Short photoperiods reduce winter energy requirements of the meadow vole, Microtus pennsylvanicus.

Adult male voles were maintained for 10 weeks in long or short photoperiods (14 or 10 hr light/day) at 23 degrees C and then for 6 days at 10 degrees C. Short photoperiod voles weighed approximately 20% less and consumed 36% and 42% fewer calories than did long-day animals at 23 degrees C and 10 degrees C, respectively. Relative energy intake (kcal/g body mass) was lower for short-than long-day voles at 10 degrees C. Short-day animals gathered more nesting material than their long-day counterparts at both ambient temperatures. Absolute as well as relative wet weights of interscapular brown adipose tissue were reduced in short-day voles and their testes were regressed. Two types of dorsal pelage hairs were longer in short- than in long-day voles; a dense undergrowth of light gray hairs, present in all short-day voles, was completely absent or only sparsely represented in the pelage of long-day animals. Photoperiod is a major proximate factor mediating winter adaptations of meadow voles: By decreasing body mass and reducing food requirements, short photoperiods presumably diminish winter foraging activity and energy devoted to thermoregulation. Increases in nest-building activity, growth of a winter pelage, and testicular regression are other changes induced by short daylengths that facilitate survival of over-wintering voles.