ESTRADIOL PHASE-SHIFTS CIRCANNUAL BODY-MASS RHYTHMS OF MALE GROUND-SQUIRRELS

Gonadectomized male golden-mantled ground squirrels (Spermophilus late ralis) were implanted with estradiol benzoate (EB)-filled or empty cap sules. Body mass was monitored before, during, and for at least 1 yr a fter hormone treatment. EB treatment during the mass-gain phase of the annual cycle significantly decelerated increases in body mass; the pe riod of the circannual rhythm (CAR) of body mass was 54 days longer in EB- than blank-treated squirrels. Hormone treatment during the mass-l oss phase accelerated mass loss; although this effect only approached statistical significance, some phase markers of the CAR were significa ntly advanced in subsequent cycles. We conclude that, as in females, e stradiol affects the waveform of the CAR of males differently at diffe rent phases of the circannual cycle. Sexual differentiation does not e liminate responsiveness of CARs of squirrels to estradiol; sex differe nces, if any, are subtle rather than absolute and, in this respect, di ffer from circadian rhythms.