There are two broad theories about what drives monogamy. Some researchers hold that in certain species, females were dispersed so widely that it would have been difficult for males to monopolize an area large enough for them to have multiple partners. Others think that monogamy evolved as males stuck around their mates to protect their offspring, in particular from being killed by rivals. Christopher Opie, an anthropologist at University College London, and his colleagues have now traced potential drivers of monogamy in 230 primate species, back to a 75-million-year-old common ancestor. The researchers compiled information about how each species behaves, such as the range of females’ territories and whether the males care for their young and guard their mates, then they ran computer simulations of the evolutionary process. “We’re effectively re-running history millions of times to see how all these behaviours would have had to have evolved in order for us to get to where we are now,” says Opie. The researchers found that mating relationships co-evolved with several behaviours. “When the mating system changed, the behaviour changed,” says Opie. But of all the behaviours, infanticide by rival males was the only one to consistently precede a shift to monogamous mating, they report today in Proceedings of the National Academy of Sciences1. The fear of infanticide alone can be postulated as a cause of monogamy in primates, Opie says; the other behaviours are consequences.
Yet the waters will be muddied by a report published today in Science2. This study considered the wider origins of monogamy in mammals. Whereas almost one-third of primate species are monogamous, fewer than one-tenth of mammals are. Tim Clutton-Brock and Dieter Lukas, both zoologists at the University of Cambridge, UK, used a previously published detailed evolutionary tree of 2,288 species of mammal3. They found that all but one of the evolutionary transitions to monogamous partnerships arose from scenarios in which females were solitary. Unable to mate with more than one female, males were probably guarding their mates as a way of maximizing their number of offspring, and any increase in paternal care was a “consequence, not a cause”, says Lukas.