They like it cold, but only in winter: Climate‐mediated effects on a hibernator

Variations in ambient temperature ( T a ) profoundly influence energy consumption in endotherms and therefore their survival and fitness. But depending on whether endotherms are hibernating or active, the same changes in T a may have opposing consequences for their energy consumption. The aim of this study was therefore to investigate how variations in T a , occurring during hibernation and during the active period of a hibernator, affect different fitness relevant traits. To understand whether changes in T a impact phenology, body mass and reproduction in a small mammalian hibernator, we analysed T a variations and detailed capture–mark–recapture data on edible dormice Glis glis collected between 1993 and 2016 in South‐West Germany. Results revealed that during hibernation a T a increase of 1°C advanced the date of first capture after hibernation by 6 days, but only if food availability during the preceding year was low. In contrast, after years of comparatively high food availability, date of first capture was not affected by T a , but dormice had a significantly lower body mass after emergence, if T a during hibernation was elevated (3–6 g per 1°C). We presume that an earlier emergence as well as a lower body mass after emergence potentially reduces the chance to survive. During the active period of edible dormice, mean spring T a did not affect the date of birth but was positively associated with litter size, potentially improving reproductive success. Results of this study highlight that, depending on the physiological state of a hibernator, an increase in T a can have positive and negative effects on fitness relevant traits. A free Plain Language Summary can be found within the Supporting Information of this article.