Simulated Drought Affects Male Reproductive Function in Deer Mice (Peromyscus maniculatus bairdii)

Reproduction stops during the breeding season of many rodent populations, including Peromyscus maniculatus. The proposition that water availability mediates the midseason breeding hiatus in male deer mice was evaluated in the present study. Paired testicular and epididymal weights (absolute and relative to body mass) were reduced after 10 wk of restricted water consumption. Spermatogenesis was also impaired by limited water intake. Blood plasma testosterone concentrations were reduced, but plasma luteinizing hormone (LH) titers were unaffected by water restriction. Finally, the number of offspring sired was reduced in mice with restricted access to water. Carcass water content did not significantly differ between animals with restricted (25% of ad lib.) or unrestricted access to water; however, body mass was reduced about 15% after chronic water intake restriction. Although reproductive function (i.e., reproductive organ mass, plasma testosterone titers, and sperm counts) was significantly depressed in water-restricted mice, the reproductive systems of about 25% of the animals were relatively unaffected by water restriction. In order to study the physiological factors associated with individual reproductive responses to water restriction, animals were rank ordered according to their epididymal sperm numbers; individuals in the top (Q₁) and bottom (Q₄) quartiles of epididymal sperm counts were further compared. Water-restricted Q₁ deer mice did not significantly differ on any parameter from mice with ad lib. access to water. No water-restricted Q₄ deer mice possessed any epididymal sperm. Deer mice in Q₄ significantly reduced paired testicular and epididymal masses as compared with animals in the top quartile. It was important that Q₄ deer mice reduced their body mass by 34%. Plasma LH titers were significantly depressed in Q₄ animals as compared with Q₁ mice or mice with ad lib. water. Body fat content was also reduced only in the Q₄ deer mice. Taken together, these findings suggest that water availability may act as an extrinsic cue to suppress fertility in deer mice independently of temperature and day length and somewhat independently from food because despite an absolute reduction in food consumption with the onset of water restriction, food intake corrected for body mass did not differ. Animals that stopped reproductive function (Q₄ mice) lost substantial body fat and total mass. Water-restricted males that maintained body fat levels also maintained reproductive function. Differential hydrolysis of adipose tissue in response to decreased testosterone titers may account for the phenotypic variation in reproductive responsiveness to water restriction. These results are discussed in terms of their ecological and evolutionary implications.