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Human cooperation varies both across and within societies, and developmental studies can inform our understanding of the sources of both kinds of variation. One key candidate for explaining within-society variation in cooperative behaviour is gender, but we know little about whether gender differences in cooperation take root early in ontogeny or emerge similarly across diverse societies. Here, we explore two existing cross-cultural datasets of 4- to 15-year-old children's preferences for equality in experimental tasks measuring prosociality (14 societies) and fairness (seven societies), and we look for evidence of (i) widespread gender differences in the development of cooperation, and (ii) substantial societal variation in gender differences. This cross-cultural approach is crucial for revealing universal human gender differences in the development of cooperation, and it helps answer recent calls for greater cultural diversity in the study of human development. We find that gender has little impact on the development of prosociality and fairness within these datasets, and we do not find much evidence for substantial societal variation in gender differences. We discuss the implications of these findings for our knowledge about the nature and origin of gender differences in cooperation, and for future research attempting to study human development using diverse cultural samples. This article is part of the theme issue ‘Cooperation among women: evolutionary and cross-cultural perspectives’.

For mammalian females, which bear the energetic costs of gestation and lactation, competition over food resources is generally expected to have a more important impact on reproductive success than competition over mates. However, there are some situations in which mammalian females compete over access to mates, limited supplies of sperm or caretaking for their offspring. Competition over both access to food and access to mates have been documented in female baboons (Papio spp.). Here, we examined the relative importance of competition over food and mates for wild female olive baboons, Papio anubis, in shaping the patterns of aggression among females. Lactating and pregnant females were more aggressive than females in other reproductive states, and sexually receptive females initiated and received relatively little aggression. There was no evidence that females competed over caretakers for their offspring or reduced future competition through reproductive suppression. Our data suggest that competition over food plays a more important role than competition over mates for female olive baboons. The body of current evidence suggests that evolution has finely tuned female baboons’ responses to the competitive pressures that they face in their local environments.

Is it possible to slow the rate of ageing, or do biological constraints limit its plasticity? We test the ‘invariant rate of ageing’ hypothesis, which posits that the rate of ageing is relatively fixed within species, with a collection of 39 human and nonhuman primate datasets across seven genera. We first recapitulate, in nonhuman primates, the highly regular relationship between life expectancy and lifespan equality seen in humans. We next demonstrate that variation in the rate of ageing within genera is orders of magnitude smaller than variation in pre-adult and age-independent mortality. Finally, we demonstrate that changes in the rate of ageing, but not other mortality parameters, produce striking, species-atypical changes in mortality patterns. Our results support the invariant rate of ageing hypothesis, implying biological constraints on how much the human rate of ageing can be slowed., The ‘invariant rate of ageing’ hypothesis suggests that the rate of ageing tends to be constant within species. Here, Colchero et al. find support for the hypothesis across primates, including humans, suggesting biological constraints on the rate of ageing.

Social bonds enhance fitness in many group-living animals, generating interest in the processes that create individual variation in sociality. Previous work on female baboons shows that early life adversity and temperament both influence social connectedness in adulthood. Early life adversity might shape sociality by reducing ability to invest in social relationships or through effects on attractiveness as a social partner. We examine how females’ early life adversity predicts sociality and temperament in wild olive baboons, and evaluate whether temperament mediates the relationship between early life adversity and sociality. We use behavioural data on 31 females to quantify sociality. We measure interaction style as the tendency to produce grunts (signals of benign intent) in contexts in which the vocalization does not produce immediate benefits to the actor. Early life adversity was negatively correlated with overall sociality, but was a stronger predictor of social behaviours received than behaviours initiated. Females who experienced less early life adversity had more benign interaction styles and benign interaction styles were associated with receiving more social behaviours. Interaction style may partially mediate the association between early life adversity and sociality. These analyses add to our growing understanding of the processes connecting early life experiences to adult sociality.

