Social monogamy – when a male and female of the species stick together for the long term, although may mate with others – is rare in mammals generally. However, it occurs in over a quarter of primate species, including humans, gibbons and many New World monkeys, such as titis.
To investigate what originally drove us to establish such pair bonds, a team led by Kit Opie of University College London and Susanne Shultz from the University of Manchester, UK, gathered data on the mating behaviour of 230 primate species. They selected behavioural traits associated with several possible evolutionary drivers of monogamy, including the risk of infanticide, the need for paternal care and the potential for guarding female mates.
Using data on the genetic relationships between the species, the team ran millions of computer simulations of the evolution of these traits to work out which came first.
All three were linked to the evolution of monogamy but only behaviours associated with infanticide actually preceded it, suggesting that this was the driver. Suckling infants are most likely to be killed by unrelated males, in order to bring the mother back into ovulation.
With pair-bonding in place, not only would a mother have a male to help protect the infant from marauding males, but there would then be the opportunity for the male to help care for it by providing extra resources. This means the infant can be weaned earlier, again reducing the chance of it being killed.
“Until recently, reconstructing how behaviour evolves has been very tricky as there are few behavioural traces in the fossil record. The statistical approach we have used allows us to bring the fossils to life and to understand the factors that have led to the evolution of monogamy in humans and other primates,” says Shultz.
Carel van Schaik, a primatologist at the University of Zurich in Switzerland says the results are solid but questions whether they can be extrapolated to humans. He says evidence suggests that humans were never really monogamous and that the monogamy we see today in many cultures is socially imposed.
Shultz counters that there is fossil evidence pointing to monogamy in australopithecines, the hominin genus from which modern humans descended.
“Although we suggest that infanticide may help explain the evolution of monogamy in humans, we do not argue that it is the only factor nor that monogamy is universal,” Shultz says. “I would suggest that where infanticide risk is high, as it would be with our ancestors, having a father provide protection and care would facilitate the evolution of the modern human extended childhood.”
Journal reference: PNAS, DOI: 10.1073/pnas.1307903110