Interrelation of endogenous annual rhythms of reproduction and hibernation in the golden-mantled ground squirrel

Abstract

1. Ten male golden-mantled ground squirrels were captured in late summer and maintained in the laboratory for nearly 2 years at constant photo-period (LD 12∶12) and temperature (23 °C). Testes were measured monthly using a new laparotomy technique, and body mass and water consumption were recorded as well. 2. Size of testes showed a precise and well marked endogenous circannual rhythm with a period of about 11 months. The testes developed over a 3-month period during the hibernation phase of the annual cycle and reached a peak in mass by the end of hibernation. Two successive circannual cycles were expressed during the experiment. 3. Circannual rhythms of body mass and water use — which are components of the overall cycle of hibernation and metabolism — persisted concurrently with the circannual testicular cycle and also showed a period of about 11 months. Analysis of phase relationships between the 3 rhythmic parameters measured shows that precise internal synchronization of these independent rhythmic functions is maintained. The annual peak in body mass occurred about 3 months before the peak in testis size. The minimum of water use — an indication of annual metabolic minimum associated with hibernation — occurred about one month before the testis peak. 4. The duration, annual phasing, and predictability of aboveground environmental conditions which favor successful reproduction have strongly shaped the evolution of reproductive performance in golden-mantled ground squirrels and have promoted endogeneity, precision, and brevity of breeding condition in the annual cycle of these animals. Further problems in the evolution of this rhythmic system and in its mechanisms are discussed.