Meat sharing among the Gombe chimpanzees: harassment and reciprocal exchange

The nature of prosociality in chimpanzees

An important debate centres around the nature of prosociality in nonhuman primates. Chimpanzees help other individuals in some experimental settings, yet they do not readily share food. One solution to this paradox is that they are motivated to help others provided there are no competing interests. However, benefits to recipients could arise as by-products of testing. Here we report two studies that separate by-product from intended helping in chimpanzees using a GO/NO-GO paradigm. Actors in one group could help a recipient by releasing a food box, but the same action for another group prevented a recipient from being able to get food. We find no evidence for helping-chimpanzees engaged in the test regardless of the effects on their partners. Illusory prosocial behaviour could arise as a by-product of task design.

Competence and the Evolutionary Origins of Status and Power in Humans

In this paper I propose an evolutionary model of human status that expands upon an earlier model proposed by Henrich and Gil-White Evolution and Human Behavior, 22,165-196 (2001). According to their model, there are two systems of status attainment in humans-"two ways to the top": the dominance route, which involves physical intimidation, a psychology of fear and hubristic pride, and provides coercive power, and the prestige route, which involves skills and knowledge (competence), a psychology of attraction to experts and authentic pride, and translates mainly into influence. The two systems would have evolved in response to different selective pressures, with attraction to experts serving a social learning function and coinciding with the evolution of cumulative culture. In this paper I argue that (1) the only one way to the top is competence because dominance itself involves competence and confers prestige, so there is no such thing as pure dominance status; (2) dominance in primates has two components: a competitive one involving physical coercion and a cooperative one involving competence-based attraction to high-ranking individuals (proto-prestige); (3) competence grants the same general type of power (dependence-based) in humans and other primates; (4) the attractiveness of high rank in primates is homologous with the admiration of experts in humans; (5) upon the evolution of cumulative culture, the attractiveness of high rank was co-opted to generate status differentials in a vast number of culturally generated domains of activity. I also discuss, in this perspective, the origins of hubristic pride, authentic pride, and nonauthoritarian leadership.

Tool transfers are a form of teaching among chimpanzees

Teaching is a form of high-fidelity social learning that promotes human cumulative culture. Although recently documented in several nonhuman animals, teaching is rare among primates. In this study, we show that wild chimpanzees (Pan troglodytes troglodytes) in the Goualougo Triangle teach tool skills by providing learners with termite fishing probes. Tool donors experienced significant reductions in tool use and feeding, while tool recipients significantly increased their tool use and feeding after tool transfers. These transfers meet functional criteria for teaching: they occur in a learner’s presence, are costly to the teacher, and improve the learner’s performance. Donors also showed sophisticated cognitive strategies that effectively buffered them against potential costs. Teaching is predicted when less costly learning mechanisms are insufficient. Given that these chimpanzees manufacture sophisticated, brush-tipped fishing probes from specific raw materials, teaching in this population may relate to the complexity of these termite-gathering tasks.

The psychology of primate cooperation and competition: a call for realigning research agendas

Cooperation and competition are two key components of social life. Current research agendas investigating the psychological underpinnings of competition and cooperation in non-human primates are misaligned. The majority of work on competition has been done in the context of theory of mind and deception, while work on cooperation has mostly focused on collaboration and helping. The current impression that theory of mind is not necessarily implicated in cooperative activities and that helping could not be an integral part of competition might therefore be rather misleading. Furthermore, theory of mind research has mainly focused on cognitive aspects like the type of stimuli controlling responses, the nature of representation and how those representations are acquired, while collaboration and helping have focused primarily on motivational aspects like prosociality, common goals and a sense of justice and other-regarding concerns. We present the current state of these two bodies of research paying special attention to how they have developed and diverged over the years. We propose potential directions to realign the research agendas to investigate the psychological underpinnings of cooperation and competition in primates and other animals.

