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.
Mating systems among birds become more varied as the amount of parental care decreases. For example, in birds with precocial chicks that can feed themselves, there is the opportunity for one partner to desert the family to start a new breeding attempt once the chicks hatch. And if one bird can both incubate and rear the chicks, then the options for unequal parental investment are even greater.
Shorebirds provide one of the classic case studies of the diversity of mating systems as they exhibit the full range, from polygamy (for example, Ruff and Pectoral Sandpiper) through monogamy to polyandry (for example, Spotted Sandpiper and some jacanas, phalaropes and painted-snipes). As expected, species with chicks that are unable to feed themselves, such as oystercatchers and thick-knees, are socially monogamous, whereas species with more ‘exotic’ mating systems have chicks that require little parental care.
In nature, all marriages tend to be expedient. As soon as one partner can raise more offspring by deserting the pair bond, selection favours that option. This rather begs the question, why don’t all shorebirds with precocial chicks that feed themselves experience strong competition between the sexes, pushing towards either polygyny or polyandry? In order to tease apart the factors underpinning different mating choices, it is informative to study the intermediate cases, where individuals vary in their choices.
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