The evolution of social monogamy has intrigued biologists for over a century. Here, we show
that the ancestral condition for all mammalian groups is of solitary individuals and that social
monogamy is derived almost exclusively from this social system. The evolution of social monogamy
does not appear to have been associated with a high risk of male infanticide, and paternal care
is a consequence rather than a cause of social monogamy. Social monogamy has evolved in
nonhuman mammals where breeding females are intolerant of each other and female density
is low, suggesting that it represents a mating strategy that has developed where males are
unable to defend access to multiple females.