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 new studies 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 is the paramount benefit of monogamy.
Separate findings published in the journal Science says keeping his mate faithful provides 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.
The PNAS paper, which analysed 230 species of primates, concludes that protecting offspring is the greatest benefit of male monogamy. By sticking close to his mate, a male reduces the risk of infanticide. That reasoning has resonance in people, too, where children who grow up without a father are more likely to die in childhood.
"This is the first time the theories for the evolution of monogamy have been systematically tested, showing that infanticide is the driver of monogamy," said anthropologist Christopher Opie of University College London. "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 per cent were socially monogamous.
They conclude that guarding against infanticide played little if any role among such mammals as wolves, gibbons and meerkats. The behaviour arose when females had zero tolerance for other females entering their turf.
That left a male little choice but to foreswear 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.