Nonhuman primates don’t have formal kinship systems, but genetic relatedness shapes patterns of residence, behavior, mating preferences, and cognition in the primate order. The goal of this article is to provide insight about the ancestral foundations on which the first human kinship systems were built. In order for evolution to favor nepotistic biases in behavior, individuals need to have opportunities to interact with their relatives and to be able to identify them. Both these requirements impose constraints on the evolution of kinship bonds in primate species. For nonhuman primates and many other mammals, maternal care generates close ties between mothers and offspring and among littermates. For species in which mother–offspring bonds extend beyond weaning, associations with the mother also generate familiarity among siblings of different ages. For species with female philopatry, mother–offspring ties provide the foundation for lifelong relationships between mothers and daughters, and dense matrilineal kin networks. However, several factors constrain the development of kinship bonds in nonhuman primates. First, dispersal strategies, which are designed to reduce inbreeding, bias kinship networks in favor of the philopatric sex. Second, hostile relationships between groups mean that kin ties are usually severed when individuals move from one group to another. Third, in species with polygynandrous mating systems, uncertainty about paternity and restricted periods of male reproductive monopoly limit the scope for development of ties among fathers and offspring, paternal siblings, or extended paternal kin networks. In this article, I explore the impact of these factors on the evolution of ancestral human kinship systems.

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships. Publisher PDF

Is it possible to slow the rate of aging, or do biological constraints limit its plasticity? We test this ‘invariant rate of aging’ hypothesis with an unprecedented collection of 39 human and nonhuman primate datasets across seven genera. We first recapitulate, in nonhuman primates, the highly regular relationship between life expectancy and lifespan equality seen in humans. We next demonstrate that variation in the rate of aging within genera is orders of magnitude smaller than variation in pre-adult and age-independent mortality. Finally, we demonstrate that changes in the rate of aging, but not other mortality parameters, produce striking, species-atypical changes in mortality patterns. Our results support the invariant rate of aging hypothesis, implying biological constraints on how much the human rate of aging can be slowed.

Adverse experiences during early life exert important effects on development, health, reproduction, and social bonds, with consequences often persisting across generations. A mother’s early life experiences can impact her offspring’s development through a number of pathways, such as maternal care, physiological signaling through glucocorticoids, or even intergenerational effects like epigenetic inheritance. Early life adversity in female yellow baboons (Papio cynocephalus) predicts elevated glucocorticoids, reduced sociality, shortened lifespan, and higher offspring mortality. If baboon mothers with more early life adversity, experience poorer condition and struggle to provide for their offspring, this could contribute to the persisting transgenerational effects of adversity. Here, we examined the effects of mothers’ early life adversity on their maternal effort, physiology, and offspring survivability in a population of olive baboons, Papio anubis. Mothers who experienced more adversity in their own early development exerted greater maternal effort (i.e., spent more time nursing and carrying) and had higher levels of glucocorticoid metabolites than mothers with less early life adversity. Offspring of mothers with more early life adversity had reduced survivability compared to offspring of mothers with less early life adversity. There was no evidence that high maternal social rank buffered the effects of early life adversity. Our data suggest early life experiences can have lasting consequences on maternal effort and physiology, which may function as proximate mechanisms for intergenerational effects of maternal experience. Animals exposed to early life adversity experience both immediate and lasting consequences. If early life adversity exerts developmental constraints that affect a mother’s ability to provide for her offspring, this could explain the transgenerational effects of early life adversity. In our study of wild olive baboons, we examined how a mother’s own early life adversity predicts her maternal effort (i.e., nursing and carrying time), maternal fecal glucocorticoid levels, and offspring outcomes. We found that female baboons who experienced more early life adversity had higher glucocorticoid levels during pregnancy and lactation, exerted more maternal effort, and produced offspring with higher mortality risk than females with less early life adversity. Our results suggest that female baboons with more early life adversity experience developmental constraints and struggle to invest in offspring, which likely contributes to persisting effects of early life adversity across generations.

