VARIATION IN REPRODUCTIVE PHOTORESPONSIVENESS IN A WILD POPULATION OFMEADOW VOLES

Individual variation in reproductive photoresponsiveness has been docu mented in laboratory colonies of several species of rodents. When main tained on short day lengths, some individuals experience complete gona dal regression, others undergo little if any gonadal regression, and s till 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 seas onal breeding in a wild population of meadow voles (Microtus pennsylva nicus). A sample of adult males was trapped in central Pennsylvania du ring the summer when all were in breeding condition. When exposed to s hort day lengths in the laboratory, these males showed the entire rang e of variation in responsiveness noted above and, correlatively, varia tion 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 le ngths in the laboratory, and thus, as might be expected, a few pregnan t and lactating females were also trapped at this time. Longitudinal s tudies over a 42-wk period revealed that the variation reflects the de gree to which the testes regress in response to short-day exposure rat her than the rate at which they regress. Finally, studies with laborat ory-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 ro dents indeed has relevance for understanding the seasonal control of r eproduction in the wild.