Nonresponding Siberian hamsters (Phodopus sungorus) do not develop the winter phenotype of white fur, low body mass (Mb) and spontaneous torpor in response to short photoperiod. However, their thermoregulatory response to fasting remains unknown. We measured body temperature and Mb of 12 nonresponders acclimated to short photoperiod and then to cold and fasted four times for 24 h. Four individuals used torpor, and in total, we recorded 19 torpor bouts, which were shallow, short and occurred at night. Moreover, fasting increased the heterothermy index in all hamsters. Low Mb was not a prerequisite for torpor use and Mb loss did not correlate with either heterothermy index or torpor use. This is the first evidence that individuals which do not develop the winter phenotype can use torpor or increase body temperature variability to face unpredictable, adverse environmental conditions. Despite the lack of seasonal changes, thermoregulatory adjustments may increase the probability of winter survival in nonresponders.

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