Winter activity and thermal performance of gopher tortoises near their southern range limit

Animals face decisions when confronting seasonal changes that challenge
homeostasis and the maintenance of thermoregulatory setpoints. Among ectotherms, a
frequent response to sub-optimal thermal change is a reduction in whole-body function
and induction of dormancy. Tradeoffs exist between both populations and species that
are context-dependent and result in optimal strategies that either favor winter
dormancy or winter activity. Using affixed autonomous GPS trackers containing
thermochrons, we investigated thermal acclimation, seasonal shifts in field thermal
performance curves (TPC) during winter, and the relationship between winter activity
and carapacial and air temperatures in 30 wild adult gopher tortoises (Gopherus
polyphemus). Gopher tortoises are 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 near the species’ southern/warm range edge where tortoises
previously have been shown to remain active throughout winter. We found that field
TPCs were significantly cold shifted during the coldest 28-day period of winter with.
Optimal temperatures based on cold-shifted TPCs were 15° C during the coldest 28-
day period and 35° C during the warmest 28-day period. We also found that while most
individuals were active throughout the coldest winter period, overall performance, as
measured by spatial velocity, was reduced. Tortoises were significantly constrained by
cold winter temperatures but maintained high levels of activity as measured by total
distances moved during the coldest period. Only seven out of 30 gopher tortoises
induced overwinter dormancy, 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.