High survival during hibernation affects onset and timing of reproduction.

The timing of reproduction is one of the most crucial life history traits, with enormous consequences for the fitness of an individual. We investigated the effects of season and timing of birth on local survival probability in a small mammalian hibernator, the common dormouse (Muscardinus avellanarius). Local monthly survival probability was lowest in the early active season (May-August, ϕ(adult) = 0.75-0.88, ϕ(juvenile) = 0.61-0.68), increased during the late active season (August-October), and highest during hibernation (October-May, ϕ(adult) = 0.96-0.98, ϕ(juvenile) = 0.81-0.94). Consequently, dormice had an extremely high winter survival probability. We observed two peaks in the timing of reproduction (June and August/September, respectively), with the majority of juveniles born late in the active season. Although early investment in reproduction seems the better life history tactic [survival probability until onset of reproduction: ϕ(born early) = 0.46, 95% confidence interval (CI) 0.28-0.64; ϕ(born late) = 0.19, 95% CI = 0.09-0.28], only females with a good body condition (significantly higher body mass) invest in reproduction early in the year. We suggest the high over-winter survival in dormice allows for a unique life history pattern (i.e., combining slow and fast life history tactics), which leads to a bimodal seasonal birth pattern: (1) give birth as early as possible to allow even the young to breed before hibernating, and/or (2) give birth as late as possible (leaving just enough time for these young to fatten) and enter directly into a period associated with the highest survival rates (hibernation) until maturity.