Role of photoperiod during pregnancy and lactation in the meadow vole, Microtus pennsylvanicus.

The influence of daylength on body mass and food intake of pregnant and lactating voles was tested by comparing animals housed in long versus short daylengths. Pregnancy rates were approximately 50% in long-day females and in voles kept in short days beginning 2 weeks before mating, but were significantly lower in voles preadapted to short days for 8 weeks before mating. Body mass and food intake increased substantially during pregnancy and lactation and the magnitude of the increase was unaffected by daylength; by contrast, body weight was significantly reduced in non-impregnated voles kept in short as compared to long days. The suppressive effects of short photoperiods on body weight were completely counteracted during pregnancy and lactation. Voles that breed during the short days of winter face extreme energetic challenges but the advantages of early breeding appear to justify the costs.