Effects of day length and endogenous control on the annual reproductive cycle of the antelope ground squirrel,Ammospermophilus leucurus

Abstract

1. Male antelope ground squirrels were removed from the field in fall, winter, spring, and summer and separate groups exposed for 50 days each to photoperiods of 16 h (LD 16∶8) and 8 h (LD 8∶16) and ambient temperature of 23 ‡C. Control samples from the field were obtained at the beginning and end of each of these experiments. A single experiment was conducted with females in winter. 2. At no time of year was there a significant difference (P>0.05) in the mass or condition of reproductive tissues of animals exposed to LD 16∶8 and those exposed to LD 8∶16. The changes in both long-day and short-day groups during each experiment paralleled the natural seasonal reproductive cycle, with a peak in late winter and early spring. 3. In association with an annual cycle of body mass,A. leucurus showed significant seasonal differences in the tendency to increase body mass and gonadal fat when held in captivity; however these responses were also unaffected by differences in experimental photoperiod. 4. Because the 50-day manipulations of photoperiod did not affect testicular activity at any season, we conclude that the annual gonadal cycle ofA. leucurus is under endogenous control. Factors that may have been involved in the evolutionary history of this adaptation are discussed.