"Why do males of some species attend to their offspring prolongedly, while others tend to spring off post-coitally?" asks Natalie Angier. The answer may relate to the varying social role of infants. "In 90 percent of mammalian species, promiscuity is common and paternity uncertain; females gestate the young internally and then provision them with breast milk, and males rarely have any evolutionary incentive to play Ward Cleaver. Yet in that remaining 10 percent, the daddy decile, we find most of the world’s primates. 'Lots of primates are suckers for babies,' said Sarah Hrdy, the primatologist and author of 'Mothers and Others' (Harvard University Press, 2009)."