Serum levels of gonadotropins and gonadal steroids, including testosterone, during the menstrual cycle of the chimpanzee (Pan troglodytes)

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Abstract
The objective of this study was to expand the data on menstrual cycle serum hormone patterns in female common chimpanzees, both in terms of the number of cycles analyzed and by the addition of data on testosterone levels. Samples were obtained from 11 unanesthetized animals trained for conscious blood withdrawal. LH, FSH, 17β‐estradiol (E2), progesterone (P), and testosterone (T) were measured by radioimmunoassay, genital swelling was recorded, and menstrual blood was noted. Concurrent midcycle elevations in LH and FSH and luteal phase elevations in progesterone suggested that the cycles were ovulatory. Detumescence of genital swelling occurred about 3 days after the midcycle LH peak, 1 day after the luteal phase nadir in E2, and 1 day after P levels exceeded 5 ng/ml. These relationships provide further support for the use of genital swelling in monitoring progress of the menstrual cycle. The hormone patterns in the chimpanzees closely resembled those of the human females, but E2 and T levels were higher. The levels of E2 and T were higher and the midcycle elevation in T was broader in the chimpanzee than in gorillas and orangutans. This is of interest because E2 and T are implicated in the regulation of mating, and chimpanzees mate over a greater portion of the cycle than the other apes. These data indicate the need for further study of hormonal contributions to the different patterns of mating in the great apes. They also support the use of the female common chimpanzee as a model for the human female in endocrine studies of the menstrual cycle.