Photoperiod influences the critical caloric intake necessary to maintain reproduction among male deer mice (Peromyscus maniculatus).

The concept of critical day length is well established among rodents; reproductive function Is maintained when day lengths are greater than some specific threshold. In addition to day length cues, seasonal breeding in deer mice can also be regulated by food availability. The caloric threshold necessary to support reproduction remains unspecified for seasonally breeding rodents. The present study examined the interaction between photoperiod and food availability on reproductive function in adult male deer mice (Peromyscus maniculatus). A critical caloric intake profile was constructed in long (16L:8D) and short (8L:16D) photoperiods; groups of deer mice in both photoperiods either received food ad libitum or 90, 80, or 70% of their individual ad libitum food intake for 10 wk. At autopsy, paired testes, epididymides, and seminal vesicles were removed and weighed. Body mass, total body fat, and total body water contents were also obtained. Short, as compared to long, day lengths inhibited the reproductive systems of male deer mice. However, food consumption interacted with photoperiod to affect reproductive function. Significant reductions in reproductive organ size as well as spermatogenic activity were observed among short-day mice after a 10% reduction in ad libitum food intake. Long-day animals required a 20% reduction in caloric intake to depress reproductive function. Body mass and total body water content were generally unaffected by either photoperiod or food consumption. Total body fat content was reduced in short- as compared to long-day mice. Individual reproductive responsiveness to short days increased as food availability decreased. Reproductive organ mass and spermatogenic activity were comparable between longday males restricted to 70% of their ad lib food intake and short-day males fed ad libitum; i.e., both groups exhibited significant reproductive regression as compared to long-day males with unrestricted access to food. However, few of these ‘regressed’ males were aspermic. In contrast, the inhibitory effects of short day lengths plus 30% restricted food intake caused virtually all mice to stop spermatogenesis. Taken together, these results confirm that short photoperiods cause statistically significant reduction in gonadal size and sperm production; however, food availability may determine whether the reduction in sperm number is functionally significant.