Humans prioritize close, positive relationships during aging, and socioemotional selectivity theory proposes that this shift causally depends on capacities for thinking about personal future time horizons. To examine this theory, we test for key elements of human social aging in longitudinal data on wild chimpanzees. Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but also socialize more with important social partners. Finally, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over the lifespan. Our findings indicate that social selectivity can emerge in the absence of complex future-oriented cognition, and provide an evolutionary context for patterns of social aging in humans.

Human cooperation is probably supported by our tendency to punish selfishness in others. Social norms play an important role in motivating third-party punishment (TPP), and also in explaining societal differences in prosocial behaviour. However, there has been little work directly linking social norms to the development of TPP across societies. In this study, we explored the impact of normative information on the development of TPP in 603 children aged 4–14, across six diverse societies. Children began to perform TPP during middle childhood, and the developmental trajectories of this behaviour were similar across societies. We also found that social norms began to influence the likelihood of performing TPP during middle childhood in some of these societies. Norms specifying the punishment of selfishness were generally more influential than norms specifying the punishment of prosocial behaviour. These findings support the view that TPP of selfishness is important in all societies, and its development is shaped by a shared psychology for responding to normative information. Yet, the results also highlight the important role that children's prior knowledge of local norms may play in explaining societal variation in the development of both TPP and prosociality.

Baboons, members of the genus Papio, comprise six closely related species distributed throughout sub-Saharan Africa and southwest Arabia. The species exhibit more ecological flexibility and a wider range of social systems than many other primates. This article summarizes our current knowledge of the natural history of baboons and highlights directions for future research. We suggest that baboons can serve as a valuable model for complex evolutionary processes, such as speciation and hybridization. The evolution of baboons has been heavily shaped by climatic changes and population expansion and fragmentation in the African savanna environment, similar to the processes that acted during human evolution. With accumulating long-term data, and new data from previously understudied species, baboons are ideally suited for investigating the links between sociality, health, longevity and reproductive success. To achieve these aims, we propose a closer integration of studies at the proximate level, including functional genomics, with behavioral and ecological studies.

There is growing evidence that social bonds have adaptive consequences for individuals in many mammalian species, including savannah baboons. While the majority of studies have shown that the strength of social bonds and the extent of social integration enhance female fitness, several other investigations have suggested that the number of social bonds may sometimes may be more important than the strength or quality of females' social relationships. Here, we examine the impact of the number of strong and weak social bonds on female fertility and longevity in a population of chacma baboons,Papio ursinus. We find no evidence that the number of social bonds consistently affects fertility or infant survival to 1 year, but offspring of females with more weak social bonds lived longer than offspring of other females. After discussing several methodological issues that may influence the analyses of the effects of the number of social bonds, we re-examine the relationship between the number of weak and strong social bonds and reproductive performance using procedures that avoid these problems. Again, we find no evidence that the number of weak or strong social bonds affects females' fertility, survival to 1 year. The effects of the number of weak social bonds on infant longevity disappear, and appear to be an artefact of the relationship between the number of weak social bonds and the number of females in the group.

According to socioecological theory, ecological pressures acting on females play an important role in shaping social organization, dispersal patterns and the nature of female–female relationships. However, there has been relatively little consideration of how females' relationships with males may influence the nature of females' relationships to other females. Here, we show that female olive baboons form well-differentiated relationships with adult males and these ties influence their relationships with other females. As in other species of savanna baboons, female olive baboons show pronounced preferences for close kin, but related females do not often share the same top-ranked male partners. Shared associations with males bring females into more frequent contact with distant kin and nonkin. The combined effects of bringing distantly related and unrelated females together and separating close kin reduced the extent of nepotistic biases among females in the study groups compared to female yellow baboons, Papio cynocephalus, and chacma baboons, Papio ursinus. These data suggest that selective pressures acting on both females and males may influence the nature and function of female–female relationships.

