The main advantage of sexual reproduction is that combining genes from two parents greatly enhances the genetic diversity of the next generation. Sex literally provides the variation on which natural selection operates.
This might seem a long way from the sex life of plovers, but bear with me. If the main advantage of sex – with all its distracting angst and aggression – is to create diversity, it stands to reason that promiscuity should be the norm. All other things being equal, you should have sex with many different individuals to increase the likelihood that at least some of your offspring will survive, whatever hurdles are presented by an ever-changing world.
Because humans are largely monogamous, we tend to forget this fundamental premise of sex. But it is humans who are unusual; most animals and plants are promiscuous. For example, more than 90 per cent of mammals are polygamous. Monogamy is thought to have evolved among humans because it benefited males to remain with their partners to protect their offspring, largely from infanticide by other males.
Birds too are unusual in being largely monogamous. This results from the high degree of parental care needed to raise a brood of chicks to independence. Of course we now know that many birds are only socially monogamous; like humans, both sexes might seek out extra-pair mating opportunities but remain with a social partner for the sake of the children. Such cuckoldry only adds further tension to an already tricky partnership.
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