This work analyzes male-female relationships outside the context of mating in a troop of wild baboons. Barbara Smuts used long-term friendships between males and females, documented over a two-year period, to show how social interactions between members of friendly pairs differed from those of other troop mates, Her findings, now enhanced by 15 years of field studies, suggest that the evolution of male reproductive strategies in baboons can only be understood by considering the relationship between sex and friendship: female baboons prefer to mate with males who have previously engaged in friendly interaction with them and their offspring. Smuts suggests that female choice may promote male investment in other species, and she explores the relevance of her findings for the evolution of male-female relationships in humans.