PHOTOPERIOD, AMBIENT-TEMPERATURE, AND FOOD AVAILABILITY INTERACT TO AFFECT REPRODUCTIVE AND IMMUNE FUNCTION IN ADULT MALE DEER MICE (PEROMYSCUS-MANICULATUS)

Winter is often stressful. Increased energetic demands in winter and c oncurrent reductions in energy availability can lead to an energetic i mbalance and compromise survival. To increase the odds of surviving wi nter, individuals of some nontropical rodent species have evolved mech anisms to enhance immune function in advance of harsh winter condition s. Short day lengths provide a proximate cue for enhancement of immune function, an adaptive functional response to counter environmental st ress-induced reduction in immune function. In the present study, photo period, ambient temperature, and food availability were manipulated an d reproductive function and cell-mediated immunity were assessed in ad ult male deer mice (Peromyscus maniculatus). Mice maintained in short days regressed their reproductive systems and displayed enhanced immun e function compared to long-day animals. Reduced food availability ele vated corticosterone concentrations and suppressed reproductive and im mune function, whereas ambient temperature alone had no effect on cell -mediated immunity. The suppressive effect of food restriction on repr oductive and immune function was overcome by maintaining animals in sh ort days. However, short-day, food-restricted mice maintained at low a mbient temperatures displayed reduced reproductive and immune function compared to animals maintained at mild temperatures. Taken together, these results suggest that short-day enhancement of immune function ca n counteract some, but not all, of the immunosuppressive effects of wi nter stressors. These data are consistent with the hypothesis that imm une function is enhanced in short days to counteract stress-mediated i mmune suppression occurring during winter.