Seasonal control of energy requirements for thermoregulation in the djungarian hamster (Phodopus sungorus), living in natural photoperiod

Abstract

Summary. Djungarian dwarf hamsters, Phodopus s. sungorus, were kept in natural photoperiodic conditions throughout the year, either inside at a constant Ta of 23 ~ or outside subjected to seasonally varying Ta. Comparisons were made between hamsters from both conditions to evaluate the significance of seasonal changes in photoperiod and/or Ta as environmental cues for seasonal acclimatization in Phodopus. Basal metabolic rate was lowest in July (1.68 ml/g.h) and highest in January (2.06 ml/g-h in Phodopus living outside), combined with a decrease in Tic from 26 ~ in July to 20 ~ in January. This was parallelled by seasonal changes in body weight (summer 42 g, winter 25 g), fur colouration, fur depth and the occurrence of short daily torpor. At Ta below thermoneutrality total energy requirements for thermoregulation in winter acclimatized Phodopus were found 36% lower than summer values (e.g. at 0 ~ T, in summer 1,160roW, in winter 760 mW), which were effected by a combined strategy of reducing body weight (19%) together with improvements of thermal insulation of the body surface (17%)i All seasonal changes were similar in Phodopus living inside or outside, suggesting that seasonal changes in photoperiod and not seasonal changes in Ta is the overriding controller for the environmental cueing of seasonality in energy requirements for thermoregulation.