1. 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.
2. 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.
3. 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.
4. 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 to 6g per 1°C). We presume that an earlier emergence as well as a lower body mass after emergence potentially reduce the chance to survive.
5. During the active period of edible dormice, mean spring T_a a did not affect the date of birth but was positively associated with litter size, potentially improving reproductive success.
6. 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.

Data were collected in a capture - mark - recapture study.

Ambient temperature data are retrieved from the weather station Stuttgart-Echterdingen, DWD (Deutscher Wetterdienst; www.dwd.de).