Evolution of mammalian monogamy remains mysterious

Why some mammalian species choose to spend their lives with the same mates has long baffled scientists — and will probably continue to do so as two new massive studies present contradictory results.

One group of researchers says monogamy evolved in primates to counter the threat of males killing babies to boost their siring success. The other team concludes that mammals, including primates, become monogamous when females live far away from one another.

The differences in the studies have raised eyebrows. “They do seem to be saying the opposite thing,” says Anthony Di Fiore, an evolutionary anthropologist at the University of Texas at Austin. “It’s interesting because they use very, very similar methods,” Di Fiore says.

The two groups also disagree on whether the research has implications for why humans evolved fidelity to mates.

Both teams investigated the evolution of social monogamy, which researchers define as males and females living in breeding pairs. It does not necessarily mean that each animal is always faithful and never mates outside the pair.

Social monogamy is normal for birds but rare in mammals. That’s because birds of both sexes can participate in parenting duties such as incubating eggs and feeding chicks, but male mammals can’t help gestate or breastfeed a baby. During the long period when a mother mammal is occupied with parenting, an opportunistic father can take off to sire more offspring with other females.

But around 9 percent of mammal species, such as wolves and beavers, live in pairs in which the male sticks by his mate. This living arrangement is more common among primate species, about a quarter of which live in pairs.

To determine what factors drove the evolution of monogamy, Dieter Lukas and Tim Clutton-Brock from the University of Cambridge in England collected information about more than 2,500 species of mammals — nearly half of all mammalian species. The researchers used published reports to classify each species as monogamous or not, and then noted whether that species practices infanticide and whether the females live in discrete territories. Using this dataset, the researchers reconstructed the likely evolutionary history of mammalian monogamy. The team concludes in the Aug. 2 Science that monogamy evolved independently 61 times, almost always when females lived far from one another.

In those situations, Lukas says, males have difficulty mating with multiple females. By sticking with one female and guarding her from amorous advances from other males, he might produce more offspring than if he attempted to spread himself around.

The other group, led by Kit Opie of University College London, performed a similar evolutionary reconstruction but focused on 230 primates. These researchers conclude July 29 in the Proceedings of the National Academy of Sciences that the trigger for the evolution of monogamy was high rates of infanticide by males.

In nonmonogamous species such as gorillas, males may benefit from killing other males’ babies because losing a baby forces the mother to enter her fertile period sooner. But males that hang around their mate and offspring can defend them from roving killers, so monogamy could have evolved as a counter-strategy, Opie and colleagues suggest. Today, monogamous primates have very low rates of infanticide, and in some cases, such as in titi monkeys native to South America, infanticide has never been observed at all.

Opie is confident that infanticide drove many primates to live in pairs. “It solves the puzzle. It finishes the debate,” Opie says. “Or we certainly hoped, before we heard about the other paper, that it would finish the debate.”

The debate is, of course, far from over. “We don’t find any support that infanticide has been important for the evolution of monogamy across mammals,” Lukas says. In his team’s dataset, monogamy is as likely to have evolved from an ancestor that did not practice infanticide as from one that did. This was also true when they examined only primates.

But Opie says that widely spaced female territories can’t be the cause of the switch to monogamy in primates, because in his team’s analysis, females shifted into discrete territories after the evolution of monogamy.

The groups also disagree about implications for human evolution. Opie says that humans evolved to live in monogamous pairs to minimize the threat of infanticide. Humans were part of his team’s analysis. “We treated them just the same as all the other primates, because that’s what they are,” Opie says.

But Lukas and his team say their own results have little bearing on humans. Humans evolved from an ancestor that lived in social groups, so their theory about monogamy evolving when females live far apart doesn’t apply. Besides, he says, humans may not actually have evolved monogamy at all. In many traditional societies, one man may take several wives.

The dispute might stem from the way the groups classify the key behaviors: monogamy, infanticide and female territories, says Charles Nunn, an evolutionary anthropologist at Duke University.

Opie’s methods were also slightly better at handling the blurry lines between types of mating systems, Nunn says, whereas Lukas’ team “really wants to pin each species into one cubby hole.” For example, Opie’s team classified the gray bamboo lemur, which has some variation in its mating habits, as both monogamous and polygynous, while Lukas’ team classified this species as not monogamous. “The devil is going to be in the details,” Nunn says.

Both teams emphasize that they do not yet know why their conclusions on primates differed, but they have exchanged data and agree they need to work together to iron out the details.

Ultimately, the factors that led to monogamy might differ among species. “Many times we forget that this is not math,” says evolutionary anthropologist Eduardo Fernandez-Duque of the University of Pennsylvania. “It’s unlikely that one size will fit all.”