of these birds were there to forage for invertebrates and maybe seeds. Small spiders, beetles, and other insects are commonly observed around burrow openings, so it makes sense that birds would take advantage of the opportunity. In some cases, particularly with the Carolina Wrens, we can see the birds moving dead leaves and picking up small objects off the ground. The resolution does not allow us to identify what is being collected, but it matches foraging behavior that I’ve seen from passerine birds elsewhere. Also, in two separate instances I’ve witnessed Bachman’s Sparrow flee my presence by hopping down into tortoise burrows, rather than flying or running away. So it’s possible some of these observations may show birds approaching the burrow as a form of cover from a potential threat that’s off camera.
Predator and Prey or Just Uneasy Roommates?
This is a very short but fun series of images that takes place over a span of 6 minutes. We can see an adult Eastern Indigo Snake crawl into the burrow and about 3 minutes later, this small mouse (Peromyscus sp.) is captured in mid-air as it’s fleeing the burrow! Then another couple minutes later we see the indigo exit the burrow. Now it’s probably pretty safe to assume that the mouse fled because of the snake, but was the snake actually attempting to prey on the mouse, or was the mouse being there just coincidence? It’s not uncommon for indigo snakes to briefly visit burrows and then immediately move on. However, indigo snakes in Georgia will occasionally eat during the winter, so it’s very possible this indigo was looking for some winter calories and continued the hunt outside the burrow. Either way, this is a fun series of photos!
Breakfast and a Sunbath
I’m not exactly surprised by this observation, but more so disappointed that I hadn’t thought about this before. Tortoise burrows are often located in spots that have low tree canopy and therefore receive a lot of sunlight. We also know that animals use burrows for shelter, including an abundance of invertebrates. So it only makes sense that lizards would take advantage of this microhabitat. Both Green Anoles and Eastern Fence Lizards remain active on warm winter days in the Coastal Plain, and we documented them both using tortoise burrows. While some fence lizards appeared to simply emerge from the burrow for daytime activity, other individuals and this anole are seen approaching the burrow from elsewhere. They proceeded to move between the exposed roots, the oak leaf litter, the bare apron, and the sandy rim of the burrow. This cycle repeated several times, with both lizards frequently cocking their heads to the side between movements, and sometimes disappearing into the leaf litter before re-emerging, presumably in pursuit of prey. It’s also worth noting that the fence lizard spends considerable time basking in the sun, and the anole changes color throughout. While the anole is on the roots and in the leaf litter, its color is primarily brown, but whenever it sits on the sandy rim of the burrow in the direct sun, it turns vibrant green. These photos all suggest to me that both lizards sought out burrows to both thermoregulate and to hunt.
The Other Burrow Architect
Nine-banded Armadillos are a common sight throughout Gopher Tortoise and Eastern Indigo Snake range. Armadillos are fully capable of digging their own burrows, which also provide beneficial wildlife habitat, but they aren’t above using or sometimes even commandeer tortoise burrows. This particular tortoise burrow was active and housed a resident tortoise, but that didn’t stop the armadillo from poking around inside and excavating some soil near the opening. Over the course of this project an armadillo visited this burrow several times, but never stayed very long. Since we never see it disappear inside the burrow for very long, it’s possible the armadillo was simply foraging for invertebrates in the soil near the burrow opening, rather than intending to take up residence.
Cat and mou….squirrel?
Here we have several images of mammals that visit tortoise burrow aprons, inspect the burrow openings, but then never actually enter. I’m not sure what each individual animal is doing, but Eastern Cottontails and Eastern Fox Squirrels repeatedly visited burrows and showed interest in the openings. In all the time I’ve spent scoping tortoise burrows, I’ve never seen either species inside a burrow, so it’s possible they were simply smelling the entrance for any whiff of potential predators that might be sheltering inside. Adult Eastern Diamondback Rattlesnakes, a common tortoise burrow commensal, specialize in hunting rabbits, but they could also certainly take a squirrel if the opportunity presented itself. However, rattlesnakes aren’t the only predator they need to look out for. The day after the last fox squirrel observation, a Bobcat approached the burrow, crawled inside the opening, re-emerged, then laid down on the burrow apron for 3 minutes. I can’t help but wonder if the squirrel’s presence the day before had anything to do with the bobcat entering the burrow. No matter the reason, seeing a Bobcat going inside a tortoise burrow is just too cool, and I think this has been my favorite series of photos to come out of this project!