Critical issues in experimental studies of prosociality in non-human species

Prosociality and acts of altruism are defined as behaviours which benefit another with either no gain or some immediate cost to the self. To understand the evolutionary origins of these behaviours, in recent years, studies have extended to primate species; however, studies on non-primates are still scarce. In light of the fact that phylogenetic closeness to humans does not appear to correlate with prosocial tendencies, but rather differences in the propensity towards prosociality may be linked to allomaternal care or collaborative foraging, it appears that convergent selection pressures may be at work in the evolution of prosociality. It would hence seem particularly important to extend such studies to species outside the primate clade, to allow for comparative hypothesis testing of the factors affecting the evolution of prosocial behaviours. In the current review, we focus on the experimental paradigms which have been used so far (i.e. the prosocial choice task, helping paradigms and food-sharing tests) and highlight the strengths and weaknesses of each method. In line with the aim of encouraging a broader comparative approach to the topic of prosociality, particular emphasis is placed on the methodological issues that need to be taken into account. We conclude that although a number of the paradigms used so far may be successfully applied to non-primate species, there is a need to simplify the cognitive demands of the tasks and ensure task comprehension to allow for a 'fair' comparative approach of prosocial tendencies across species.

Food begging and sharing in wild bonobos (Pan paniscus): assessing relationship quality?

Food transfers are often hypothesised to have played a role in the evolution of cooperation amongst humans. However, they also occur in non-human primates, though no consensus exists regarding their function(s). We document patterns of begging for food and success rates as well as associated factors that may influence them for wild bonobos at LuiKotale, Democratic Republic of Congo. Our data, collected over 1074 observation hours, focus on 260 begging events (outside mother-offspring dyads) of which 37 % were successful. We find no support for the "reciprocity hypothesis"-that food is exchanged for grooming and/or sexual benefits; and only weak support for the "sharing under pressure" hypothesis-that food is transferred as a result of harassment and pays off in terms of nutritional benefits for the beggar. Instead, our data support the "assessing-relationships" hypothesis, according to which beggars gain information about the status of their social relationship with the possessor of a food item. This seems to hold particularly true for the frequent, albeit unsuccessful begging events by young females (newly immigrated or hierarchically non-established) towards adult females, although it can be observed in other dyadic combinations independent of sex and age.

Bystanders, parcelling, and an absence of trust in the grooming interactions of wild male chimpanzees

The evolution of cooperation remains a central issue in socio-biology with the fundamental problem of how individuals minimize the risks of being short-changed (‘cheated’) should their behavioural investment in another not be returned. Economic decisions that individuals make during interactions may depend upon the presence of potential partners nearby, which offers co operators a temptation to defect from the current partner. The parcelling model posits that donors subdivide services into parcels to force cooperation, and that this is contingent on opportunities for defection; that is, the presence of bystanders. Here we test this model and the effect of bystander presence using grooming interactions of wild chimpanzees. We found that with more bystanders, initiators gave less grooming at the beginning of the bout and were more likely to abandon a grooming bout, while bouts were less likely to be reciprocated. We also found that the groomer’s initial investment was not higher among frequent groomers or stronger reciprocators, suggesting that contrary to current assumptions, grooming decisions are not based on trust, or bonds, within dyads. Our work highlights the importance of considering immediate social context and the influence of bystanders for understanding the evolution of the behavioural strategies that produce cooperation.

The evolution of altruistic social preferences in human groups

In this paper, we consider three hypotheses to account for the evolution of the extraordinary capacity for large-scale cooperation and altruistic social preferences within human societies. One hypothesis is that human cooperation is built on the same evolutionary foundations as cooperation in other animal societies, and that fundamental elements of the social preferences that shape our species' cooperative behaviour are also shared with other closely related primates. Another hypothesis is that selective pressures favouring cooperative breeding have shaped the capacity for cooperation and the development of social preferences, and produced a common set of behavioural dispositions and social preferences in cooperatively breeding primates and humans. The third hypothesis is that humans have evolved derived capacities for collaboration, group-level cooperation and altruistic social preferences that are linked to our capacity for culture. We draw on naturalistic data to assess differences in the form, scope and scale of cooperation between humans and other primates, experimental data to evaluate the nature of social preferences across primate species, and comparative analyses to evaluate the evolutionary origins of cooperative breeding and related forms of behaviour.