Summary In polygynous and polygynandrous species, there is often intense male-male competition over access to females, high male reproductive skew, and more male investment in mating effort than parenting effort [ 1 ]. However, the benefits derived from mating effort and parenting effort may change over the course of males’ lives. In many mammalian species, there is a ∩-shaped relationship between age, condition, and resource holding power as middle-aged males that are in prime physical condition outcompete older males [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ] and sire more infants [ 9 , 10 , 11 , 12 ]. Thus, males might derive more benefits from parenting effort than mating effort as they age and their competitive abilities decline [ 13 ]. Alternatively, older males may invest more effort in making themselves attractive to females as mates [ 14 ]. One way that older males might do so is by developing relationships with females and providing care for their offspring [ 14 , 15 ]. Savannah baboons provide an excellent opportunity to test these hypotheses. They form stable multi-male, multi-female groups, and males compete for high ranking positions. In yellow and chacma baboons (Papio cynocephalus and P. ursinus), there is a ∩-shaped relationship between male age and dominance rank [ 12 ], and high rank enhances paternity success [ 12 , 16 ]. Lactating female baboons form close ties (“primary associations” hereafter) with particular males [ 15 , 16 , 17 , 18 , 19 , 20 ], who support them and their infants in conflicts [ 15 , 19 ] and buffer their infants from rough handling [ 20 ]. Females’ primary associates are often, but not always, the sires of their current infants [ 16 , 20 , 21 , 22 ].

Darwin viewed cooperation as a perplexing challenge to his theory of natural selection. Natural selection generally favors the evolution of behaviors that enhance the fitness of individuals. Cooperative behavior, which increases the fitness of a recipient at the expense of the donor, contradicts this logic. William D. Hamilton helped to solve the puzzle when he showed that cooperation can evolve if cooperators direct benefits selectively to other cooperators (i.e. assortment). Kinship, group selection and the previous behavior of social partners all provide mechanisms for assortment (Figure 1), and kin selection and reciprocal altruism are the foundation of the kinds of cooperative behavior observed in many animals. Humans also bias cooperation in favor of kin and reciprocating partners, but the scope, scale, and variability of human cooperation greatly exceed that of other animals. Here, we introduce derived features of human cooperation in the context in which they originally evolved, and discuss the processes that may have shaped the evolution of our remarkable capacity for cooperation. We argue that culturally-evolved norms that specify how people should behave provide an evolutionarily novel mechanism for assortment, and play an important role in sustaining derived properties of cooperation in human groups.

In most species, individual attributes such as size, strength, and condition influence individuals’ ability to monopolize resources [resource-holding power (RHP)] and establish high-ranking positions in dominance hierarchies (1). But there are some taxa in which dominance rank becomes decoupled from individual differences in RHP. For example, in the eusocial wasp Liostenogaster flavolineata , females queue for the breeding position, with older females having priority over younger ones (2). In large provisioned groups of rhesus macaques ( Macaca mulatta ), male rank is based on the length of residence in the group (3). In some cercopithecine primates (i.e., macaques, baboons, and vervets), females inherit their mothers’ ranks and form corporate matrilineal dominance hierarchies in which members of matrilines hold adjacent ranks (4). Attribute-based hierarchies tend to fluctuate as individuals’ physical abilities change over time and as high-ranking individuals are challenged and replaced by more-powerful rivals (1). In contrast, the conventional matrilineal dominance hierarchies of baboons, macaques, and vervets are remarkably stable over long periods of time (4). In PNAS, Strauss and Holekamp (5) explore the dynamics and long-term consequences of matrilineal dominance hierarchies among female spotted hyenas ( Crocuta crocuta ). Spotted hyenas live in large multimale, multifemale groups, or clans (6). Females normally remain in their clans throughout their lives, while males disperse when they reach sexual maturity. In contrast to most other social vertebrates, females outrank males. Clans are composed of multiple matrilines, which forage separately but collectively defend a shared territory. Spotted hyenas feed primarily on large and medium-sized prey that they kill themselves but may scavenge when opportunities arise (7). There has been striking convergence in the patterns and mechanisms of maternal rank inheritance in monkeys and spotted hyenas. In both cases, two inheritance “rules” typically … [↵][1]1Email: joansilk{at}gmail.com. [1]: #xref-corresp-1-1

