This project addresses two fundamental questions about primate biology. First, why do adult males and females share enduring bonds? In the majority of mammalian species, the sexes interact affiliatively around the period of female estrus or fertility; after that, males and females typically pursue largely separate lives. In primates, however, permanent association between males and nonfertile (anestrous) females is not only more common than in mammals generally, it is the rule in virtually all monkeys and apes. The evolutionary reasons for the ubiquity and variability of female-male relations in primates remain unknown, however. Second, why is infanticide a reproductive strategy of some males, but not others? Infanticide, the killing of dependent infants (usually by males), is widespread but variably expressed in mammals. These two apparently unrelated questions are united in a new theory that proposes that males and females form social relationships with one another in order to protect young against infanticide. The researchers will test this hypothesis in a study of two closely related savanna baboons that share similarities as well as important differences. The olive baboons (Papio cynocephalus anubis) of Kenya and the chacma baboons (P. cynocephalus ursinus) of Botswana both live in large, multi-male, multi-female groups in which lactating females maintain close bonds with particular males--popularly known as "friendships". Infanticide by males, however, is common in chacma baboons, but rare in olives baboons. By comparatively studying both populations, the researchers will answer the following questions: (1) how do males benefit from friendship behavior: are males the genetic fathers of their friends' infants, or, as argued for olive baboons, do they care for unrelated infants because mothers will then choose them as sires of future offspring?; (2) how do females benefit from friendships with males: do they obtain male anti-infanticide protection in chacma baboons, and some other benefit (such as protection from male harassment directed at females) in the less infanticidal olive baboon?; (3) do bonds between males and females vary in light of the male's paternity of the female's infant and due to the different benefits females may derive from friendships in the two populations? (4) does infanticide increase the reproductive success of male chacma baboons?; (5) why do some (but not all) male chacma baboons become infanticidal when reaching alpha status in the dominance hierarchy, whereas few male olive baboons ever do? Ultimately, the study of a nonhuman primate may improve our understanding of social relationships between the sexes within our own species.