Thermoregulation and habitat selection in wood turtles Glyptemys insculpta: chasing the sun slowly

P>It is widely accepted that reptiles are able to regulate behaviourally their body temperature (T-b), but this generalization is primarily based on studies of lizards and snakes in the temperate zone. Because the precision of T-b regulation may vary considerably between taxa and over geographical ranges, studies of semi-terrestrial turtles in climatic extremes are relevant to the understanding of reptilian thermoregulation. We studied thermoregulation in 21 free-ranging wood turtles (Glyptemys insculpta) at the northern limit of their range in Quebec, using miniature data loggers to measure their internal T-b and external temperature (T-ext) continuously. We simultaneously recorded the available operative environmental temperature (T-e) using 23 physical models randomly moved within each habitat type, and we located turtles using radiotelemetry. The habitat used by wood turtles was thermally constraining and the target temperature (T-set) was only achievable by basking during a short 5-h time window on sunny days. Wood turtles did show thermoregulatory abilities, as determined by the difference between turtle T-b distribution and the null distribution of T-e that resulted in T-b closer to T-set. Although most individuals regulated their T-b between 09.00 h and 16.00 h on sunny days, regulation was imprecise, as indicated by an index of thermoregulation precision (vertical bar T-b - T-set vertical bar). The comparison of habitat use to availability indicated selection of open habitats. The hourly mean shuttling index (vertical bar T-ext - T-b vertical bar) suggested that turtles used sun/shade shuttling from 09.00 to 16.00 h to elevate their T-b above mean T-e. Based on laboratory respirometry data, turtles increased their metabolic rate by 20-26% over thermoconformity, and thus likely increased their energy gain which is assumed to be constrained by processing rate at climatic extremes.