In polygynous species, there is often intense intra-sexual competition over access to females and high reproductive skew, and males invest more in mating effort than parenting effort. However, the benefits of investments in mating effort and parenting effort may change systematically over the course of males’ lives. In many polygynous mammalian species, there is an ∩-shaped relationship between age, condition, and resource holding power as middle-aged males that are in prime physical condition outcompete older and younger males. As males age, and their competitive abilities decline, males might benefit from reallocating investment from mating effort to parenting effort. We examined the effects of male age and paternity on the likelihood that male olive baboons, Papio anubis, would invest in parenting effort. Male baboons compete for high ranking positions in the group, and there is an ∩-shaped relationship between male age and dominance rank. High ranking males sire more infants than lower ranking males. Males and lactating females often form close ties (primary associates), and females’ primary associates are likely to be the sires of their infants. Males are attentive to their primary associates’ distress, support them in conflicts, and buffer their infants from rough handling and harassment. Here, we show that males are more likely to become the primary associates of lactating females as they age and their competitive abilities decline. These results suggest that fitness benefits derived from male parental investment may favor an extended post-reproductive lifespan in species in which males can identify their own offspring and positively affect their welfare.

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.

Across the animal kingdom, long-term social relationships outside the context of reproductive pair bonds are rare. However, they have been demonstrated in some mammals including primates, cetaceans, and social carnivores. The ontogeny of such relationships is likely to depend on the benefits individuals can gain by cultivating them. Previous studies demonstrated that young mountain gorillas (Gorilla beringei beringei) have strong relationships with adult males, but little is known about the longevity of these bonds. Here, we examine the temporal stability of proximity relationships between coresident adult male and maturing gorillas in the habituated population monitored by the Dian Fossey Gorilla Fund International's Karisoke Research Center in Volcanoes National Park, Rwanda. We used spatial proximity data to assess the strength of relationships between adult males and juveniles, and tracked these relationships as the juveniles matured into subadults (3–4 yr later; n = 229 dyads) and then young adults (7 yr later; n = 42 dyads). The proximity relationships of juveniles of both sexes predicted their proximity relationships with adult males in both subadulthood and young adulthood. However some young adult males who had lost their top adult male proximity partner from juvenilehood developed new relationships with older males that had risen in the dominance hierarchy. These data suggest that (1) kin selection may play a more important role in social relationships between potential fathers and adult female offspring than previously suspected, and (2) when maturing males' foremost adult male social partners remain available to them, their relationships can be stable past the age at which younger males become reproductive competitors.

Recent studies have proposed that social norms play a key role in motivating human cooperation and in explaining the unique scale and cultural diversity of our prosociality. However, there have been few studies that directly link social norms to the form, development and variation in prosocial behaviour across societies. In a cross-cultural study of eight diverse societies, we provide evidence that (1) the prosocial behaviour of adults is predicted by what other members of their society judge to be the correct social norm, (2) the responsiveness of children to novel social norms develops similarly across societies and (3) societally variable prosocial behaviour develops concurrently with the responsiveness of children to norms in middle childhood. These data support the view that the development of prosocial behaviour is shaped by a psychology for responding to normative information, which itself develops universally across societies.

