The energy budget of captive Siberian hamsters, Phodopus sungorus, exposed to photoperiod changes: mass loss is caused by a voluntary decrease in food intake.

Unlike most nonhibernating small mammals, Siberian hamsters (Phodopus sungorus) undergo a pronounced mass loss, almost exclusively as white adipose tissues, in response to a switch from long- to short-photoperiod exposure. This mass loss can be caused by an increase in the rate of energy expenditure, a decrease in the rate of energy intake, or a decrease in assimilation efficiency. In order to determine how they relate to photoperiod-induced mass loss, we measured these energy budget components every 2 wk on 12 captive Siberian hamsters exposed to 8 wk of long photoperiod followed by 12 wk of short photoperiod. Body mass decreased shortly after short-photoperiod exposure. This was accompanied by an immediate decrease in the rate of energy intake and, after a 2-wk delay, by a decrease in the rate of energy expenditure. The overall cumulative decrease in energy intake (376 kJ) could account for the mass loss (249 kJ) observed during short-photoperiod exposure. Regression analyses indicated that only the rate of energy intake was significantly related to the rate of mass change. Therefore, we conclude that photoperiod-induced mass loss in Siberian hamsters is caused by a decrease in the rate of food intake, rather than by changes in energy expenditure or in assimilation efficiency.