Longevity is a major component of variation in fitness in long-lived iteroparous species [1-4]. Among female baboons, variation in breeding lifespan accounts for approximately 50% of the variation in lifetime fitness [5, 6]. However, we know little about the causes of variation in longevity in primates or other long-lived mammals. Savannah baboons form strong, equitable, and enduring relationships with specific female partners, particularly with close relatives and agemates [7-10]. The quality of females' social relationships influences their ability to cope with stressful events [11-13] and is associated with variation in female reproductive success [9, 14]. Here we show that dominance rank and the quality of close social bonds have independent effects on the longevity of female chacma baboons (Papio hamadryas ursinus). High-ranking females live longer than lower-ranking females. In addition, females who form stronger and more stable social bonds with other females live significantly longer than females who form weaker and less stable relationships. These data extend our understanding of the adaptive value of social bonds in baboons and complement a growing body of evidence that indicates that social bonds have adaptive value in a range of taxa, from mice to humans [9, 14-19].

Sociality has evolved in many animal taxa, but primates are unusual because they establish highly differentiated bonds with other group members. Such bonds are particularly pronounced among females in species like baboons, with female philopatry and male dispersal. These relationships seem to confer a number of short-term benefits on females, and sociality enhances infant survival in some populations. However, the long-term consequences of social bonds among adult females have not been well established. Here we provide the first direct evidence that social relationships among female baboons convey fitness benefits. In a group of free-ranging baboons, Papio cynocephalus ursinus, the offspring of females who formed strong social bonds with other females lived significantly longer than the offspring of females who formed weaker social bonds. These survival benefits were independent of maternal dominance rank and number of kin and extended into offspring adulthood. In particular, females who formed stronger bonds with their mothers and adult daughters experienced higher offspring survival rates than females who formed weaker bonds. For females lacking mothers or adult daughters, offspring survival was closely linked to bonds between maternal sisters. These results parallel those from human studies, which show that greater social integration is generally associated with reduced mortality and better physical and mental health, particularly for women.

Analyses of the pattern of associations, social interactions, coalitions, and aggression among chacma baboons (Papio hamadryas ursinus) in the Okavango Delta of Botswana over a 16-year period indicate that adult females form close, equitable, supportive, and enduring social relationships. They show strong and stable preferences for close kin, particularly their own mothers and daughters. Females also form strong attachments to unrelated females who are close to their own age and who are likely to be paternal half-sisters. Although absolute rates of aggression among kin are as high as rates of aggression among nonkin, females are more tolerant of close relatives than they are of others with whom they have comparable amounts of contact. These findings complement previous work which indicates that the strength of social bonds enhances the fitness of females in this population and support findings about the structure and function of social bonds in other primate groups.

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The apparent rarity of contingent cooperation in animals has convinced many investigators that such reciprocity is unimportant, stimulating consideration of alternative explanations for cooperation, such as by-product mutualism and biological markets motivated by the likelihood of immediate reward. Nevertheless, there is also limited evidence that animals do sometimes rely on memory of recent interactions when behaving altruistically toward others. Here we describe a playback experiment conducted on wild female baboons, suggesting that contingent cooperation may occur among unrelated individuals, even when there is a temporal delay between the two cooperative acts. Hearing the recruitment call of an unrelated recent grooming partner caused subjects to move in the direction of the loudspeaker and approach their former partner, particularly when this partner had an infant. When the subject and her partner were close kin no such effect was observed. Subjects' responses were not influenced by any type of recent interaction, because prior grooming and prior aggression influenced their behavior in qualitatively different ways. Similarly, their responses were not prompted only by the motivation to resume friendly interactions, because prior grooming alone did not elicit approach. Instead, subjects were most likely to approach their former grooming partner when they had also heard her recruitment call. Results suggest that at least some forms of cooperation in animals may be based on memory of specific recent interactions. D uring the last decade there has been increasing skepticism about the relevance of contingent cooperation in the social behavior of animals. Because most cooperative behavior occurs among long-term partners (often kin) for whom any single altruistic act may be relatively insignificant, many investigators are now convinced that the sort of contingent reciprocity first proposed by Trivers (1) is both rare and fragile in nature (e.g., refs. 2-3). Although there is limited experimental (e.g., refs. 4-8) and correlational (e.g., refs. 9-11) evidence that unrelated animals may sometimes rely on memory of recent interactions when behaving altruistically toward others, interpretation has been complicated by a paucity of convincing examples, the absence of important controls in some early tests (e.g., ref. 4), and several experimental studies of captive apes, suggesting a general insensitivity to contingency in cooperative tasks (e.g., refs. 12-13). These concerns have stimulated consideration of alternative explanations for cooperation among unrelated partners, such as by-product mutualism and biological markets motivated by the likelihood of immediate reward (e.g., refs. 14-16). The change in focus has also been prompted by doubts that animals possess the requisite cognitive abilities to sustain contingent cooperation. These include the ability to delay reward, to quantify past cooperative acts, to plan and anticipate future outcomes, and to detect and punish cheaters (e.g., refs. 17-20).Although these obje...