Long-term male-female bonds and bi-parental investment in offspring are hallmarks of human society. A key question is how these traits evolved from the polygynandrously mating multimale multifemale society that likely characterized the Pan-Homo ancestor. In all three species of savanna baboons, lactating females form strong ties (sometimes called "friendships") with one or more adult males. For yellow baboons (Papio cynocephalus) and chacma baboons (Papio ursinus), several lines of evidence suggest that these relationships are a form of male parenting effort. In olive baboons (Papio anubis), females are thought to preferentially mate with their "friends", and male-female bonds may thus function as a form of mating effort. Here, we draw on behavioral and genetic data to evaluate the factors that shape male-female relationships in a well-studied population of olive baboons. We find support for the parenting effort hypothesis in that sires have stronger bonds with their infants' mothers than do other males. These bonds sometimes persist past weaning age and, in many cases, the sire of the previous infant is still a close partner of the female when she nurses her subsequent offspring. We find that males who have the strongest bonds with females that have resumed cycling, but are not currently sexually receptive, are more likely to sire the female's next offspring but the estimate is associated with large statistical uncertainty. We also find that in over one third of the cases, a female's successive infants were sired by the same male. Thus, in olive baboons, the development of stable breeding bonds and paternal investment seem to be grounded in the formation of close ties between males and anestrous females. However, other factors such as male dominance rank also influence paternity success and may preclude stability of these bonds to the extent found in human societies.

Kin discrimination mechanisms are expected to evolve when they provide fitness benefits. To date, evidence for kin discrimination is mixed across taxa and mating systems even when it would apparently be beneficial. In animals with promiscuous mating systems, males were long believed to abstain from parenting behaviours partly because the costs of offspring misidentification outweighed the benefits of dual parenting. Conversely, males in monogamous systems could parent because of high paternity certainty. However, recent work has shown that in some species males parent despite high false paternity rates, and males in some promiscuous systems discriminate between their own and other males' offspring. Here we evaluate the impact of male dominance rank, paternity and age on maleeimmature relationships in wild mountain gorillas. Mountain gorillas provide an interesting context for assessing paternal kin discrimination because (1) maleeimmature relationships are strong, and (2) while their morphological characteristics suggest an evolutionary history of single-male groups, a substantial fraction contain multiple adult males. In our sample of 21 males and 49 genotyped immatures living in multimale groups monitored by the Dian Fossey Gorilla Fund's Karisoke Research Center, we found that male rank was the primary predictor of maleeimmature relationship strength. There was little evidence that paternity or age were related to relationship patterns. Maleeimmature dyads were closer social partners in 2011e12 when groups were smaller and reproductive skew lower, than comparable dyads in 2003e04 when groups were larger and skew higher. Gorillas' lack of paternal kin discrimination provides further behavioural evidence that the species' multimale social structure is evolutionarily novel. However, patterning of maleeimmature relationships and genetic paternity suggest a persistent minority of two-male groups throughout G. beringei's evolutionary history. This may help explain their ability to live in multimale, multifemale social units despite possessing morphological characteristics typical of harem systems.

Individuals that participate in exchanges with delayed rewards can be exploited if their partners don't reciprocate. In humans, friendships are built on trust, and trust enhances cooperation. New evidence suggests that close social bonds also enhance trust in chimpanzees.

Previous analyses indicate that female baboons that form strong and stable social bonds reproduce more successfully than others, and that some elements of females’ personalities are associated with the tendency to form close social bonds. Here we use a new method to confirm that females’ personalities were stable over time, although not fixed, and that matrilineal kin had personalities that were no more alike than those of other individuals. Our results indicate that personality similarities enhance the strength of social bonds among some pairs of females but not others. Strong bonds between matrilineal sisters were correlated with similarity in their personalities and ages, but mothers and daughters formed uniformly strong bonds regardless of the similarities in their personalities or ages. Among nonkin, strong bonds were correlated with similarity in age and dominance rank, but not personality. Females adjusted the behaviours that contributed to their personality scores in response to unpredictable demographic events, like the death of a close relative. Results suggest that the personality traits of female baboons do not exist in isolation but are embedded in a network of rank and kin relations. Although a female baboon has little control over her dominance rank or the presence of kin, by varying the tenor of her social interactions she can take advantage of the opportunities, or overcome the constraints, imposed by demographic circumstances. For this reason, selection may have favoured particular personality traits that are relatively independent of rank and the presence of kin.

