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

We investigated the influence of ambient temperature (T(a)) and food a vailability on seasonal timing and extent of physiological responses t o short photoperiod (SP), in particular daily torpor, in Djungarian ha msters (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 torpo r 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) a n accelerated reduction of body weights (P < 0.001). However, within S P/cold-T(a) exposed groups, individual hamsters clearly showed differe nt tendencies for torpor (torpor on 0-95% of days observed). Therefore , we evaluated the effects of small changes in T(a) on torpor frequenc y and extension by subjecting the same SP-adapted individuals to varyi ng temperatures. Lowering of T(a) from 15-degrees-C to 10-degrees-C an d 5-degrees-C caused significant (P < 0.05) increases in the incidence of torpor (20%, 33%, and 40%, respectively) and lower minimal body te mperatures during hypothermia (P < 0.05). When the same animals were s ubjected 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 a t all T(a)s. These results show that T(a) and food availability are ef fective in modifying both seasonal timing and extent of photoperiodica lly 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 flexib le physiological responses towards unpredictable climatic changes in t he environment. (C) 1993 Wiley-Liss, Inc.