Because of their dependence on ambient temperature ectothermic animals can serve as sentinels of conservation problems related to global warming. Reptiles in temperate areas are especially well suited to study such effects, as their annual and daily activity patterns directly depend on ambient temperature. This study is based on annual data spanning 68 years from a fringe population of Grass Snakes (Natrix natrix), which is the world’s northernmost oviparous (egg-laying) reptile, and known to be constrained by temperature for reproduction, morphology, and behavior. Mark-recapture analyses showed that survival probability was generally higher in males than in females and that it increased with body length. Body condition (scaled mass index) and body length increased over time, indicative of a longer annual activity period. Monthly survival was generally higher during winter (i.e., hibernation) than over the summer season. Summer survival increased over time, whilst winter survival decreased, especially during recent decades. Winter survival was lower when annual maximum snow depth was less than 15 cm, implying a negative effect of milder winters with less insulating snow cover. Our study demonstrates long-term shifts in body length, body condition and seasonal survival associated with a warming climate. Although the seasonal changes in survival ran in opposite directions and though changes were small in absolute terms, the trends did not cancel out, but total annual survival decreased. We conclude that effects of a warming climate can be diverse and pose a threat to thermophilic species in temperate regions and that future studies should consider survival change by season, preferably in a long-term approach.

This study is based on data collected by the late Carl Edelstam (Fig. 1). From 1942 to 2009 he captured, marked, and recaptured Grass snakes Natrix natrix in what is today known as the Nacka nature reserve (‘Nackareservatet’, 7.3 km², WGS84 dec N 59.29, E 18.15, located in the hemi-boreal biotic zone). Snakes were captured by hand and kept in cloth bags in the field until examination. Each snake was marked by scale-clipping and its natural color markings were recorded (i.e., the unique ventral pattern, which can be used for individual identification). On each occasion a snake was captured (with few exceptions) date, sex, body length (i.e., snout-vent length, SVL), tail length, and body mass were recorded. After examination, snakes were returned to the location where they had been captured.