Whenever a public figure cheats on his wife, pundits can be counted on to trot out the tired old claim that males are simply wired by evolution to be promiscuous.
Two studies released on Monday beg to differ. By sticking to one female, they conclude, males of many species, especially primates, can increase their chances of siring many offspring who survive long enough to reproduce – the key factor in determining whether a particular behaviour survives the brutal process of natural selection.
In fact, the evolutionary advantages to males of being monogamous are so clear that the two studies reached competing conclusions about which benefit is greater for males. According to research published in the Proceedings of the National Academy of Sciences, protecting the lives of his offspring was the paramount benefit of monogamy.
Separate findings published in the journal Science said that keeping his mate faithful provided the greatest evolutionary edge.
Both studies addressed a conundrum: because male mammals can sire so many more offspring per breeding season than females, it would seem that mating with only one female would be less adaptive for a male than spreading his seed widely.
Protecting the kids
The PNAS paper, which analysed 230 species of primates, concludes that protecting the kids is the greatest benefit of male monogamy. By sticking close to his mate a male reduces the risk of infanticide. Although the study examined only nonhuman primates, that reasoning has resonance in people, too, where children who grow up without a father in the household are more likely to die in childhood, according to government statistics.
"This is the first time that the theories for the evolution of monogamy have been systematically tested, conclusively showing that infanticide is the driver of monogamy," said anthropologist Christopher Opie of University College London, lead author of the PNAS paper, which analyzed 230 species of primate. "This brings to a close the long-running debate about the origin of monogamy in primates."
Not so fast, according to authors of the Science paper. Zoologists Dieter Lukas and Tim Clutton-Brock of the University of Cambridge examined the social structure of 2 545 species of mammals, of which 9% are socially monogamous. That was defined as a system in which a male mates with only one female and they "usually stay together until one dies", Lukas told reporters on Monday.
The Cambridge scientists conclude that guarding against infanticide played little if any role in the development of monogamy among such mammals as tamarins, marmosets, beavers, wolves, jackals and meerkats. Instead, the behaviour arose when females spread out over large territories and had zero tolerance for other females entering their turf.
That left a male little choice but to stick close to his mate, foreswearing others, because philandering risked leaving her to the sweet entreaties of a rival.
"Monogamy arose where guarding a single female was a male's best reproductive strategy," said Clutton-Brock.
Once monogamy set in for this reason, the care that faithful fathers provide their offspring can make the behaviour even more evolutionarily advantageous, since it increases their brood's chances of surviving.
In fact, the two studies are not that far apart, starting with their basic conclusion that male monogamy can be a winning strategy, evolutionarily, and thus be favoured by natural selection – the "survival of the fittest" winnowing process that shapes species. Both studies also found that paternal care "is a consequence, not a cause, of monogamy", said Clutton-Brock, but one that made it even more beneficial: with monogamy, males are more likely to help care for their offspring.
That not only protects against murderous marauders but takes some of the burden of childcare from mom, preserving her health and allowing her to bear more healthy offspring, which count as additional evolutionary points for the faithful dad too.
It will take more research to sort out which evolutionary force – preventing infanticide or guarding against cuckoldry – had the strongest effect on the development of monogamy among some mammals.
The research is even less able to speak to monogamy – or the lack of it – in people. The Cambridge team believes that "humans are not socially monogamous", said Lukas, while the PNAS authors classify humans as both monogamous and polygamous, depending on their historical and social circumstances.
"Humans are such unusual animals, depending so excessively on culture, which changes so many of the ground rules of evolution," said Clutton-Brock.
"Human monogamy might have arisen when females were separated and solitary, and a male needed to stick close to one mate to guard her," added Lukas, "but it is also possible that monogamy is a very recent, cultural arrangement of marriage within groups."
Whatever the initial reason and whenever it evolved, monogamy has been a boon for Homo sapiens. The study pointing to protection from infanticide as the prime force in male monogamy "allows us to peer back into our evolutionary past to understand the factors that were important in making us human," said Susanne Shultz of the University of Manchester, a co-author of the PNAS paper. "Once fathers decide to stick around and care for young, mothers can then change their reproductive decisions and have more, brainy offspring."