Abstract
Sex is not about reproduction; sex is about (re-)combination of DNA. Sex, not reproduction, always involves physical contact between two individuals; to achieve this, strategies of sexual behavior evolved. Sexual behavior, therefore, did not evolve as part of a reproductive strategy, but evolved to enable exchange of genetic material. In multicellular organisms the situation is more complicated than in unicellular organisms, as it is impossible for each cell within a multicellular body to have sex with another cell. Hence, evolution selected a system in which the possibility to have sex was limited to only one cell-line: the germ cells. As a result, sex adopted the character of fertilization, and sex and reproduction became inseparably linked. Still, in some species, including humans, sexual behavior still exhibits features of its evolutionary past: in humans (like in bonobo's) most sexual activity and many sexual behavioral patterns have nothing to do with reproduction (masturbation, homosexual behavior, for example); in humans, sexual behavior also became associated with other strategic objectives, such as intensifying the pair bond, expression of love or power. Different genders - male and female - evolved, and each gender evolved typical gender-related sexual and reproductive strategies as well. In most multicellular species, these strategies became inextricably mixed, and sexual behavior increasingly more - and in most species even exclusively - 'served' the interests of reproduction: sexual behavior became more or less synonymous with reproductive behavior. In most species, the 'mix' of sexual and reproductive strategies evolved into typical gender-related patterns of behavior, that is, in typical 'sex-roles'. Often, males are bigger and more 'beautiful' (= more intensely ornamented) than females; males compete with each other for access to females; males court females, while females choose males ('female choice'). However, ecological circumstances may cause a reversal of sex-roles, resulting in a situation in which females are bigger and more intensely ornamented than males, females compete for access to males, females court males and only males invest in care for the young, provided they are relatively certain of their paternity. Also, as in the case of the spotted hyena, females may be highly virilized and be socially dominant. This 'sex-role reversal' is seen, e.g., when males are relatively rare due to high predation pressure, or when the process of reproduction is very risky for the same reason: then it is 'better' that males, with their plenty of sperm, are wasted, than females with their few, precious eggs. It can be argued, with women being the fair sex, exhibiting competitive behavior and with an actively displaying courtship, and with men investing heavily in their offspring, meanwhile taking all (cultural) kinds of measures to guarantee their paternity, that humans, too, exhibit some degree of sex-role reversal.
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