Conclusions

The concept of loss of oestrus as it has been applied to the evolution of human sexuality is flawed, and its use should be discontinued. Heape's original definitions of oestrus in mammals are of very little value in discussions of primate sexual behaviour, and especially so where the monkeys and apes are concerned. Female anthropoids are not dependent upon the secretion of ovarian hormones for the expression of sexual behaviour. Beach's three-pronged approach to defining female sexuality (i.e. female proceptivity, receptivity, and attractiveness) has much greater explanatory power than ill-defined statements concerning the presence or absence of oestrus. Comparative studies indicate that a rigid neuroendocrine control of female receptivity and proceptivity during the peri-ovulatory period was lost in ancestral anthropoids. Thus, extant monkeys and apes, as well as women, lack oestrus, whilst it is still present among the prosimian primates.

These observations do not mean that ovarian hormones have no effects upon sexual behaviour or attractiveness in anthropoid primates. On the contrary, numerous studies have established that such effects exist in monkeys and apes. Examples are presented and discussed in this chapter. Effects of ovarian hormones upon female proceptivity (sexual invitations to males) and attractiveness (e.g. sexual skin swellings) are more pronounced than hormonal effects upon receptivity (willingness to accept the male and allow copulation).

Cyclical changes in sexual behaviour also occur during the human menstrual cycle. However, they tend to be even more subtle, and situation dependent, than in many monkeys or apes. The hypothesis that some women alter their mating strategies during the fertile phase of the menstrual cycle in order to conceive with a short-term male partner having 'better genes' than a long-term (e.g. marriage) partner is evaluated and rejected. Instead, it is proposed here that evolutionary psychology has created an artificial dichotomy between long-term versus short-term patterns of female mate choice, and over-interpreted relatively small cyclic fluctuations in women's preferences for certain masculine traits.

The notion that 'pair-bonding' was facilitated by loss of oestrus during human evolution, or that 'concealed ovulation' evolved in order to confuse paternity and reduce the likelihood of male infanticide, is likewise rejected here. There is currently no convincing explanation as to why the neuroendocrine mechanisms controlling proceptivity and receptivity are less dependent on ovarian hormones in anthropoids than most mammals. The explanation is likely to involve selection for female choice, and alternative mating tactics in non-human primate ancestors of Homo sapiens that had complex social organizations and relatively large brains. Such selection occurred long before the homin-ids appeared in Africa, so that absence of a peri-ovulatory oestrus would have been part of the evolutionary inheritance of the australopithecines, and of the genus Homo.

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