Stable heterosexual associations in a promiscuous primate

Close associations between adult males and females are rare among group-living, nonmonogamous mammals but may mark the transition from life in promiscuous bisexual groups to groups of pairs or pair living. Although heterosexual friendships have been described in baboons, these are short-term affairs serving as protection against infanticide and thus tightly linked to the presence of vulnerable infants. Long-term association may be adaptive in situations of low male monopolization potential where it pays to invest in a particular female partner instead of spreading the effort among many females. Using long-term data, we investigated male–female and male–infant associations in wild Assamese macaques, Macaca assamensis. Group-wide and individual male–female associations were stable for at least 2 or 3 years. Association during the mating season but not before the mating season predicted male mating success, lending support to the ‘friends with benefits’ but not the ‘mating effort’ hypothesis. Mating success in turn predicted male–female association at birth as well as male–infant association before weaning. In support of the ‘paternal care hypothesis’ paternity was an independent predictor of male–infant association beyond weaning age, creating potential for true paternal care. We thus postulate that particular demographic and life history circumstances may favour male–female friendships by creating a positive feedback between male–female–infant associations driven by paternal care and male–female associations promoted by increased mating access to drive the evolution of long-term male–female bonds.