Meat sharing between male and female Guinea baboons (<i>Papio papio</i>)

Abstract. Meat sharing in non-human primates has been linked to a variety of functions, including harassment reduction, mate provisioning and status enhancement. We present observational data regarding male prey capture and male–female meat sharing in wild Guinea baboons. Guinea baboons live in a multilevel society that comprises units of males with associated females and, sometimes, secondary males. Several males of different units maintain strong bonds, resulting in the formation of parties within gangs. Female–male relationships persist irrespective of female reproductive states, yet females may also switch between males at all stages of the reproductive cycle. Our data show that males capture and kill a variety of prey, including hares and antelope. Males shared meat passively only with females in their social and reproductive units. The occurrence of oestrus females in the gang did not influence whether or not sharing would occur in that males did not share with oestrus females unless an affiliative relationship already persisted, indicating that short-term currency exchanges of meat for sex are unlikely. We hypothesise that males may benefit from feeding tolerance by retaining females, while females may increase access to potentially nutritious and rare food sources. Alternatively, females may prefer males that are generally less aggressive and thus also more likely to share meat. Long-term data will be needed to ultimately distinguish between the two accounts. Although there is no evidence that males intentionally provide necessary resources to particular females during times of high energetic demands and decreased foraging efficiency, as has been found in humans, and meat sharing is generally rare, it may have subtle, yet important effects on the maintenance of bonds in Guinea baboons.

On potential links between inequity aversion and the structure of interactions for the evolution of cooperation

Despite the fact that most models of cooperation assume equal outcomes between individuals, in real life it is likely rare that this is the case. Does it make a difference for our understanding of the evolution of cooperation? Following a taxonomy of cooperation concepts that focuses on costs and benefits, we explore this question by considering the degree to which inequity aversion may provide one mechanism to stabilize cooperation. We suggest a key role for inequity aversion in some contexts in both biological markets and direct reciprocity, and highlight the potentially unique role of positive inequity aversion for human reputation games. Nevertheless, a key challenge is to determine how different animal species perceive the payoff structure of their interactions, how they see their interaction with their partners, and the degree to which simpler mechanisms, like contrast effects or the associative learning seen in optimal foraging, may produce similar outcomes.

Reproductive state and rank influence patterns of meat consumption in wild female chimpanzees (Pan troglodytes schweinfurthii)

An increase in faunivory is a consistent component of human evolutionary models. Animal matter is energy- and nutrient-dense and can provide macronutrients, minerals, and vitamins that are limited or absent in plant foods. For female humans and other omnivorous primates, faunivory may be of particular importance during the costly periods of pregnancy and early lactation. Yet, because animal prey is often monopolizable, access to fauna among group-living primates may be mediated by social factors such as rank. Wild chimpanzees (Pan troglodytes) across Africa habitually consume insects and/or vertebrates. However, no published studies have examined patterns of female chimpanzee faunivory during pregnancy and early lactation relative to non-reproductive periods, or by females of different rank. In this study, we assessed the influence of reproductive state and dominance rank on the consumption of fauna (meat and insects) by female chimpanzees of Gombe National Park, Tanzania. Using observational data collected over 38 years, we tested (a) whether faunivory varied by reproductive state, and (b) if high-ranking females spent more time consuming fauna than lower-ranking females. In single-factor models, pregnant females consumed more meat than lactating and baseline (meaning not pregnant and not in early lactation) females, and high-ranking females consumed more meat than lower-ranking females. A two-factor analysis of a subset of well-sampled females identified an interaction between rank and reproductive state: lower-ranking females consumed more meat during pregnancy than lower-ranking lactating and baseline females did. High-ranking females did not significantly differ in meat consumption between reproductive states. We found no relationships between rank or reproductive state with insectivory. We conclude that, unlike insectivory, meat consumption by female chimpanzees is mediated by both reproductive state and social rank. We outline possible mechanisms for these patterns, relate our findings to meat-eating patterns in women from well-studied hunter-gatherer societies, and discuss potential avenues for future research.

