The Annual Cycle and Fat Storage in Two Populations of Golden-Mantled Ground Squirrels

Abstract

The annual hibernation-reproduction cycle in two populations of captive golden-mantled ground squirrels ( Spermophilus lateralis ) was studied in the laboratory. One population was from an altitude of 2900 meters in Colorado, where the winters are long and severe; the other was from an altitude of 970 meters in Oregon, where the winters are relatively short and mild. In the laboratory the cycles of all animals were reduced to less than 1 year in length, averaging 48 weeks. The shortest were in females from the higher altitude, which averaged 43 weeks. As a result, cycles of these females drifted out of phase with those of males from the same population and with the calendar year. The cycle consisted of five stages: 1) gonadal involution and fat storage; 2) fat storage only; 3) hibernation and gonadal quiescence; 4) hibernation and gonadal recrudescence; and 5) reproductive readiness. The involution and fattening stages of the cycle were longer in ground squirrels from the high altitude than in those from the lower altitude. Fat storage and body size were greater in the ground squirrels from 2900 meters. In both populations males stored more fat than did the females. The animals from the higher altitude were torpid, at room temperature, more often than those from the lower altitude. The differences in fat storage, body size, and frequency of torpor substantiate that ground squirrels from the high altitude are better adapted to long winters than those from the low altitude.