COLD-EXPOSURE AND FOOD RESTRICTION FACILITATE PHYSIOLOGICAL-RESPONSES TO SHORT PHOTOPERIOD IN DJUNGARIAN HAMSTERS (PHODOPUS-SUNGORUS)

We investigated the influence of ambient temperature (T(a)) and food availability on seasonal timing and extent of physiological responses to short photoperiod (SP), in particular daily torpor, in Djungarian hamsters (Phodopus sungorus). Exposure of hamsters to cold temperature (T(a) = 5-degrees-C), relative to warm T(a) (23-degrees-C), resulted in: 1) a significant advance (P < 0.05) of the first occurrence of torpor among cold-exposed hamsters (days 52-97 vs. days 83-99 in SP); 2) a higher (P < 0.01) incidence of torpor (48% vs. 20% torpid animals/day); 3) a higher (P < 0.05) degree of molt into the winter pelt; and 4) an accelerated reduction of body weights (P < 0.001). However, within SP/cold-T(a) exposed groups, individual hamsters clearly showed different tendencies for torpor (torpor on 0-95% of days observed). Therefore, we evaluated the effects of small changes in T(a) on torpor frequency and extension by subjecting the same SP-adapted individuals to varying temperatures. Lowering of T(a) from 15-degrees-C to 10-degrees-C and 5-degrees-C caused significant (P < 0.05) increases in the incidence of torpor (20%, 33%, and 40%, respectively) and lower minimal body temperatures during hypothermia (P < 0.05). When the same animals were subjected to 24-48 h lasting periods of food restriction (60% of the ad libitum intake), torpor frequency further increased 1.8-to 2.6-fold at all T(a)s. These results show that T(a) and food availability are effective in modifying both seasonal timing and extent of photoperiodically controlled adaptations. This integration of multiple environmental cues, combined with a pronounced within-species variability of winter adjustments, indicates that Djungarian hamsters are capable of flexible physiological responses towards unpredictable climatic changes in the environment. (C) 1993 Wiley-Liss, Inc.