'Impact hunters' catalyse cooperative hunting in two wild chimpanzee communities

Even when hunting in groups is mutually beneficial, it is unclear how communal hunts are initiated. If it is costly to be the only hunter, individuals should be reluctant to hunt unless others already are. We used 70 years of data from three communities to examine how male chimpanzees 'solve' this apparent collective action problem. The 'impact hunter' hypothesis proposes that group hunts are sometimes catalysed by certain individuals that hunt more readily than others. In two communities (Kasekela and Kanyawara), we identified a total of five males that exhibited high hunt participation rates for their age, and whose presence at an encounter with red colobus monkeys increased group hunting probability. Critically, these impact hunters were observed to hunt first more often than expected by chance. We argue that by hunting first, these males dilute prey defences and create opportunities for previously reluctant participants. This by-product mutualism can explain variation in group hunting rates within and between social groups. Hunting rates declined after the death of impact hunter FG in Kasekela and after impact hunter MS stopped hunting frequently in Kanyawara. There were no impact hunters in the third, smaller community (Mitumba), where, unlike the others, hunting probability increased with the number of females present at an encounter with prey.

Rats prefer mutual rewards in a prosocial choice task

Pro-sociality, i.e., the preference for outcomes that produce benefits for other individuals, is ubiquitous in humans. Recently, cross-species comparisons of social behavior have offered important new insights into the evolution of pro-sociality. Here, we present a rodent analog of the Pro-social Choice Task that controls strategic components, de-confounds other-regarding choice motives from the animals' natural tendencies to maximize own food access and directly tests the effect of social context on choice allocation. We trained pairs of rats—an actor and a partner rat—in a double T-maze task where actors decided between two alternatives only differing in the reward delivered to the partner. The “own reward” choice yielded a reward only accessible to the actor whereas the “both reward” choice produced an additional reward for a partner (partner condition) or an inanimate toy (toy Condition), located in an adjacent compartment. We found that actors chose “both reward” at levels above chance and more often in the partner than in the toy condition. Moreover, we show that this choice pattern adapts to the current social context and that the observed behavior is stable over time.

Egalitarian despots: hierarchy steepness, reciprocity and the grooming-trade model in wild chimpanzees, Pan troglodytes

Biological market theory models the action of natural selection as a marketplace in which animals are viewed as traders with commodities to offer and exchange. Studies of female Old World monkeys have suggested that grooming might be employed as a commodity to be reciprocated or traded for alternative services, yet previous tests of this grooming-trade model in wild adult male chimpanzees have yielded mixed results. Here we provide the strongest test of the model to date for male chimpanzees: we use data drawn from two social groups (communities) of chimpanzees from different populations and give explicit consideration to variation in dominance hierarchy steepness, as such variation results in differing conditions for biological markets. First, analysis of data from published accounts of other chimpanzee communities, together with our own data, showed that hierarchy steepness varied considerably within and across communities and that the number of adult males in a community aged 20-30 years predicted hierarchy steepness. The two communities in which we tested predictions of the grooming-trade model lay at opposite extremes of this distribution. Second, in accord with the grooming-trade model, we found evidence that male chimpanzees trade grooming for agonistic support where hierarchies are steep (despotic) and consequent effective support is a rank-related commodity, but not where hierarchies are shallow (egalitarian). However, we also found that grooming was reciprocated regardless of hierarchy steepness. Our findings also hint at the possibility of agonistic competition, or at least exclusion, in relation to grooming opportunities compromising the free market envisioned by biological market theory. Our results build on previous findings across chimpanzee communities to emphasize the importance of reciprocal grooming exchanges among adult male chimpanzees, which can be understood in a biological markets framework if grooming by or with particular individuals is a valuable commodity.

Task design influences prosociality in captive chimpanzees (Pan troglodytes)

Chimpanzees confer benefits on group members, both in the wild and in captive populations. Experimental studies of how animals allocate resources can provide useful insights about the motivations underlying prosocial behavior, and understanding the relationship between task design and prosocial behavior provides an important foundation for future research exploring these animals' social preferences. A number of studies have been designed to assess chimpanzees' preferences for outcomes that benefit others (prosocial preferences), but these studies vary greatly in both the results obtained and the methods used, and in most cases employ procedures that reduce critical features of naturalistic social interactions, such as partner choice. The focus of the current study is on understanding the link between experimental methodology and prosocial behavior in captive chimpanzees, rather than on describing these animals' social motivations themselves. We introduce a task design that avoids isolating subjects and allows them to freely decide whether to participate in the experiment. We explore key elements of the methods utilized in previous experiments in an effort to evaluate two possibilities that have been offered to explain why different experimental designs produce different results: (a) chimpanzees are less likely to deliver food to others when they obtain food for themselves, and (b) evidence of prosociality may be obscured by more “complex” experimental apparatuses (e.g., those including more components or alternative choices). Our results suggest that the complexity of laboratory tasks may generate observed variation in prosocial behavior in laboratory experiments, and highlights the need for more naturalistic research designs while also providing one example of such a paradigm.

