Variation in reproductive photoresponsiveness in a wild population of meadow voles.

Individual variation in reproductive photoresponsiveness has been documented in laboratory colonies of several species of rodents. When maintained on short day lengths, some individuals experience complete gonadal regression, others undergo little if any gonadal regression, and still others show intermediate levels of responsiveness. In the present research, a combination of laboratory and field studies explored the potential importance of this kind of variation for the control of seasonal breeding in a wild population of meadow voles (Microtus pennsylvanicus). A sample of adult males was trapped in central Pennsylvania during the summer when all were in breeding condition. When exposed to short day lengths in the laboratory, these males showed the entire range of variation in responsiveness noted above and, correlatively, variation in the loss in body weight induced by this treatment. A sample of males trapped in the wild just before the winter solstice showed the same distribution of variation as did the males housed on short day lengths in the laboratory, and thus, as might be expected, a few pregnant and lactating females were also trapped at this time. Longitudinal studies over a 42-wk period revealed that the variation reflects the degree to which the testes regress in response to short-day exposure rather than the rate at which they regress. Finally, studies with laboratory-born voles demonstrated that the variation is independent of age. In total, these studies demonstrate that the variation in reproductive photoresponsiveness previously seen only in laboratory colonies of rodents indeed has relevance for understanding the seasonal control of reproduction in the wild.