Chimpanzee reproductive senescence: a possible model for evolution of the menopause.

Reproductive senescence was studied in 2 female Pan troglodytes and one Pan paniscus over 40 yr old. Menstrual cycle data for these animals, in the last 3 yr, when compared to that of the same animals in previous years and to records obtained between 1967 and 1980 on 51 Pan troglodytes between 18-39 yr old, demonstrated increased length of menstrual cycles, as shown by decreases in frequency of menses. Oestrone glucuronide and pregnanediol glucuronide were measured in 24-h urine samples by radioimmunoassay. The pattern of excretion differed only slightly from that of younger animals. One Pan troglodytes had reduced oestrogen levels, but normal gonadotropin levels. The single ovulatory peak of LH normally seen in younger animals was replaced by multiple peaks in the Pan troglodytes. Menopause was observed in the Pan paniscus as documented by cessation of menstrual cyclicity, elevation of gonadotropin levels, and exaggerated response to injection of 100 microgram of gonadotropin-releasing hormone. The FSH: LH ratio was reversed in this animal. Histology of the ovaries of the Pan paniscus revealed a paucity of primary and developing follicles, and an increase in fibrous tissue. The Pan troglodytes appear to be peri-menopausal. This data suggests that the chimpanzee may serve as a model for certain phases of reproductive senescence in the human.