The emergence of human prosociality: aligning with others through feelings, concerns, and norms

The fact that humans cooperate with nonkin is something we take for granted, but this is an anomaly in the animal kingdom. Our species' ability to behave prosocially may be based on human-unique psychological mechanisms. We argue here that these mechanisms include the ability to care about the welfare of others (other-regarding concerns), to "feel into" others (empathy), and to understand, adhere to, and enforce social norms (normativity). We consider how these motivational, emotional, and normative substrates of prosociality develop in childhood and emerged in our evolutionary history. Moreover, we suggest that these three mechanisms all serve the critical function of aligning individuals with others: Empathy and other-regarding concerns align individuals with one another, and norms align individuals with their group. Such alignment allows us to engage in the kind of large-scale cooperation seen uniquely in humans.

Sharing fruit of Treculia africana among western gorillas in the Moukalaba-Doudou National Park, Gabon: preliminary report

We report the first 18 observed cases of fruit (Treculia africana) transfer among western lowland gorillas (Gorilla gorilla gorilla) in Moukalaba-Doudou National Park, Gabon. The fruit transfer occurred during our observations of a habituated group of gorillas in 2010 and 2013. Pieces of the fruits were transferred among adults and immatures, and three cases involved a silverback male. Once an individual picked up a fallen fruit of Treculia africana, other members of the group approached the possessor, who laid pieces of the fruits nearby and tolerated the others getting them. Agonistic interaction was rarely observed between the possessor and the non-possessor. Only the silverback male seemed to force another gorilla, a subadult male, to relinquish the fruit on the ground. He tolerated an adult female taking a piece of fruit on his leg and copulated with her on the following days. From these preliminary observations, most interactions over the fruit of Treculia africana among western gorillas in Moukalaba were not active transfer by the possessor but probably passive sharing. They were not only interpreted as a means of acquiring foraging skills by immatures (Nowell and Fletcher 2006) but also similar to behaviors observed in chimpanzees and bonobos in various social contexts.

Problem solving in the presence of others: how rank and relationship quality impact resource acquisition in chimpanzees (Pan troglodytes)

In the wild, chimpanzees (Pan troglodytes) are often faced with clumped food resources that they may know how to access but abstain from doing so due to social pressures. To better understand how social settings influence resource acquisition, we tested fifteen semi-wild chimpanzees from two social groups alone and in the presence of others. We investigated how resource acquisition was affected by relative social dominance, whether collaborative problem solving or (active or passive) sharing occurred amongst any of the dyads, and whether these outcomes were related to relationship quality as determined from six months of observational data. Results indicated that chimpanzees obtained fewer rewards when tested in the presence of others compared to when they were tested alone, and this loss tended to be greater when paired with a higher ranked individual. Individuals demonstrated behavioral inhibition; chimpanzees who showed proficient skill when alone often abstained from solving the task when in the presence of others. Finally, individuals with close social relationships spent more time together in the problem solving space, but collaboration and sharing were infrequent and sessions in which collaboration or sharing did occur contained more instances of aggression. Group living provides benefits and imposes costs, and these findings highlight that one cost of group living may be diminishing productive individual behaviors.

The ultra-social animal

In evolutionary perspective, what is most remarkable about human sociality is its many and diverse forms of cooperation. Here, I provide an overview of some recent research, mostly from our laboratory, comparing human children with their nearest living relatives, the great apes, in various tests of collaboration, prosocial behavior, conformity, and group-mindedness (e.g., following and enforcing social norms). This is done in the context of a hypothetical evolutionary scenario comprising two ordered steps: a first step in which early humans began collaborating with others in unique ways in their everyday foraging and a second step in which modern humans began forming cultural groups. Humans' unique forms of sociality help to explain their unique forms of cognition and morality. © 2014. The Authors. European Journal of Social Psychology published by John Wiley & Sons, Ltd.

