Circannual body weight rhythms of ground squirrels: role of gonadal hormones

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Abstract
The influence of ovarian and testicular hormones on circannual body weight cycles was assessed in golden-mantled ground squirrels (S. lateralis), maintained in a LD [light/dark] 14:10 h photoperiod at 23.degree. C. Animals were gonadectomized or sham operated at 47 days of age and implanted with empty capsules or capsules filled with estradiol benzoate (EB) or testosterone propionate (TP). Body weight, food intake and reproductive condition were recorded at weekly intervals. Gonadectomized and intact squirrels of both sexes manifested robust circannual rhythms of body weight; cyclic secretion of gonadal hormones was not necessary for the expression of this circannual cycle. Intact and gonadectomized males weighed substantially more than their female counterparts but only during part of the annual cycle. This sex difference waxed and waned on a circannual basis in the absence of concurrent gonadal hormone secretions. EB phase-delayed attainment of peak body weight in ovariectomized squirrels but did not affect timing of trough body weight. EB increased the amount of food consumed by ovariectomized squirrels particularly during the weight gain phase. TP accelerated attainment of trough body weight in orchidectomized animals. Gonadal hormones modify the magnitude and timing of body weight peaks and troughs but are not necessary for the generation or expression of the circannual body weight cycle or for the sex difference in this cycle.