Winter activity and thermal performance of gopher tortoises near their southern range limit
Description
Animals face a range of decisions when confronting seasonal temperature change that challenges homeostasis and the maintenance of thermoregulatory setpoints. A frequent acclimatization response to seasons with sub-optimal thermal environments, such as winter, is a reduction in organismal function and induction of dormancy. Tradeoffs exist between species and within populations that result in strategies that either favor winter dormancy or sustained activity through winter. Our study focused on a wild population of Gopher tortoises (Gopherus polyphemus), a species that is naturally distributed along the lower southeastern coastal plain of North America (i.e. a warm-temperate to sub-tropical climate range). Our study population was located near the species southern (i.e., warm) range limit and we investigated seasonal shifts in field thermal performance curves (TPC) in response to shifts in environmental temperature between seasons (i.e., coldest period of time during winter vs. warmest period of time during summer) using affixed autonomous GPS trackers containing temperature sensors. We also tested the relationship between winter activity and temperatures to test which strategy (i.e., dormancy vs activity) this study population utilized during the coldest period of the year. We found that field TPCs were significantly cold shifted during the coldest 28-day period of winter. Optimal temperatures during the coldest and warmest 28-day periods were 15°C and 35°C, respectively. Although most individuals were active throughout the coldest winter period, overall performance, as measured by spatial velocity, was reduced. Tortoises were therefore significantly constrained by cold winter temperatures but maintained high levels of activity as measured by distances moved during the coldest period. Only seven out of 30 gopher tortoises induced overwintering, and in those individuals, the overwintering period was shorter than previously observed in the species further north. Moreover, the proportion of dormant tortoises in our study population was significantly lower than in populations further north. Overall, we found important differences in thermal biology between populations of gopher tortoises that suggest this species facultatively utilizes numerous strategies among populations for mitigating the constraints of winter.
Files
Steps to reproduce
Research methodology is described in the paper "Winter activity and thermal performance of gopher tortoises near their southern range limit" in the Journal of Thermal Biology. To reproduce the data analysis, run the Thermal_Analysis.R script found in the repository. File_Descriptions.txt describes the contents of each file in this repository .
Institutions
- Eckerd CollegeFL, Saint Petersburg