Thermoregulatory behaviour of gravid and non‐gravid female grass snakes (Natrix natrix) in a thermally limiting high‐latitude environment

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AbstractAll else being equal, ectotherms should maintain body temperatures (Tbs) favourable for temperature‐sensitive biological functions, such as digestion and locomotion. Physical environments in the temperate zone are often thermally variable, however, thus making it difficult to maintain optimum Tbs. Radiotelemetry and a semi‐natural enclosure were used to monitor Tbs of grass snakes Natrix natrix at Canterbury, Kent, U.K. Operative temperatures (Tes) were measured using snake models (copper‐pipe models) placed in a variety of microhabitats to determine availability of thermoregulatory opportunities. A modification of Huey & Slatkin's (1976) regression model, piecewise regression, was used to evaluate thermoregulatory behaviour. Grass snakes could achieve high mean Tbs (> 30°C) most often during midday (08:00–20:00), but only for 55–61% of the study period. Overall, non‐gravid snakes maintained higher and less variable mean daytime Tbs than gravid snakes. From piecewise regression, it was determined that grass snakes initiated thermoregulation at a Te of 38.44 °C, corresponding to a Tb of 27.7 °C. Three main conclusions can be drawn from this study: (1) thermoregulatory opportunities for grass snakes were limited; (2) nonetheless, when conditions were sufficiently warm, there was clear evidence that grass snakes could thermoregulate; (3) contrary to expectation, gravid females actually maintained lower and more variable Tbs than non‐gravid females. The consequences of these thermoregulatory patterns for the fitness of snakes have yet to be determined.