Behavioral ecologists have devoted considerable effort to identifying the sources of variation in individual reproductive success.[1-5] Much of this work has focused on the characteristics of individuals, such as their sex and rank. However, many animals live in stable social groups and the fitness of individuals depends at least in part on the outcome of their interactions with other group members. For example, in many primate species, high dominance rank enhances access to resources and reproductive success.[4, 5] The ability to acquire and maintain high rank often depends on the availability and effectiveness of coalitionary support.[6] Allies may be cultivated and coalitions may be reinforced by affiliative interactions such as grooming, food sharing, and tolerance.[7, 8] These findings suggest that if we want to understand the selective pressures that shape the social behavior of primates, it will be profitable to broaden our focus from the characteristics of individuals to the properties of the relationships that they form with others. The goal of this paper is to discuss a set of methods that can be used to quantify the properties of social relationships.

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 origins of human reciprocity. Here, we explore how reciprocity develops in a sample of American children ranging from 3 to 7.5years 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 5.5years of age children consistently match the previous behavior of their partners.

Item does not contain fulltext Dozens of studies show that bystanders are less likely to help victims as bystander number increases. However, these studies model one particular situation, in which victims need only one helper. Using a multi-player dictator game, we study a different but common situation, in which a recipient’s welfare increases with the amount of help, and donors can share the burden of helping. We find that dictators transfer less when there are more dictators, and recipients earn less when there are multiple dictators. This effect persisted despite mechanisms eliminating uncertainty about other dictators’ behavior (a strategy method and communication). In a typical public goods game, people seem to transform the situation into an assurance game, willing to contribute if certain others will too. Despite similarities, people do not treat a recipient’s welfare like a public good. Instead, people seem to transform the situation into a prisoner’s dilemma, refusing to help whatever others do. 13 p.

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.

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.

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.

Friendship is a core aspect of human social life. Friends form long-term, cooperative relationships, and provide material and emotional support for one another. Previous research in social psychology suggests that people prefer balanced relationships over unbalanced relationship with friends, but at the same time, friendship is defined by an absence of direct reciprocity and careful tracking of favors given and received. The goal of this study was to distinguish between differences in tracking and tolerance of imbalances among friends and strangers. We conducted parallel behavioral economic experiments in three different urban cultural settings, USA, Japan and China, in an effort to expand our understanding of the dynamics of human friendship. Across all sites, we found that subjects monitored their friends less carefully than they monitored strangers, were more generous to friends than to strangers and that friends were more tolerant of imbalances in payoffs than strangers were. Although there were differences in the extent of tracking among friends and strangers, tracking did occur among friends. Our study extends the understanding on the dynamics of human friendship and emphasizes the need for further investigation on how friends balance vigilance and tolerance in real-life interactions.

A growing body of evidence shows that humans are remarkably altruistic primates. Food sharing and division of labor play an important role in all human societies, and cooperation extends beyond the bounds of close kinship and networks of reciprocating partners. In humans, altruism is motivated at least in part by empathy and concern for the welfare of others. Although altruistic behavior is well-documented in other primates, the range of altruistic behaviors in other primate species, including the great apes, is much more limited than it is in humans. Moreover, when altruism does occur among other primates, it is typically limited to familiar group members—close kin, mates, and reciprocating partners. This suggests that there may be fundamental differences in the social preferences that motivate altruism across the primate order, and there is currently considerable interest in how we came to be such unusual apes. A body of experimental studies designed to examine the phylogenetic range of prosocial sentiments and behavior is beginning to shed some light on this issue. In experimental settings, chimpanzees and tamarins do not consistently take advantage of opportunities to deliver food rewards to others, although capuchins and marmosets do deliver food rewards to others in similar kinds of tasks. Although chimpanzees do not satisfy experimental criteria for prosociality in food delivery tasks, they help others complete tasks to obtain a goal. Differences in performance across species and differences in performance across tasks are not yet fully understood and raise new questions for further study.