Winter activity and thermal performance of gopher tortoises in their southern range edge

Animals face myriad decisions when confronting challenges to the maintenance of thermoregulatory setpoints and a frequent response to seasonal thermal challenges is a reduction in function and dormancy induction. Tradeoffs exist that result in strategies that either favor dormancy or sustained activity through thermal challenges, such as winter. Our study focused on a population of gopher tortoises (Gopherus polyphemus), a species that is 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 edge of the species' southern range. Using affixed autonomous GPS trackers containing temperature sensors, we investigated shifts in field thermal performance curves (TPC) in response to shifts in environmental temperature between seasons and 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. Field TPCs were significantly cold shifted during the coldest 28-day period of winter and 80% thermal performance breadth (B80) was greatest during the coldest winter period. Although most individuals were active throughout the coldest period, overall performance, as measured by spatial velocity, was reduced. Tortoises, however, maintained high activity levels as measured by distances moved during the coldest period. Only seven out of 30 individuals induced dormancy, and in those individuals, the mean overwintering period was only 17 days; both proportion of individuals dormant and dormancy duration of gopher tortoises in this study was lower than has been previously observed in other studies. This study indicates this species facultatively utilizes numerous strategies for mitigating winter constraints.