MONDAY, July 29, 2013 (HealthDay News) -- Love songs praise the virtues of monogamy, but a new study suggests that long-term coupling began as a way to protect babies from murder by unrelated men.
According to the researchers who tracked the evolution of monogamy among primates, humans may have adopted the practice to protect their vulnerable young.
"This is the first time that the theories for the evolution of monogamy have been systematically tested, conclusively showing that [prevention of] infanticide is the driver of monogamy," study author Christopher Opie said in a statement.
Opie said his team's study, published online July 29 in the Proceedings of the National Academy of Sciences, "brings to a close the long-running debate about the origin of monogamy in primates." Opie is a post-doctoral research fellow in the department of anthropology at University College London, in England.
Opie said the findings also suggest that other benefits of monogamy -- guarding females from rival mates or allowing males to partake in childrearing, for example -- were both the result of the adoption of monogamy, not part of the cause.
As the researchers pointed out, a look around the animal kingdom shows that monogamy is a relatively rare phenomenon. Less than three percent of mammals engage in the practice.
Researchers have long sought to understand why monogamy became the norm for humans. There were three predominant theories: that a man sticks with one woman to block other male interlopers (and thereby improve his own breeding opportunities); that the relatively slow maturation of human babies requires two committed parents to split the burden; and that monogamy stopped unrelated men from killing a woman's infant (done so she could stop nursing and be free again to breed).
In the new study, Opie and his team analyzed mammalian traits across 230 species. They looked at key factors such as the development of paternal childrearing behaviors, maternal movement and living patterns, and the threat of male infanticide.
A key finding was that, among humans, only the practice of infanticide by unrelated males was present before humans took up monogamy. This suggests that long-term coupling arose as a method to stamp out the killing of infants by rogue males.
A second study, published July 29 in the journal Science, came to a slightly different conclusion -- at least for mammals as a whole. This time, a British team led by Dieter Lukas and Tim Clutton-Brock, of the University of Cambridge, wanted to discover why some mammal species pair up for life.
Using a complex genetic analysis of more than 2,500 species, they found that the key may lie in whether females of the species live in close proximity to each other or are spread far apart. If the latter is the case, males simply cannot guard multiple females from other males, so monogamy becomes the norm.
The team said the findings may not apply to humans, however, because human evolution involved "cultural adaptations" that set the species apart from other mammals, Clutton-Brock said in a university news release.
And not all experts agreed with Opie's findings. Catherine Salmon is an associate professor of psychology and chairwoman of the department of psychology at the University of Redlands in Redlands, Calif. She was doubtful that the adoption of monogamy by humans could be tied to any one factor.
"I don't think child protection as one factor [in the mix] is at all surprising," she said. "Monogamy would offer a huge advantage in terms of protection from predators -- human or otherwise. But the idea that monogamy is mainly infanticide-driven is surprising, and I'm not entirely convinced."
"For one, although our infants are very vulnerable, I don't see the adaptive advantage for unrelated human males to commit infanticide," Salmon said. "In lions, it makes sense because when the unrelated male kills another's offspring, that brings the lioness into heat again, making her available to him. And if you're a lion, that's really the way to get in there and make your babies. But no one has ever demonstrated that humans work like that."
"The other thing," she added, "is that if human males were highly infanticidal, you would expect there would still be some remnant of that drive in human males today. But while human males are perhaps not as captivated by babies as females, they're not particularly infanticidal."
Salmon said, however, that men do seem to have "remnant" drives that predispose them to promiscuity.
"Men can ... be most successful reproductively by having sex with as many women as possible," she said. "Of course, many guys entertain sexual thoughts that they don't act upon. But many do, as we can see with the case of Anthony Weiner in New York. That's a remnant that is plain to see."
Find out more about the science of evolutionary psychology at the University of California, Santa Barbara.