Torpor use and body mass gain during pre-hibernation in late-born juvenile garden dormice exposed to food shortage

Prior to hibernation, juvenile hibernators have to sustain both somatic growth and fattening to reach a sufficient body mass to survive the following winter season. This high demand for energy is especially challenging for juveniles born late in the season, since they might already experience reduced food availability and decreasing temperatures. In this study, we asked whether late-born juvenile garden dormice can use torpor to counteract intermittent food shortages and, if so, whether the use of torpor enables them to compensate the energy deficit and to maintain rates of body mass gain similar to well-fed juveniles. We measured daily torpor use, food intake, and body mass in weaned late-born juvenile dormice exposed to intermittent fasting (n = 5) or fed ad libitum (n = 4) under natural photoperiod and ambient temperature during 6 weeks prior to hibernation (13 September-! November). We found that fasted juveniles frequently used torpor and despite eating less food grew at a significantly greater rate compared to juveniles fed ad libitum. Torpor frequency was positively related to body mass gain, but this effect was statistically significant only in the fasted group. Consequently, fasted juvenile dormice reached a critical body mass and entered into hibernation 1 week earlier than ad libitum fed juveniles. If fasted juveniles reached a similar pre-hibernation fat content compared to animals fed ad libitum, and how any difference in fat versus fat-free mass might affect hibernation propensity and winter survival, remains to be determined.