Ectotherms are unable to internally regulate their body temperatures, instead depending on the environment as a source of heat. These animals must either conform to environmental temperatures or thermoregulate to raise body temperatures above environmental temperatures through behavior, physiology, or both. For reptiles, this typically means some type of basking behavior where high body temperatures can be obtained relative to the air temperature. Body temperature is directly linked to a host of ecological and physiological processes, including things like foraging efficiency, digestion efficiency, and disease resistance (e.g., Ayers and Shine 2003). Navigating complex thermal environments to maintain body temperature within some optimum range is a key aspect of the life history of reptiles, and ultimately has implication for both individual and species level processes.
Freshwater turtles are a good example of a group that generally inhabit complex thermal environments, consisting of relatively stable water temperatures and more variable air temperatures. Basking behavior in freshwater turtles is one of the most recognizable forms of thermoregulation, and it is not uncommon to see a bunch of turtles clustered together on a single log. For species like the Spotted Turtle (Clemmys guttata) whose range extends across a broad latitudinal gradient (Florida to Canada), populations in different parts of the range experience markedly different thermal environments. Along with coauthors, Benjamin Stegenga and Dirk Stevenson, I recently published research in the Journal Copeia quantifying the environmental temperatures experienced by Spotted Turtles through an active season in Georgia.
This paper highlighted work that was completed during 2016 as part of a large study on Georgia’s Spotted Turtles. We attached radio transmitters and iButton temperature loggers to the carapace of adult Spotted Turtles in two populations. We considered shell temperature to be a reasonable proxy for body temperature, which is common in freshwater turtle research. We also recorded environmental (water and air) temperatures from multiple locations at each site using temperature loggers placed in the environment. All temperature loggers recorded temperature measurements every 90 minutes from approximately late March through late December. For our analyses, we focused only on daytime temperatures because Spotted Turtles are primarily diurnal and could be thermoregulating by basking during the day.
Over the course of the study, we recorded approximately 50,000 measurements of Spotted Turtle shell temperatures during the daytime hours. Our results showed that both shell and environmental temperatures often fell within the published optimal range for Spotted Turtles of 20–26°C (Yagi and Litzgus 2013). Spotted Turtles appeared to generally conform to environmental temperatures, especially when there was water available at the site. At Site 1, weekly mean shell and water temperature were highly correlated throughout the study period (P < 0.0001, R2 = 0.99). Interestingly, individuals at Site 2 only had access to intermittent water for most of the year, spending more time exposed to air temperatures. This difference in wetland hydrology meant that turtles at Site 2 experienced more temperature fluctuations than turtles at Site 1 (mean variance in shell temperature across individuals = 24.7 at Site 1 and 36.3 at Site 2).
We identified 1,050 occasions where turtles at Site 1 appeared to be basking (less than 5% of total observations at this site) based on recorded water temperatures at the same time (see below figure). Unsurprisingly, these behaviors were concentrated in the spring and fall when water temperatures were lowest. Furthermore, we detected a significant effect of sex on shell temperatures reached during the period surrounding egg development. Gravid female turtles reached higher shell temperatures than male turtles over this two-week period. These results suggest that individuals in southern populations of Spotted Turtles do occasionally thermoregulate, but that these behaviors are most common during short periods.
Overall, this project indicated that environmental temperatures experienced by Spotted Turtles in Georgia are highly suitable and often fall within or near the optimal range for the species. High summer temperatures corresponded to a reduction in activity and a general period of low water levels in both populations (Chandler et al. 2019). It is possible that longer activity periods and higher temperatures in these populations could limit the distribution of Spotted Turtles at the southern end of their range, especially when combined with hydrologic factors. However, additional research is needed to better understand the mechanisms that make Spotted Turtles one of the rarest turtles in southern Georgia and northern Florida.
Ayers, D. Y., and R. Shine. 2003. Thermal influences on foraging ability: Body size, posture and cooling rate of an ambush predator, the python Morelia spilota. Functional Ecology 11:342–347.
Chandler, H. C., B. S. Stegenga, and D. J. Stevenson. 2019. Movement and space use in southern populations of Spotted Turtles (Clemmys guttata). Southeastern Naturalist 18:602–618.
Chandler, H. C., B. S. Stegenga, and D. J. Stevenson. 2020. Thermal ecology of Spotted Turtles (Clemmys guttata) in two southern populations. Copeia 108:737–745. https://doi.org/10.1643/CE-19-315
Yagi, K. T., and J. D. Litzgus. 2013. Thermoregulation of spotted turtles (Clemmys guttata) in a beaver-flooded bog in southern Ontario, Canada. Journal of Thermal Biology 38:205–213.