Human Sexual Dimorphism Opposites Attract

'There is love at first sight, not love at first discussion.'

Anon.

In the Descent of Man, Darwin observed that among mammals

The greater size, strength and pugnacity of the male, his special organs of offence, as well as his special means of defence, have been acquired or modified through the special form of selection I have called sexual.

He also stressed that, as well as inter-male competition: 'there is another and more peaceful kind of contest, in which the males endeavour to excite and allure the females by various charms.'

Darwin's emphasis upon the effects of sexual selection on masculine body size, weaponry, attractive adornments and displays is consistent with the fact that females usually represent a limiting resource for males. Females invest a huge amount physiologically in reproduction and in the care of their offspring, so they are expected to be more 'choosy', whilst males may be expected to compete among themselves and to invest more in attempts to attract and control prospective mates. This is not to imply, however, that sexual selection does not act upon females. In the last chapter, the importance of female sexual skin swellings in mandrills, talapoins, chimpanzees, and other Old World anthropoids was discussed in relation to changes in female sexual attractiveness during the menstrual cycle. These extraordinary oestrogen-dependent swellings are prime examples of the effects of sexual selection acting upon females, to enhance their 'charms' during the 'more peaceful kind of contest' required to attract members of the opposite sex

(Figure 6.6). Darwin (1876) was intrigued by these structures, and he recalled that 'in my Descent of Man no case interested and perplexed me so much as the brightly-coloured hinder ends and adjoining parts of certain monkeys'. Even in this context, however, he emphasized the functions of brightly coloured areas of skin in male monkeys (notably in the male mandrill) rather than exploring the effects of sexual selection in relation to female sexual attractiveness.

In this chapter, I shall examine the question of human sexual dimorphism, and explore the extent to which sexual selection may have influenced the evolution of the physique, facial traits and secondary sexual adornments of men and women. Given the limited information available to him in the nineteenth century, Darwin made important observations and advanced new hypotheses concerning the effects of sexual selection upon the evolution of human morphology. Although the huge potential to build upon Darwin's insights was neglected for many years, this field has been revitalized by the advent of modern research in the fields of evolutionary psychology and anthropology. A critical appraisal of some of these recent advances is included here; the goal being to understand how far current sex differences in our morphology might reflect the effects of sexual selection in the remote past, during the course of human (or pre-human) evolution. How far are the morphological traits of men and women consistent with an evolutionary history which involved sexual selection within monogamous, polygynous, or multi-male/multi-female mating systems? I am not concerned here with sex differences in olfactory cues. Although body odours have been implicated in mate choice and attractiveness in human beings (Wedekind 2007), given the current state of knowledge, I consider that studies of sexual dimorphism in bodily and facial cues provide greater insights into the likely origins of human mating systems.

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