The influence of natural photoperiod on seasonal torpor expression of two opportunistic marsupial hibernators

Many mammals use torpor throughout the year but the individual contributions of environmental variables to seasonal changes in torpor expression are often difficult to tease apart. In many mammals, torpor is most often used opportunistically in response to decreased ambient temperature (T-a) and food availability, but information on how seasonally changing photoperiod per se influences torpor patterns is scant. Therefore, we quantified patterns of torpor use in response to natural photoperiod in captive marsupial pygmy-possums held at near-constant T-a with a stable food supply over a period of 19 months. Western pygmy-possums (Cercartetus concinnus) and eastern pygmy-possums (C. nanus) used spontaneous torpor in every month of the year; in total we measured >1100 individual torpor bouts. Torpor bout duration was >60 % longer in winter than in summer and increased with decreasing day length for both species. Interestingly, the duration of torpor appeared to be adjusted at both the beginning and end of bouts because the time of entry into and rewarming from torpor relative to sunrise and sunset, respectively, changed with season. We propose that this reflects a synchronisation of torpor timing with foraging periods in the wild, which would enable animals to maintain a high body mass year-round by maximising both energy savings via torpor and energy input via food consumption. Our study suggests that photoperiod makes a significant contribution to the seasonal change in torpor bout duration of small hibernating mammals that use torpor throughout the year.