While there has been a good deal of interest in the evolution of monogamy, we still know little about why mate fidelity has evolved in mammals. A recent review article called "Evolution of mammalian monogamy remains mysterious: Two large studies reach opposing conclusions about why males stay with females" nicely summarizes available data about the evolution of monogamy from two detailed analyses published in prestigious professional journals, and, it turns out, renowned researchers disagree. They also disagree about whether their findings have anything to do with human mating systems, with researchers in one group claiming that humans may not really be monogamous at all.
Monogamy is very common in birds but rare in mammals. Overall, fewer than 10% of mammals exhibit monogamy, but about 25% of primates do. In birds, both parents can help to raise young by incubating eggs or feeding chicks, whereas in mammals males can't directly partake in prenatal care or breastfeed young. Meerkats are among the rare mammalian exceptions (see teaser image here).
In the recent studies using similar methods of analysis in which social monogamy was defined as "males and females living in breeding pairs", "One group of researchers says monogamy evolved in primates to counter the threat of males killing babies to boost their siring success. The other team concludes that mammals, including primates, became monogamous when females live far away from one another." The first conclusion is based on the idea that when males kill other males' babies, infanticide causes a female to become fertile sooner than she would if her baby survived (please click here to see a summary of this study; see also). The second conclusion argues that when females live far apart, males can't easily mate with more than one female (the abstract of this study can be seen here and a discussion of both can also be seen here and here).
Who knows what's really going on?
These sorts of debates are what keep science alive. Experts can and do disagree and that's what fuels future studies. And, as evolutionary anthropologist Eduardo Fernandez-Duque of the University of Pennsylvania notes, “It’s unlikely that one size will fit all.” We need to be very careful about grand theories because different factors may lead to the evolution of social behavior in different species.
Stay tuned for more on this most interesting and important topic.