Humans regularly engage in prosocial behavior that differs strikingly from that of even our closest living relatives, the chimpanzees (Pan troglodytes). In laboratory settings, chimpanzees are indifferent when given the opportunity to deliver valued rewards to conspecifics, while even very young human children have repeatedly been shown to behave prosocially. Although this broadly suggests that prosocial behavior in chimpanzees differs from that of young human children, the methods used in prior work with children have also differed from the methods used in studies of chimpanzees in potentially crucial ways. Here we test 92 pairs of 3-8-year-old children from urban American (Los Angeles, CA, USA) schools in a face-to-face task that closely parallels tasks used previously with chimpanzees. We found that children were more prosocial than chimpanzees have previously been in similar tasks, and our results suggest that this was driven more by a desire to provide benefits to others than a preference for egalitarian outcomes. We did not find consistent evidence that older children were more prosocial than younger children, implying that younger children behaved more prosocially in the current study than in previous studies in which participants were fully anonymous. These findings strongly suggest that humans are more prosocial than chimpanzees from an early age and that anonymity influences children's prosocial behavior, particularly at the youngest ages.

Cooperation between nonrelatives is common in humans. Reciprocal altruism is a plausible evolutionary mechanism for cooperation within unrelated pairs, as selection may favor individuals who selectively cooperate with those who have cooperated with them in the past. Reciprocity is often observed in humans, but there is only limited evidence of reciprocal altruism in other primate species, raising questions about the origens of human reciprocity. Here, we explore how reciprocity develops in a sample of American children ranging from 3 to 7.5 years of age, and also compare children's behavior to that of chimpanzees in prior studies to gain insight into the phylogeny of human reciprocity. Children show a marked tendency to respond contingently to both prosocial and selfish acts, patterns that have not been seen among chimpanzees in prior studies. Our results show that reciprocity increases markedly with age in this population of children, and by about 5.5 years of age children consistently match the previous behavior of their partners.

We conducted experiments on two populations of chimpanzees, Pan troglodytes, to determine whether they would take advantage of opportunities to provide food rewards to familiar group members at little cost to themselves. In both of the experiments described here, chimpanzees were able to deliver identical rewards to themselves and to other members of their social groups. We compared the chimpanzees' behaviour when they were paired with another chimpanzee and when they were alone. If chimpanzees are motivated to provide benefits to others, they are expected to consistently deliver rewards to others and to distinguish between the partner-present and partner-absent conditions. Results from both experiments indicate that our subjects were largely indifferent to the benefits they could provide to others. They were less likely to provide rewards to potential recipients as the experiment progressed, and all but one of the 18 subjects were as likely to deliver rewards to an empty enclosure as to an enclosure housing another chimpanzee. These results, in conjunction with similar results obtained in previous experiments, suggest that chimpanzees are not motivated by prosocial sentiments to provide food rewards to other group members.

In a wide range of taxa, including baboons, close social bonds seem to help animals cope with stress and enhance long-term reproductive success and longevity. Current evidence suggests that female baboons may benefit from establishing and maintaining highly individuated relationships with a relatively small number of partners. Here, we extend previous work on the stability of female baboons' social relationships in three different ways. First, we assess the stability of females' social relationships in two distinct and geographically distant sites using the same method. Second, we conduct simulations to determine whether females' social relationships were more stable than expected by chance. Third, we examine demographic sources of variance in the stability of close social bonds. At both sites, females' relationships with their most preferred partners were significantly more stable than expected by chance. In contrast, their relationships with less preferred partners were more ephemeral, often changing from year to year. While nearly all females experienced some change in their top partners across time, many maintained relationships with top partners for several years. Females that lived in smaller groups and had more close kin available had more stable social relationships than those that lived in larger groups and had fewer close kin available. Keywords baboon; bond stability; female bond; Papio hamadryas ursinus; reproductive success; stress A growing body of evidence suggests that close social bonds with same-sex partners help animals cope with stress and enhance long-term reproductive success and longevity. Female rats, Rattus norvegicus, that have well-balanced affiliative relationships exhibit lower glucocorticoid levels than those with less symmetric relationships (Yee et al. 2008). In chacma baboons, Papio hamadryas ursinus, or P. ursinus, grooming relationships enhance females' ability to cope with various sources of stress, such as the immigration of potentially infanticidal males or instability of the dominance hierarchy (Engh et al. 2006a; Crockford et

Because female reproductive success in social mammals is determined largely by parental rather than mating effort, intra-sexual competition among females has typically been assumed to occur primarily over food. Recently, however, renewed attention has been paid to the importance of other sources of variation in female fitness, with a concomitant focus on a broader definition of intra-sexual selection that encompasses both competition for resources and competition for mates and social partners. We present behavioural and demographic data gathered over 15 years on a group of wild chacma baboons (Papio hamadryas ursinus) which show that females incur several costs when living in groups with many females and comparatively few males: increased intrasexual aggression, less stable bonds with female partners, and increased mortality. Female-female aggression was higher in years when the adult sex ratio was more highly skewed toward females, with low-ranking female exhibiting particularly high rates of aggression toward other females. Females' social bonds with other females also became less stable in years when the group contained many females. Finally, female mortality rates were highest in years when the group contained comparatively more females and fewer males. The negative correlation between the number of males and female mortality suggests that the increase in mortality was due to increased predation rather than food competition. Results indicate that intra-sexual competition for social partners, mates, paternal investment, and perhaps also male protection occurs even in species where females exhibit low reproductive skew.