Winter Snakes
In addition to indigo snakes, many other snake species in this part of the country don’t go completely dormant in the winter. They may still move across the landscape, bask, or even eat during spells of warm weather. Many of the upland species will also frequent Gopher Tortoise burrows for shelter and possibly for finding prey, so I wasn’t surprised to see several show up on our cameras. However, I noticed that where I observed other snakes might not have been very random. We detected indigo snakes at 6 of our 10 cameras, but only observed a single Eastern Coachwhip using a burrow that had previously been occupied by indigos. On the other hand, the burrow in these photos was not used by indigos at all for during our study, but we observed 3 other snake species using it: a Corn Snake, an Eastern Coachwhip, and a Southern Black Racer. Since indigo snakes are accomplished predators of other snakes, it makes sense that other snakes (even smaller indigos) would want to avoid crossing paths with the “Emperor of the Forest”. Were these snakes actively avoiding burrows that indigos had used or is this just a coincidence? Or were there other favorable habitat features or an abundance of prey near this burrow that these other snakes were taking advantage of? And could the racer have been drawn to the burrow by the scent left by the smaller juvenile coachwhip the day before? Both racers and coachwhips readily prey on other snakes, so it’s possible the racer was trying to track down a winter meal.
Pair of Indigos
Here we get a very brief glimpse of what I assume is a potential breeding pair of indigos. While we can’t be positive this is a male-female pair or that successful breeding took place, it is very likely. These photos were taken in peak breeding season when males are actively seeking out females. I’m quite confident it isn’t two males, because adult males don’t tolerate each other and regularly engage in combat over territory and access to females during the winter. It is possible these are two female snakes, however, with over a decade of surveying for indigos, I have never found multiple female indigo snakes utilizing the same burrow or basking together. The second snake that emerges appears to be larger and more robust than the first, which is another good indication that it’s a male. Unfortunately, we never see any obvious interactions between the two snakes, and despite seeing multiple snakes use this burrow throughout the rest of the season, we never again get confirmation that more than one snake is using the burrow at a time.
Parasitoid Wasp and Wolf Spider
Although invertebrates are some of the most prolific burrow inhabitants, they are undoubtedly the hardest to capture with this sort of camera setup. Nevertheless, we did capture images of various beetles, spiders, bees, and even ants. However, the most interesting observation was of this parasitoid wasp dragging a paralyzed wolf spider back across the burrow apron towards the mouth. While we don’t see where the spider ultimately ends up, I’m guessing the wasp buried it somewhere out of sight inside the burrow. Many wasps deposit their eggs on paralyzed spiders, caterpillars, or crickets, so when the wasp larvae hatch, they have sufficient food until they metamorphose into adults. I’ve commonly seen parasitoid wasps around burrow entrances when conducting visual surveys, but never one actively dragging prey towards the burrow. I always wondered if they were entering the burrows to hunt or to dig their own nest chambers, so it’s cool to get confirmation that at least some wasps are using tortoise burrows as nest sites.
House Trained Snakes
The heading might sound funny, but I think many people see snakes as simple, mindless creatures, when in fact, many of their behaviors are deliberate and serve important purposes. Here we see an indigo snake emerging from a burrow on two different occasions, and in each instance, the snake expels waste immediately after emerging from the burrow. This may seem like a pretty insignificant observation, but I think it’s worth noting. I can’t say that indigos never poo inside burrows, but this snake repeated this behavior, and I’ve found snake poop just outside of tortoise burrows on many occasions. I feel like this could be a common and fairly advantageous behavior. By leaving the burrow to do their business, snakes are keeping their winter refuge clean and not promoting additional bacterial growth in a space where they’re spending prolonged amounts of time. And since snakes are highly dependent on smell, this could also serve as some sort of olfactory signal to other indigo snakes.
The Landlord
And last but certainly not least, we have the Gopher Tortoise. The architect and entire reason why these important habitat features exist. Throughout most of the winter, Gopher Tortoises remain fairly inactive. We only saw them occasionally on camera until late February or March when the temperatures began warming up. At that point, the tortoises began spending large spans of time basking on the apron, but also devoted some energy to clearing their burrow entrances and browsing on nearby forbs. Unfortunately, we had to pull our cameras just as these sandhill tanks were ramping up their activity. I’m sure if we had left the cameras running, we’d likely have observations of social behavior, courtship, and even interactions between tortoises and their commensals. Maybe we’ll revisit the world of tortoise burrows and commensal species in a future project, but for now I’ll be content with all the photos we did capture. And I can be grateful I don’t have another million photos to sort through this spring. Whew.
Learn more about Gopher Tortoises on our focal species page.
