The evolution of monogamy

Although there select variations on the theme, monogamy is largely the favoured form of reproductive unit in humans. Such things of course do not happen by accident. Although they may be justified and adopted by cultures and religions, there are deeper forces driving monogamy and now researchers from the University of Oxford, University of Manchester, UCL and the University of Auckland have identified what those forces are.

For some time there have been theories as to why monogamy may have been selected in evolutionary terms. After all, the strategy of two parents investing lots of effort into to raising one or two offspring is very different to, say, some fish species that fertilise hundreds of eggs and leave the young to fend for themselves relying on weight of numbers to ensure survival of their genes.

When you put all of your eggs into one child the stakes are high. Infants are most vulnerable when they are fully dependent on their mother because females delay further conception while nursing slowly developing young. This leads to the threat from unrelated males, who can bring the next conception forward by killing the infant. Sharing the costs of raising young both shortens the period of infant dependency and can allow females to reproduce more quickly. An additional benefit of sharing the burden of care is that females can then have more costly young. Growing a big brain is expensive in evolutionary terms and requires that the offspring mature slowly. Caring fathers can help alleviate the burden of looking after young with long childhoods and may explain how large brains could evolve in humans.

So the theories as to why monogamy evolved come down to: providing paternal care when the cost of raising offspring is high, guarding solitary females from rival males, and reducing infanticide risk where males can provide protection against rival males. Now though, researchers believe they know exactly what factor made monogamy so evolutionarily successful.

The researchers gathered data across 230 primate species and plotted family a tree of the relationships between those species. They then used statistical methods to run various models of evolution across that family tree and from this analysis found the answer: monogamy evolved because it reduced the risk of other males killing the infants. Shared parental care and protection of the female were found to be results of monogamy, not the cause.

It may not be a very flattering underpinning for the state of holy matrimony, but it works.