Here we examine the effects of maternal kinship, reciprocity, and dominance rank on the social relationships of female baboons (Papio cynocephalus ursinus) in a well-habituated, free-ranging group in the Okavango Delta of Botswana. These data are useful for testing comparative hypotheses about the ecological and demographic factors that shape the evolution of social organization in primates and other animals. In this group, adult females had well-differentiated grooming relationships with one another, and limited their grooming to a relatively small subset of available partners. Although there were 19 adult females in the group, the average female groomed only 8 other females, and devoted at least 5% of her grooming to only four other females. Females groomed maternal kin at significantly higher rates and for significantly longer periods than they groomed other females. The bias in favor of maternal kin was not an artifact of a general attraction toward females of adjacent rank. How...

Darwin was struck by the many similarities between humans and other primates and believed that these similarities were the product of common ancestry. He would be even more impressed by the similarities if he had known what we have learned about primates over the last 50 years. Genetic kinship has emerged as the primary organizing force in the evolution of primate social organization and the patterning of social behaviour in non-human primate groups. There are pronounced nepotistic biases across the primate order, from tiny grey mouse lemurs ( Microcebus murinus ) that forage alone at night but cluster with relatives to sleep during the day, to cooperatively breeding marmosets that rely on closely related helpers to rear their young, rhesus macaque ( Macaca mulatta ) females who acquire their mother's rank and form strict matrilineal dominance hierarchies, male howler monkeys that help their sons maintain access to groups of females and male chimpanzees ( Pan troglodytes ) that ...

Humans are an exceptionally cooperative species, but there is substantial variation in the extent of cooperation across societies. Understanding the sources of this variability may provide insights about the forces that sustain cooperation. We examined the ontogeny of prosocial behavior by studying 326 children 3–14 y of age and 120 adults from six societies (age distributions varied across societies). These six societies span a wide range of extant human variation in culture, geography, and subsistence strategies, including foragers, herders, horticulturalists, and urban dwellers across the Americas, Oceania, and Africa. When delivering benefits to others was personally costly, rates of prosocial behavior dropped across all six societies as children approached middle childhood and then rates of prosociality diverged as children tracked toward the behavior of adults in their own societies. When prosocial acts did not require personal sacrifice, prosocial responses increased steadily...

There is much interest in the evolutionary forces that favored the evolution of large brains in the primate order. The social brain hypothesis posits that selection has favored larger brains and more complex cognitive capacities as a means to cope with the challenges of social life. The hypothesis is supported by evidence that shows that group size is linked to various measures of brain size. But it has not been clear how cognitive complexity confers fitness advantages on individuals. Research in the field and laboratory shows that sophisticated social cognition underlies social behavior in primate groups. Moreover, a growing body of evidence suggests that the quality of social relationships has measurable fitness consequences for individuals.

Although vocal production in non-human primates is highly constrained, individuals appear to have some control over whether to call or remain silent. We investigated how contextual factors affect the production of grunts given by wild female chacma baboons, Papio ursinus, during social interactions. Females grunted as they approached other adult females 28% of the time. Supporting previous research, females were much more likely to grunt to mothers with young infants than to females without infants. Grunts also significantly increased the likelihood of affiliative interactions among all partners. Notably, however, grunts did not simply mirror existing social bonds. Instead, they appeared to perform a very different function: namely, to serve as signals of benign intent between partners whose relationship is not necessarily close or predictable. Females were less likely to grunt to their mothers or adult daughters-the individuals with whom they shared the closest and least aggressive bonds-than to other females. In contrast, patterns of grunting between sisters were similar to those between nonkin, perhaps reflecting sisters' more ambivalent relationships. Females grunted at higher rates to lower-ranking, than to higher-ranking, females, supporting the hypothesis that grunts do not simply signal the signaler's level of arousal or anxiety about receiving aggression, but instead function as signals of benign intent. Taken together, results suggest that the grunts given by female baboons serve to reduce uncertainty about the likely outcome of an interaction between partners whose relationship is not predictably affiliative. Despite their limited vocal repertoire, baboons appear to be skilled at modifying call production in different social contexts and for different audiences.

Vocal signals often play an important role in synchronizing the activities of group members, coordinating decisions about when and where to travel, and facilitating social interactions in which there are potential conflicts of interest. Here, we show that when female olive baboons (Papio anubis) give low amplitude grunts after approaching other females, they are less likely to behave aggressively toward their partners and more likely to handle their partners' infants and interact affiliatively with them. In addition, females are more likely to grunt after they approach lower ranking females than after they approach higher ranking females and are less likely to grunt after they approach their own mothers and daughters than after they approach other females. These patterns, which are strikingly similar to patterns previously reported in chacma baboons (Papio ursinus) support the hypothesis that grunts function as signals of benign intent. Moreover, they suggest that actors' decisions about whether to grunt or remain silent are influenced by the social context, particularly their partners' likely response to their approach. Taken together, the patterning of grunts in olive and chacma baboon suggests that these vocalizations play an important in reducing uncertainty about actors' intentions and facilitate nonaggressive social interactions.