EFFECT OF REPRODUCTIVE FUNCTION ON COLD TOLERANCE IN DEER MICE

Thermoregulatory responses were evaluated in male deer mice (Peromyscus maniculatus nebrascensis) after exposure to short photoperiod and either warm or cold ambient temperature (T(a)). Deer mice were chosen for this study because males exhibit differential reproductive responses to short day length (SD); this difference has a genetic basis, and both phenotypes are found within natural breeding populations. Deer mice undergoing SD-induced gonadal regression significantly improved their cold limit to -32.9-degrees-C after exposure to SD/warm T(a) and to -47.4-degrees-C after SD/cold T. exposure, relative to long day length/warm T(a) controls (-17.4-degrees-C). In contrast, deer mice maintaining reproductive function despite SD exposure significantly improved cold limit to -27.2-degrees-C only after exposure to SD/cold T(a), relative to controls (-16.3-degrees-C). Maximum norepinephrine-induced nonshivering thermogenesis (NST) did not vary with reproductive state, indicating differences in cold tolerance were not due to capacity to produce heat by NST. Comparison between phenotypes of heat production during cold tolerance tests indicated that greater cold tolerance among mice exhibiting SD-induced gonadal regression can be accounted for by 1) lower rates of heat loss and 2) greater improvement of heat production. These findings suggest a functional relationship between reproductive function and seasonal thermoregulatory adjustments and indicate a significant cost to breeding during the winter months.