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Ectotherms are dependent on their environment as a source of heat to regulate body temperature. However, environmental conditions can often be hazardous (too hot or too dry), necessitating use of shelter sites to aid in thermoregulation and survival. Indigo snakes present an interesting case study of shelter site use in a terrestrial snake because of the latitudinal differences apparent in indigo snake ecology. In the northern portion of their range, indigo snakes primarily use Gopher Tortoise burrows as winter refugia but will occasionally shelter in stump holes, mammal burrows, and under decaying logs (Hyslop et al. 2009). This contrasts with indigo snakes in Peninsular Florida that are generally believed to use a wider variety of habitat types and be less strongly tied to longleaf pine uplands with tortoise burrows. However, studies quantifying this difference are lacking, in part because of the challenges associated with studying indigo snakes in Florida.
Because locating indigo snakes using standard survey techniques is difficult in Florida, radio telemetry data offers the most reliable approach for estimating fine-scale habitat use. In particular, snakes can be tracked to specific sites that are being used as refugia. A recent study by Bolt et al. (2023) combined data from two radio telemetry studies on indigo snakes in Peninsular Florida (Breininger et al. 2011; Bauder and Barnhart 2014) to examine how shelter site selection may differ from previous studies in southern Georgia. The authors identified occasions in these existing datasets when snakes were located at shelter sites, and sites were classified as either a tortoise burrow, woody or man-made debris, or other type of underground hole (e.g., mammal burrows or root channels). Each shelter site was also assigned to a specific habitat type using either on-the-ground-delineations of habitat or GIS-based habitat layers. The authors then used this dataset to model how shelter site use varied across seasons, across habitat types, and between males and females.
The combined dataset included observations from 102 individuals, which were tracked for a median of 268 days. The median number of shelter site observations per snake was 18. Across all observations of shelter site use, the median percentage of shelter types was 38% tortoise burrows, 25% other holes, and 25% for debris. For individuals located at least 30 times, the median percentage of shelter site use was 50% tortoise burrows, 26.5% other holes, and 17.5% for debris. There were 24 snakes that never used tortoise burrows even though they had access to burrows within their home range. These individuals used 44% other holes, 21% brush piles, 21% man-made debris, and 15% woody debris. Finally, across all data, mean tortoise burrow use was 0.43 in spring, 0.23 in summer, 0.34 in fall, and 0.52 in winter.
There was strong support for seasonal and surrounding habitat effects on shelter site use in indigo snake populations. In general, tortoise burrow use was highest in xeric habitats when compared to more mesic habitats and was highest across habitat types during the cooler months of the year. In xeric habitats, females tended to use tortoise burrows more frequently than males, but in mesic habitats this trend was reversed, with males using tortoise burrows more frequently than females. In southern Georgia, tortoise burrows may play an important role in reproduction (Stevenson et al. 2021), but this link may not be as important in Peninsular Florida. Importantly, these general trends were similar across analyses using both on-the-ground classifications of habitat and GIS-derived delineations of habitat, indicating that the general conclusions are robust.
Overall, the results from this work reinforce the idea that indigo snake shelter site use shifts along a north to south gradient across the indigo snake range. Indigo snakes in Peninsular Florida still commonly used tortoise burrows but the use of other types of refugia was more frequent than in southern Georgia. Furthermore, there were some individuals that were never observed using tortoise burrows even though there were burrows available to them. Because indigo snakes appear to be more generalist in their habitat use within Peninsular Florida, tortoise burrows should not be the only factor used to identify whether or not habitat could be suitable. This region is experiencing rapid habitat loss along with an expanding human population. Preserving and connecting large, unfragmented habitat patches continues to be an important goal for indigo snake conservation.
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Literature Cited
Bauder, J.M., and P. Barnhart. 2014. Factors affecting the accuracy and precision of triangulated radio telemetry locations of Eastern Indigo Snakes (Drymarchon couperi). Herpetological Review 45:590–597.
Breininger, D.R., M.R. Bolt, M.L. Legare, J.H. Drese, and E.D. Stolen. 2011. Factors influencing home-range sizes of Eastern Indigo Snakes in central Florida. Journal of Herpetology 45:484–490.
Hyslop, N.L., R.J. Cooper, and J.M. Meyers. 2009a. Seasonal shifts in shelter and microhabitat use of Drymarchon couperi (Eastern Indigo Snake) in Georgia. Copeia 2009:458–464.
Stevenson, D.J., N.L. Hyslop, C. Layton, J. Godlewski, and F.H. Snow. 2021. Nesting sites of the Eastern Indigo Snake (Drymarchon couperi) in Georgia. Southeastern Naturalist 20:345–352.