Natural cooperators: food sharing in humans and other primates

The study of cooperation is rich with theoretical models and laboratory experiments that have greatly advanced our knowledge of human uniqueness, but have sometimes lacked ecological validity. We therefore emphasize the need to tie discussions of human cooperation to the natural history of our species and its closest relatives, focusing on behavioral contexts best suited to reveal underlying selection pressures and evolved decision rules. Food sharing is a fundamental form of cooperation that is well-studied across primates and is particularly noteworthy because of its central role in shaping evolved human life history, social organization, and cooperative psychology. Here we synthesize available evidence on food sharing in humans and other primates, tracing the origins of offspring provisioning, mutualism, trade, and reciprocity throughout the primate order. While primates may gain some benefits from sharing, humans, faced with more collective action problems in a risky foraging niche, expanded on primate patterns to buffer risk and recruit mates and allies through reciprocity and signaling, and established co-evolving social norms of production and sharing. Differences in the necessity for sharing are reflected in differences in sharing psychology across species, thus helping to explain unique aspects of our evolved cooperative psychology.

Food sharing is linked to urinary oxytocin levels and bonding in related and unrelated wild chimpanzees

Humans excel in cooperative exchanges between unrelated individuals. Although this trait is fundamental to the success of our species, its evolution and mechanisms are poorly understood. Other social mammals also build long-term cooperative relationships between non-kin, and recent evidence shows that oxytocin, a hormone involved in parent-offspring bonding, is likely to facilitate non-kin as well as kin bonds. In a population of wild chimpanzees, we measured urinary oxytocin levels following a rare cooperative event--food sharing. Subjects showed higher urinary oxytocin levels after single food-sharing events compared with other types of social feeding, irrespective of previous social bond levels. Also, urinary oxytocin levels following food sharing were higher than following grooming, another cooperative behaviour. Therefore, food sharing in chimpanzees may play a key role in social bonding under the influence of oxytocin. We propose that food-sharing events co-opt neurobiological mechanisms evolved to support mother-infant bonding during lactation bouts, and may act as facilitators of bonding and cooperation between unrelated individuals via the oxytocinergic system across social mammals.

Does sympathy motivate prosocial behaviour in great apes?

Prosocial behaviours such as helping, comforting, or sharing are central to human social life. Because they emerge early in ontogeny, it has been proposed that humans are prosocial by nature and that from early on empathy and sympathy motivate such behaviours. The emerging question is whether humans share these abilities to feel with and for someone with our closest relatives, the great apes. Although several studies demonstrated that great apes help others, little is known about their underlying motivations. This study addresses this issue and investigates whether four species of great apes (Pongo pygmaeus, Gorilla gorilla, Pan troglodytes, Pan paniscus) help a conspecific more after observing the conspecific being harmed (a human experimenter steals the conspecific’s food) compared to a condition where no harming occurred. Results showed that in regard to the occurrence of prosocial behaviours, only orangutans, but not the African great apes, help others when help is needed, contrasting prior findings on chimpanzees. However, with the exception of one population of orangutans that helped significantly more after a conspecific was harmed than when no harm occurred, prosocial behaviour in great apes was not motivated by concern for others.

Reciprocity explains food sharing in humans and other primates independent of kin selection and tolerated scrounging: a phylogenetic meta-analysis

Helping, i.e. behaviour increasing the fitness of others, can evolve when directed towards kin or reciprocating partners. These predictions have been tested in the context of food sharing both in human foragers and non-human primates. Here, we performed quantitative meta-analyses on 32 independent study populations to (i) test for overall effects of reciprocity on food sharing while controlling for alternative explanations, methodological biases, publication bias and phylogeny and (ii) compare the relative effects of reciprocity, kinship and tolerated scrounging, i.e. sharing owing to costs imposed by others. We found a significant overall weighted effect size for reciprocity of r = 0.20-0.48 for the most and least conservative measure, respectively. Effect sizes did not differ between humans and other primates, although there were species differences in in-kind reciprocity and trade. The relative effect of reciprocity in sharing was similar to those of kinship and tolerated scrounging. These results indicate a significant independent contribution of reciprocity to human and primate helping behaviour. Furthermore, similar effect sizes in humans and primates speak against cognitive constraints on reciprocity. This study is the first to use meta-analyses to quantify these effects on human helping and to directly compare humans and other primates.

