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

Summary1 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. 2 We studied thermoregulation in 21 free‐ranging wood turtles ( Glyptemys insculpta ) at the northern limit of their range in Québec, 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. 3 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 (|  T b  –  T set  |). 4 The comparison of habitat use to availability indicated selection of open habitats. The hourly mean shuttling index (|  T ext  –  T b  |) suggested that turtles used sun/shade shuttling from 09.00 to 16.00 h to elevate their T b above mean T e . 5 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.