Anthropologists have found that polyandry—the union of one woman and more than one man—is a rarity in humans. Across thousands of studied societies, just a few dozen polyandrous cultures exist, widely scattered around the world. For the most part, the guess is that cultural factors are at work. Among rodents, however, the practice is both widespread and well understood: it cuts down on infanticide. Males who have not sired with a given female will kill her newborns to prevent the spread of his rival’s genes, and to free her from the burden of raising another’s young in favour of his own.
In a classic sexual arms-race case, the practice of polyandry won out. Males cannot distinguish their own young from a rival’s, so a female that gives birth to young from more than one male will have protected them all from any individual father’s aggression, lest he threaten his own offspring.
Natural selection, then, should have weeded out monogamy in rodents. But in the house mouse, Mus musculus, that has not happened; females can choose one or many mates. In a study published in Behavioral Ecology Yannick Auclair, of the University of Zurich, and colleagues, may have figured out why.
Mr Auclair set up and then meticulously followed the progress of a mouse colony over three years. He kept count of the litters laid, and whether they were raised in solitary nests (those that consisted of only one female and her family) or communal nests (in which females shared maternal duties, irrespective of who sired their offspring). For those pups that survived into adolescence, he took tissue samples to determine paternity by genetic analysis.
The final tally noted 146 survivors and 254 deaths among the pups. Scratch marks on the bodies suggested that almost all deaths were due to infanticide. What was more interesting, however, were the survival rates between different kinds of litters. Polyandrous litters survived well in both solitary and communal nests, but monandrous ones survived significantly more in communal nests than in solitary ones. The reason, the team concludes, is “socially mediated polyandry”. A nest full of pups from many mothers and fathers was as safe as a nest with one mother and many fathers’ pups. Females derived the protective benefits of polyandry without actually having to expend the effort to carry it out.
The authors suggest that socially mediated polyandry might apply to many more species that engage in communal care of the young, including a small percentage of mammals such as rodents. That makes it a rich seam for investigation by evolutionary biologists. The explanation for a smattering of polyandry in humans, however, remains a matter of guesswork for anthropologists.