Daily torpor in summer- and winter-acclimated hamsters: are thermal biology and balancing energy budgets under a different seasonal physiological control?

Djungarian hamsters (Phodopus sungorus), originating from Asian steppes, change fur color from brown in summer to white in winter when they reduce body mass and size of reproductive organs. The species also enters energy-conserving daily torpor, characterized by a substantial temporal reduction of metabolic rate (MR) and body temperature (Tb). However, spontaneous daily torpor (food ad libitum) is only used by winter-acclimated hamsters. In contrast, short and shallow torpor induced by food restriction may also occur in summer- acclimated hamsters. To better understand the seasonal physiology of torpor, we examined patterns of spontaneous and induced torpor in two groups of hamsters, one was maintained under natural photoperiod in winter (‘winter’ hamsters) the other on a constant long summer photoperiod (‘summer’ hamsters); both were maintained in unheated rooms with an ambient temperature (Ta) of 9 ± 2 °C during the time of measurements in late winter. Largely white winter hamsters (n = 9 of 10), entered spontaneous torpor frequently (51.5% of days), the heaviest individual did not enter torpor. Conversely, none of the brown summer hamsters (n = 8) entered spontaneous torpor. Induced torpor at Ta 15 °C was observed in both groups (9 of 10 winter hamsters, 2 of 8 summer hamsters), but against predictions, torpor in summer hamsters was on average deeper (minimum Tb 19.4 ± 1.1 °C) than in winter hamsters (minimum Tb 22.2 ± 1.6 °C). Other variables of torpor were similar although the bout duration tended to be longer in summer hamsters. These data suggest that the seasonal change of physiology of P. sungorus, largely caused by photoperiod, does not concern thermal energetics or depth of torpor. These appear to remain similar throughout the year for survival of acute energetic challenges, and this also applies to some hibernators. Rather, seasonality of torpor seems to reflect largely the proclivity of displaying torpor to overcome likely long-term energy challenges that in cold climates are mainly faced in winter.