Environmental Cues Evoke Differential Responses in Pituitary-Testicular Function in Deer Mice*

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Abstract
Pituitary-testicular function was assessed in deer mice (Peromyscus maniculatus bairdii) after exposure to external cues predicting seasonal climatic changes. Mice were exposed to either a 14-h light, 10-h dark cycle (L:D;14:10) or L:D;10:14 at 0, 23, or 32 C for 80–90 days. Spermatogenic arrest occurred in about one fourth of the mice exposed to short photoperiods at 23 C. Exposure to L:D;10:14 at 0 C significantly (P < 0.05) enhanced the proportion of mice with atrophic testes over that induced at 23 C. In contrast, L:D;14:10 at 0 C failed to affect gonadal function. Modest to severe testicular dysfunction was evident in at least 80% of mice exposed to 32 C at either L:D;14:10 or L:D;10:14. Certain mice (∼20%), however, failed to undergo testicular regression even after exposure to L:D;10:14 at 0, 23, or 32 C. Graded restrictions in food intake, over 4 weeks, triggered testicular atrophy in about 70% of the males whose gonads failed to regress after provocative exposure to photic and thermal cues. Testicular involution, regardless of the external cue, was associated with an unequivocal lowering of plasma LH and testosterone concentrations. These results, taken together, offer new and important insights about previously unrecognized variation in pituitary-testicular responses among deer mice exposed to environmental cues reminiscent of breeding and nonbreeding conditions at temperate latitudes. The physiological implications inherent in this variation are that individual males from the same population exhibit profound differences in their reproductive responses to external cues. Thus, our evidence suggests that seasonal reproductive performance is probably occasioned by one or more external cues depending, among other factors, on photoperiod, temperature, food availability, and the individual's sensitivity to these cues.