Reactions of reproductively photoresponsive versus unresponsive meadow voles to simulated winter conditions

At least some populations of meadow voles (Microtus pennsylvanicus) comprise individuals that vary greatly in the degree to which their reproduction can be controlled by day length. Some individuals respond to the short days of winter with complete gonadal inhibition, others are insensitive to this cue and thus have the capacity to reproduce opportunistically during the winter, and still others are intermediate in their responsiveness. The relative costs and benefits associated with some of the nonreproductive dimensions of these different strategies are explored. The two extreme phenotypes, reproductively photoresponsive and unresponsive individuals, were exposed in the laboratory to winter versus summer conditions, as defined by photoperiod, temperature, and quality of diet. This was done in cages that required the voles to leave their nests and subject themselves to ambient conditions in order to feed. The winter condition exerted a potent influence on body mass, body fat, food intake, nest building, pelage depth, and the amount and temporal pattern of feeding, as well as reproductive potential. The results suggest that the major nonreproductive advantage enjoyed by the photoregulated phenotype is a decrease in body mass and hence a decrease in required foraging time that anticipates harsh winter conditions. The opportunists also may lose mass in response to harsh conditions, but this is a direct and immediate response for which they may be poorly prepared.