Academics from the Universities of Manchester, Oxford, Auckland, and University College London have revealed a new understanding of why primates live in monogamous pairs. According to their study, the key drive factor that led to the evolution of monogamy in many primates is the threat of infants being killed by unrelated males. The team also found that males became more likely to care for their offspring following the emergence of monogamy, as they began protecting infants and sharing the burden of childcare.
Infants are most vulnerable when fully dependent on their mother because females delay further conception while nursing. This leads to threats from unrelated males, who hope to bring the next conception forward by killing the infant. When raising the young is a shared process, the dependency period is shortened, and females reproduce sooner.
Sharing the burden of care also allows for more intelligent infants. Growing a ‘big’ brain is expensive and requires offspring to mature slowly. Caring fathers help to alleviate the burden of looking after the young, allowing for longer childhoods, which may explain the evolution of large brains in the human lineage.
To uncover this evolutionary pathway, the team gathered data across 230 primate species, plotting them on a family tree of the species’ relationships. They then used computers to ‘re-run’ evolution millions of times to see whether different behaviours appeared at similar points in evolutionary time, and if so, which behaviour evolved first. They found that male infanticide tended to cause the switch from a multi-male mating system to monogamy, while other suggested causes, such as bi-parental care and solitary ranging by females, were the result of monogamy rather than a cause. Faculty researcher Dr Susanne Shultz said:
“What makes this study so exciting is that it allows us to peer back into our evolutionary past to understand the factors that were important in making us human. Once fathers decide to stick around and care for their young, mothers can then change their reproductive decisions and have more, brainier offspring.”