Why Monogamy Evolved in Mammals

In groups of meerkats, the majority of individuals are the offspring of a single, life-long mated pair.
Image courtesy of Dieter Lukas

Male primates may have become monogamous to protect their offspring from being killed by rival males, a new study finds. However, others disagree, saying monogamy evolved in mammals so that males could guard their mates.

A team of British and Australian researchers compared data across 230 primate species over 75 million years, and found that the threat of infanticide -- specifically, the threat of baby primates being killed by unrelated males -- likely triggered monogamy.

Since infants are dependent on their mothers throughout childhood, and since female primates typically delay further conception while they are nurturing their young, male competitors may see advantages in doing away with babies that their rivals have sired, said study lead author Christopher Opie, a postdoctoral research fellow in the department of anthropology at the University College London in the United Kingdom. (8 Humanlike Behaviors of Primates)

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"For a male who knows he's not the father of an infant, it can pay for him to kill that infant, because then he can make sure the female comes back into ovulation. And he can mate with her," Opie told LiveScience. "It's a way for males to try to increase their genes that are passed into the next generation."

The researchers examined the prevalence of infanticide across different primate species over time and found links between this threat and the onset of monogamy.

"When we looked across all 230 species, we saw that infanticide evolved at different points, but in all cases, it had already evolved by the time monogamy evolved," Opie said. The results were published online today (July 29) in the journal Proceedings of the National Academy of Sciences (PNAS).

Another study out today, however, suggests monogamy may have evolved to protect females against competition from other females.

Neither study purports to explain monogamy in people. "We are cautious about making any definite statement about monogamy in humans," study researcher Tim Clutton-Brock of the University of Cambridge said in a press briefing, adding that when it comes to monogamy, "humans are obviously fantastically variable."

Primate Family Tree

Only 3 percent to 5 percent of all mammals bond for life, but researchers have long debated the evolution of monogamy, with scientists trying to pinpoint when in history animals displayed monogamous tendencies -- and why.

To trace monogamy's evolutionary pathway, Opie and his colleagues constructed a giant family tree based on genetic data of the relationships among the species of primates. The researchers then used statistical models to identify where behavioral changes -- such as the emergence of paternal care of offspring or the ranging patterns of females -- likely occurred throughout the primates' evolutionary history.

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"We effectively simulate evolution millions of times across the family tree and get probabilities for how each of the behaviors would change over time," Opie explained.

This technique resembles the one used by famed American statistician Nate Silver when he predicts the results of presidential elections, and the method used by Google when it produces search engine results, Opie said.

The models determined that male infanticide coincided with the switch from behavior in which females mated with multiple males, to monogamy in primates. The results also suggest that other behaviors, such as paternal care, resulted from monogamy. (The Animal Kingdom's Most Devoted Dads)

"In all the species where males provide care, monogamy already evolved in those species," Opie said. "So, we can see an evolutionary pathway where infanticide evolved first, then as one of the responses to that, monogamy evolved, and then in those species -- but not all -- paternal care evolved."

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