Loss of oestrus and concealed ovulation in human evolution - The case against the sexual-selection hypothesis

The assumption that absence of oestrus and of manifestations of ovulation is specific to humans has given rise to various proposals of a role for selection pressures in the evolution of these features in the form of sexual selection or other behavioural adaptations. Analysis of the sexual behaviour of nonhuman primates and humans indicates, however, that constant receptivity is not unique to humans and that human sexual behaviour is not independent of the phases of the menstrual cycle. Quantitative differences in the distribution of sexual behaviour between humans and the nonhuman primates in question may be the result of many morphological, ecological, and cultural factors of which those differences are side effects. In the case of the postulated selection pressures on the disappearance of visual manifestations of ovulation, the rather unlikely chimpanzee model of anogenital swelling in the early Hominidae may be replaced by an early-hominoidal model in which the swelling was relatively small. Its reduction in anthropogenesis may have been caused by bipedal locomotion, the cost of water accumulation, hyperaemia of the area, and an increase in adipose tissue. Furthermore, olfactory communication in the context of sexual behaviour in the climatic conditions of the African savannah would have been sufficient for detection of the fertile periods of the menstrual cycle. Thus, assuming the existence of direct selection pressures on sexual behaviour in the Plio/Pleistocene evolution of the Homininae seems unjustified.