Why would male mammals choose one special female when they could have many? Two groups of U.K. researchers trying to answer that question have reached two different conclusions.
But they both agree on one thing — the benefits of two-parent families aren't the main motivation for monogamy.
Scientists have long wondered why a small percentage of mammal species, including beavers, wolves, meerkats and many monkeys such as marmosets, are socially monogamous — that is, a single male mates with only a single female during each breeding cycle.
Why males would "stick around instead of finding other females" is puzzling because female mammals spend long periods of time pregnant or nursing young, when they are typically unable to conceive more offspring, said Dieter Lucas, a University of Cambridge researcher who is the lead author of one of the two new studies.
That is time that a male could use to mate with other females in order to improve his reproductive success, Lucas said at a news conference organized by Science, the journal where the study is being published this week.
There are a few popular theories about why monogamy evolved:
Animals form pairs because the offspring are more likely to survive when they are cared for by two parents — something that occurs in most animals that are socially monogamous.
Males form pairs to prevent the female from mating with other males.
Males form pairs to protect their offspring from other males, who might kill the babies to improve their chance of reproducing with the mother.
Lukas and his colleague Tim Clutton-Brock classified 2,500 species of mammals as solitary, socially monogamous, or group living, and found that about nine per cent were socially monogamous. Other known mammal species were excluded from the study because not enough information was available to classify them. The researchers used the family tree of mammals, based on their genetic relationships, to figure out how the animals' common ancestors behaved and therefore when social monogamy arose relative to other behaviours.
They concluded that monogamy typically evolved when females started roaming widely and defending large territories from competing females in order to protect valuable, rich and uncommon food sources, such as fruit or game.
"Females in socially monogamous species live at low density so that males cannot successfully defend more than one female," said Lukas.
Later, some of those animals evolved to live in larger family groups that include the adult offspring of a monogamous pair, but typically only one pair in each social group breeds.
Lukas and Clutton-Brock's study found that males didn't start becoming involved in parental care of their offspring until after monogamy was already established.
The latter fact is consistent with the findings of a study led by Christopher Opie, an anthropologist at University College London, which involved similar analysis of smaller subgroup of mammals — primates, which include humans, apes, monkeys and lemurs.
However, that study found no correlation between monogamy and the females' ranges and territories.
Instead, the study published this week in the Proceedings of the National Academy of Sciences journal found monogamy evolved after males began killing the offspring of other males. They concluded that this kind of infanticide is what prompts primates — possibly even humans — to switch to a monogamous mating system.
They noted that the long period of time that primate infants, including human babies, are nursing and dependent on their mothers increases the time that they're vulnerable to infanticide, and that may be why monogamy is more common among primates (around 27 per cent of primate species are monogamous) than other mammals.
On the other hand, Clutton-Brock said the University of Cambridge study found "no evidence of a close association between monogamy and male infanticide" in either mammals overall or primates only.
Unlike the University College London study, the University of Cambridge study purposely excluded humans.
"I'm far from convinced that humans are monogamous," Clutton-Brock said, noting that mating systems in different human cultures cover the full range of animal mating systems across species and that human behaviour is heavily influenced by culture in ways that the behaviour of other animals is not.
Lukas added that even if humans form monogamous pairs, they don't meet the definition of "social monogamy" used in the study because it requires that pairs of animals have non-overlapping ranges, whereas human couples live in larger groups. That "simply does not occur in other animals," he said.
Clutton-Brock and Lukas said they did not know why Opie's study arrived at such a different conclusion about the origin of monogamy in mammals, although they suggested it might have to do with the smaller sample size or the way the different species were classified.
"Until a few days ago, we had no idea that this paper existed," he added at the news conference.
Otherwise, the two research groups would have met beforehand to figure out how the differences may have arisen. As it is, he said, "We have had no chance to compare notes and to compare methods."
However, they are hoping to do so in the near future.