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The female Spotted Hyena's urinary-genital system is unique among mammals: there is no vagina, and the clitoris is modified so that it is as large, and as erectile, as the male's penis - only the shape of the glans makes it possible to tell the sexes apart. The female urinates, mates and gives birth through this modified clitoris (it is inverted for mating). It is thought that the development of this structure depends on a masculisation process triggered by the action of androgens of the developing female cub before or soon after birth. The evolutionary origins and function of this unique organ are not yet known. Birth through the clitoris is very difficult, and in addition the internal birth canal is contorted because of the unusual geometry of the external organs. In captivity, many cubs of primiparous mothers are stillborn because of the long labour times involved; in the wild, survival rates of females seem to fall sharply around the age of first giving birth, suggesting that the process is hazardous for the mother also. This suggests that at some point there must have been powerful selective pressures driving the evolution of masculinisation.