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.

Fossils and molecular data are two independent sources of information that should in principle provide consistent inferences of when evolutionary lineages diverged. Here we use an alternative approach to genetic inference of species split times in recent human and ape evolution that is independent of the fossil record. We first use genetic parentage information on a large number of wild chimpanzees and mountain gorillas to directly infer their average generation times. We then compare these generation time estimates with those of humans and apply recent estimates of the human mutation rate per generation to derive estimates of split times of great apes and humans that are independent of fossil calibration. We date the human-chimpanzee split to at least 7-8 million years and the population split between Neanderthals and modern humans to 400,000-800,000 y ago. This suggests that molecular divergence dates may not be in conflict with the attribution of 6-to 7-million-y-old fossils to the human lineage and 400,000-yold fossils to the Neanderthal lineage.hominin | molecular dating | primate | speciation O ver 40 y ago, Sarich and Wilson used immunological data to propose that humans and African great apes diverged only about 5 million y ago, some three to four times more recently than had been assumed on the basis of the fossil record (1). Although contentious at the time (e.g., ref. 2), this divergence has since been repeatedly estimated from DNA sequence data at 4-6 million years ago (Ma) (3-8). However, this estimate is incompatible with the attribution of fossils older than 6 Ma to the human lineage. Although the assignment of fossils such as the ∼6 Ma Orrorin (9) and the 6-7 Ma Sahelanthropus (10) to the human lineage remains controversial (11), it is also possible that the divergence dates inferred from DNA sequence data are too recent.The total amount of sequence differences observed today between two evolutionary lineages can be expressed as the sum of two values: the sequence differences that accumulated since gene flow ceased between the lineages ("split time") and the sequence differences that correspond to the diversity in the common ancestor of both lineages. The extent of variation in the ancestral species may be estimated from the variance of DNA sequence differences observed across different parts of the genome between the species today, which will be larger the greater the level of variation in the ancestral population. By subtracting this value from the total amount of sequence differences, the sequence differences accumulated since the split can be estimated. The rate at which DNA sequence differences accumulate in the genome ("mutation rate") is needed to then convert DNA sequence differences into split times.In prior research, mutation rates have been calculated using species split times estimated from the fossil record as calibration points. For calculating split times between present-day humans and great apes, calibration points that assume DNA sequence differences between humans and orangutans...

Animals that maintain cooperative relationships show gains in longevity and offspring survival. However, little is known about the cognitive or hormonal mechanisms involved in cooperation. Indeed, there is little support for a main hypothesis that non-human animals have the cognitive capacities required for bookkeeping of cooperative exchanges. We tested an alternative hypothesis that cooperative relationships are facilitated by an endocrinological mechanism involving oxytocin, a hormone required for bonding in parental and sexual relationships across mammals. We measured urinary oxytocin after single bouts of grooming in wild chimpanzees. Oxytocin levels were higher after grooming with bond partners compared with nonbond partners or after no grooming, regardless of genetic relatedness or sexual interest. We ruled out other possible confounds, such as grooming duration, grooming direction or sampling regime issues, indicating that changes in oxytocin levels were mediated by social bond strength. Oxytocin, which is thought to act directly on neural reward and social memory systems, is likely to play a key role in keeping track of social interactions with multiple individuals over time. The evolutionary linkage of an ancestral hormonal system with complex social cognition may be the primary mechanism through which long-term cooperative relationships develop between both kin and non-kin in mammals.