Perseverance and food sharing among closely affiliated female chimpanzees

Chimpanzees (Pan troglodytes) have been frequently observed to share food with one another, with numerous hypotheses proposed to explain why. These often focus on reciprocity exchanges for social benefits (e.g., food for grooming, food for sex, affiliation, kinship, and dominance rank) as well as sharing based on begging and deterring harassment. Although previous studies have shown that each of these hypotheses has a viable basis, they have only examined situations in which males have preferential access to food whereby females are required to obtain the food from males. For example, studies on male chimpanzee food sharing take advantage of successful crop-raids and/or acquisitions of meat from hunting, situations that only leave females access to food controlled by male food possessors. This begs the question how and with whom might a female chimpanzee in sole possession of a high-quality food item choose to share? In two large captive groups of chimpanzees, we examined each of the hypotheses with female food possessors of a high-quality food item and compared these data to a previous study examining food transfers from male chimpanzees. Our results show that alpha females shared significantly more with closely affiliated females displaying perseverance, while kinship and dominance rank had no effect. This positive interaction between long-term affiliation and perseverance shows that individuals with whom the female possessor was significantly affiliated received more food while persevering more than those with neutral or avoidant relationships towards her. Furthermore, females with avoidant relationships persevered far less than others, suggesting that this strategy is not equally available to all individuals. In comparison to the mixed-sex trials, females chose to co-feed with other females more than was observed when the alpha male was sharing food. This research indicates that male and female chimpanzees (as possessors of a desired food item) share food in ways influenced by different factors and strategies.

Does food sharing in vampire bats demonstrate reciprocity?

Claims of reciprocity (or reciprocal altruism) in animal societies often ignite controversy because authors disagree over definitions, naturalistic studies tend to demonstrate correlation not causation, and controlled experiments often involve artificial conditions. Food sharing among common vampire bats has been a classic textbook example of reciprocity, but this conclusion has been contested by alternative explanations. Here, we review factors that predict food sharing in vampire bats based on previously published and unpublished data, validate previous published results with more precise relatedness estimates, and describe current evidence for and against alternative explanations for its evolutionary stability. Although correlational evidence indicates a role for both direct and indirect fitness benefits, unequivocally demonstrating reciprocity in vampire bats still requires testing if and how bats respond to non-reciprocation.

Chimpanzees share food for many reasons: the role of kinship, reciprocity, social bonds and harassment on food transfers

There is currently great interest in the phylogenetic origins of altruistic behaviour within the primate order. Considerable attention has been focused on chimpanzees, Pan troglodytes, because they are our closest living relatives and participate in a wide range of collective activities, including hunting and food sharing. Food sharing is of particular importance because it plays a critical role in the human foraging niche, but food sharing among adults is rare in nonhuman primates. Some research suggests that chimpanzees selectively share meat with reciprocating partners and allies, while other work indicates that chimpanzees primarily share to reduce harassment from other group members (tolerated theft). We examined the effects of kinship, relationship quality, reciprocity and the intensity of solicitations on the pattern of food transfers in six captive groups of chimpanzees. We observed events that occurred after the chimpanzees were provisioned with large frozen juice disks. These disks share some properties with prey carcasses: they are a valued, but limited, resource; they take a considerable period of time to consume; they can be monopolized by one individual, but bits can be broken off and transferred to others. Our analyses suggest that food transfers serve multiple functions for chimpanzees. Individuals may use food transfers to enhance the welfare of closely related group members, strengthen social relationships with favoured partners and reduce the costs of persistent solicitations.

Competition over personal resources favors contribution to shared resources in human groups

Members of social groups face a trade-off between investing selfish effort for themselves and investing cooperative effort to produce a shared group resource. Many group resources are shared equitably: they may be intrinsically non-excludable public goods, such as vigilance against predators, or so large that there is little cost to sharing, such as cooperatively hunted big game. However, group members' personal resources, such as food hunted individually, may be monopolizable. In such cases, an individual may benefit by investing effort in taking others' personal resources, and in defending one's own resources against others. We use a game theoretic “tug-of-war” model to predict that when such competition over personal resources is possible, players will contribute more towards a group resource, and also obtain higher payoffs from doing so. We test and find support for these predictions in two laboratory economic games with humans, comparing people's investment decisions in games with and without the options to compete over personal resources or invest in a group resource. Our results help explain why people cooperatively contribute to group resources, suggest how a tragedy of the commons may be avoided, and highlight unifying features in the evolution of cooperation and competition in human and non-human societies.

