The effects of long ‘summer’ (22L:2D) and short ‘winter’ (2L:22D) photoperiod on post-weaning growth and food consumption of the collared lemming were examined. Growth was described by Gompertz equations. After 10 weeks, lemmings maintained under short photoperiods developed white winter pelage and heavy bifid claws; body mass and estimated asymptotic mass were significantly greater than for lemmings reared under long photoperiod. Sexual dimorphism in body size was observed within treatments, males growing larger than females. There were no significant differences in the overall growth rate constant k as a result of either sex or photoperiod treatment. However, instantaneous growth rates (dW/dt) were significantly higher in ‘winter’ lemmings. Mass at weaning was a significant determinant of adult mass at 90 days.

Cumulative food consumption at 90 days was not significantly different between photoperiod groups, even though ‘winter’ lemmings gained more than double the mass of their ‘summer’ counterparts over the post-weaning period. Increased gut length observed for winter morphs may act as the physiological mechanism promoting greater gross energy efficiency.

The present study suggests that the overwintering strategy of the collared lemming with respect to body mass changes and energy requirements differs considerably from that of other microtines.