Scientists have long wondered why a small minority of mammals, including humans, have evolved into monogamous creatures, and two studies provide new information but give different answers.
One group of scientists that looked only at primates found that the impulse for males to protect their offspring from infanticide by rival males was the trigger for monogamy. That study was published in Proceedings of the National Academy of Sciences.
The other study, which focused on more than 2,500 species of mammals, said males form pairs with females to protect their mates. That situation arose, the study published in the journal Science said, because females lived spread apart from one another, making the risk of leaving a vulnerable female too great.
For researchers tackling the monogamy question, here was the fundamental puzzle: Males, by sticking with one partner, seemed to lose out on the chance to father lots of children; gestation periods, after all, can be long in female mammals. That explains why most mammalian species don't follow the one-partner rule. But for the roughly 5 percent that do, what caused monogamy to evolve?
Both groups of researchers studied the DNA sequences of animals alive today and traced the evolutionary tree to answer the question. They tracked how species were related and when species branched off.
One long-standing hypothesis -- that having a father on hand to help raise and protect the child swayed mammals toward monogamy -- was thoroughly debunked by both groups. A two-parent system is a consequence, not a cause, of staying faithful, they concluded.
"First, you become monogamous, and then you are stuck, so you might as well help raise the child," said Eduardo Fernandez-Duque, a University of Pennsylvania anthropologist who was not involved in the studies. He called the wealth of new data "very exciting."
The Science paper said females started living far from one another as they competed for a better diet.
"Females changed their diet to foods of higher quality that were clumped and defended that food more aggressively," University of Cambridge zoologist Dieter Lukas said. This lead to large, exclusive territories, each containing one female, rather than territories that overlapped.
The males had no choice but to follow that distribution. A male mammal could not successfully defend more than one female because of risk of injury or predation, and then he would lose the paternity he had just gained, Lukas said.
However, the researchers found no association between monogamy and infanticide, which the PNAS paper cited as the primary reason monogamy evolved.
That paper looked at 230 species of primates, about a quarter of which are monogamous; their analysis included humans, classifying them as both monogamous and polygynous, a mating system involving one male with two or more females.
"Infanticide is a real problem, particularly for social species," said University College London anthropologist Christopher Opie, senior author of the PNAS paper.
Living in an advanced social system requires a large brain to deal with the complexities of relationships, Opie said. The downside of a big brain is slower infant development and longer lactation periods to foster brain growth -- meaning more opportunities for a rival male to kill the child and impregnate the female.
This gives males an evolutionary advantage for sticking with the child, to ward off intruding males.
Even though the primary incentive for mammals becoming monogamous differed, "quite a number of (the Science and PNAS papers') conclusions are similar," said Tim Clutton-Brock, senior author of the Science paper and a University of Cambridge zoologist. He called it a "chance phenomenon" that both groups were investigating such a similar problem.
Fernandez-Duque said how species were classified in each study could possibly explain the differences in the results. The Cambridge group focused more on the social behavior of animals by separating species into three groups: solitary, socially monogamous and group-living.
However, the other group used mating system as its classification, tagging each type of primate as monogamous, polygynous or "promiscuous, meaning multiple males and multiple females," Opie said.
He said he finds an issue with the Cambridge classification because of its focus on social, rather than mating, habits.
"You can't have a breeding system that is solitary," he said. "You can't do that on your own."
Also, the Science paper included evolutionary trees from a wide variety of mammals, including wolves, jackals, beavers, meerkats and primates.