Manipulated into giving: when parasitism drives apparent or incidental altruism

Altruistic acts involve the actor donating fitness to beneficiaries at net cost to itself. In contrast, parasitic acts involve the actor extracting benefit from others at net cost to the donors. Both behaviours may have the same direct net-cost transferral of fitness from donor to beneficiary; the key difference between parasitism and altruism is thus who drives the interaction. Identifying the evolutionary driver is not always straightforward in practice, yet it is crucial in determining the conditions necessary to sustain such fitness exchange. Here, we put classical ecological competition into a novel game-theoretic framework in order to distinguish altruism from parasitism. The distinction depends on the type of interaction that beneficiaries have among themselves. When this is not costly, net-cost transferrals of fitness from the donor are strongly altruistic, and sustained only by indirect benefits to the donor from assortative mixing. When the interaction among beneficiaries is costly, however, net-cost transferrals of fitness from the donor are sustainable without assortative mixing. The donor is then forced into apparent or incidental altruism driven by parasitism from the beneficiary. We consider various scenarios in which direct and indirect fitness consequences of strong altruism may have different evolutionary drivers.

Food-snatching behavior of free-ranging Japanese macaques observed on Shodoshima Island: a preliminary report

We investigated the characteristics of a particular food-snatching behavior in which one individual forced another's mouth open and grabbed the food, as performed by free-ranging Japanese macaques (Macaca fuscata) in Choshikei Monkey Park on Shodoshima Island, western Japan. We conducted a survey in late June 2012 and observed one of two monkey troops, comprising 214 monkeys. We recorded the age classes and sexes of the individuals who performed the snatching behavior and were snatched from, and examined the effects of provisioned food distribution and quantity on the frequency of snatching trials and success. During the survey, we recorded 747 snatching trials, of which 609 were successful, all of which were performed by seven individuals: one adult male and six adult females. The snatching behavior occurred only during provisioning. The target animals were primarily juveniles (650 trials, 578 successful), while cases in which food was snatched from adult females (93 trials, 30 successful) and subadult females (4 trials, 1 success) were less frequent. Among the juveniles, small juveniles had food snatched more frequently than large juveniles. The higher frequency of snatching trials against juveniles was likely due to their subordinate nature. Neither the distribution nor quantity of the provisioned foods had significant effects on the number of snatching trials and successes, while the time elapsed after provisioning had significant negative effects, attributed to a decrease in the number of wheat grains left within the mouth pouch of the potential target animals.

Five-year olds, but not chimpanzees, attempt to manage their reputations

Virtually all theories of the evolution of cooperation require that cooperators find ways to interact with one another selectively, to the exclusion of cheaters. This means that individuals must make reputational judgments about others as cooperators, based on either direct or indirect evidence. Humans, and possibly other species, add another component to the process: they know that they are being judged by others, and so they adjust their behavior in order to affect those judgments - so-called impression management. Here, we show for the first time that already preschool children engage in such behavior. In an experimental study, 5-year-old human children share more and steal less when they are being watched by a peer than when they are alone. In contrast, chimpanzees behave the same whether they are being watched by a groupmate or not. This species difference suggests that humans' concern for their own self-reputation, and their tendency to manage the impression they are making on others, may be unique to humans among primates.

Theft in an ultimatum game: chimpanzees and bonobos are insensitive to unfairness

Humans, but not chimpanzees, punish unfair offers in ultimatum games, suggesting that fairness concerns evolved sometime after the split between the lineages that gave rise to Homo and Pan. However, nothing is known about fairness concerns in the other Pan species, bonobos. Furthermore, apes do not typically offer food to others, but they do react against theft. We presented a novel game, the ultimatum theft game, to both of our closest living relatives. Bonobos and chimpanzee 'proposers' consistently stole food from the responders' portions, but the responders did not reject any non-zero offer. These results support the interpretation that the human sense of fairness is a derived trait.