Pattern and source of secretion of relaxin in the reproductive cycle of the spotted hyena (Crocuta crocuta)

Female spotted hyenas are highly masculinized at birth and have no external vagina. Copulation with males and birth of young are accomplished through the central urogenital canal of the clitoris. This unusual adaptation requires remarkable changes in the elasticity of the connective tissues of the clitoris, without which neither copulation nor birth would be possible. We hypothesized that relaxin, a hormone that increases the extensibility of the connective tissues of the uterus and cervix of many other mammalian species, plays a role in the clitoral changes observed in hyenas. Serum relaxin was determined by specific RIA. Relaxin was not detected in serum of males, pubertal or nonpregnant adult females, or ovariectomized females. Immunoactive relaxin was detected in serum of juveniles at the time of initial growth of the urogenital meatus. High concentrations of immunoactive relaxin appeared in the serum of pregnant hyenas in the 2 wk preceding parturition. Immunoassays of extracts of hyena tissues and serum obtained from uterine and ovarian veins indicated that the placenta was the predominant source of relaxin, with possible ovarian contributions. Circulating relaxin decreased promptly following cesarean section near term. We conclude that relaxin secretion coincides with changes in extensibility of clitoral connective tissues 1) during growth of the clitoris in juveniles and 2) near the time of parturition in adults.