Why cooperation is not running away

A growing number of experimental and theoretical studies show the importance of partner choice as a mechanism to promote the evolution of cooperation, especially in humans. In this paper, we focus on the question of the precise quantitative level of cooperation that should evolve under this mechanism. When individuals compete to be chosen by others, their level of investment in cooperation evolves towards higher values, a process called competitive altruism, or runaway cooperation. Using a classic adaptive dynamics model, we first show that when the cost of changing partner is low, this runaway process can lead to a profitless escalation of cooperation. In the extreme, when partner choice is entirely frictionless, cooperation even increases up to a level where its cost entirely cancels out its benefit. That is, at evolutionary equilibrium, individuals gain the same payoff than if they had not cooperated at all. Second, importing models from matching theory in economics we, however, show that when individuals can plastically modulate their choosiness in function of their own cooperation level, partner choice stops being a runaway competition to outbid others and becomes a competition to form the most optimal partnerships. In this case, when the cost of changing partner tends towards zero, partner choice leads to the evolution of the socially optimum level of cooperation. This last result could explain the observation that human cooperation seems to be often constrained by considerations of social efficiency.     

Social factors determine cooperation in marmosets

We investigated which factors determine cooperative behaviour of common marmosets, Callithrix jacchus, in an instrumental task. During the approach phase each of eight individuals of a family group learned in isolation to pull a handle moving a bowl with attractive food towards its reach. In the following two experimental phases we used 16 dyads. In a dyadic training phase we assessed whether the partners were willing to manipulate the apparatus and to share food. In the subsequent cooperation test we examined whether they were willing to cooperate at a slightly modified apparatus whose solution required one individual (the producer) to pull the handle and the other (the scrounger) to grasp the bowl. Although all individuals were willing to cooperate with at least one partner, only half of the dyads solved the task in the cooperation phase. Examination of the factors that correlated with success in this phase revealed that primarily those dyads cooperated in which the dominant subject took the role of the scrounger and the subordinate took the role of the producer. However, in these successful dyads the dominant animal did not force the subordinate partner to pull the handle. Rather, the partners of cooperative dyads shared the reward and pulled equally often in both the dyadic training and the cooperation test. Thus, cooperation of marmosets in an instrumental task seems to depend on a specific distribution of roles and the tolerance of higher-ranking individuals.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Experiments on respect of possession and property in hamadryas baboons (Papio hamadryas)

The question of how useful concepts of possession and property are in analysing the relationship between baboons and objects was evaluated in five experimental situations. The results of laboratory experiments concerning the possession of a food container and experiments on possession of fruits confirmed the inhibition of the dominant partner to take food away from a lower-ranking companion. This inhibition always appeared in male-male dyads, while in male-female and female-female dyads, the extent of respect depended on the difference in dominance rank and the type of food. However, in a dominance test, where food pieces were thrown between both partners, the dominant did not allow the subordinate to take a single piece. Given the choice, males preferred neutral food-cans over cans previously used by their partner. In experiments in a field enclosure, whole one-male units preferred their own feeding place over that of another unit. The outcome of the two experiments concerning choice does not support the alternative explanation involving ‘familiar’ versus ‘new’, but does support the property hypothesis.     

Food transfers in capuchin monkeys: an experiment on partner choice

Although most primates live in groups, experiments on reciprocity usually test individuals in dyads. This could hide the processes emerging in richer social settings, reducing the ecological validity of the results. We run an experiment on reciprocal food transfers testing capuchin monkeys (Cebus apella) in triads, so that subjects could choose to allow access to their food to either of their two partners. We tested the hypothesis that partner choice was related to a comparison of long-term social bonds with the two partners, more than to a comparison of recent food transfer events from the two partners. The results confirmed this hypothesis, thus supporting the notion that reciprocal partner preferences are based on long-term accounts of benefits that have been exchanged.     

Spontaneous categorization along competence in partner and leader evaluations

Successful cooperation requires a partner both willing and capable of contributing to a joint endeavour. Accordingly, partner choice psychology should include mechanisms to distinguish between people with good and bad intentions, and between people who are competent and incompetent. While it is well established that intentions influence partner choice, the literature offers mixed evidence concerning people's ability to gauge competence in social interactions. Theoretical accounts in leadership-followership psychology and food-sharing imply that partner competence can influence the estimated future benefits from cooperation. The available empirical evidence, however, is limited to leadership evaluations in the political science literature. This paper thus investigates if people have dedicated cognitive mechanisms, which have evolved to categorize potential social partners on competence. It looks at competence both in regular social partnerships and leader-follower relations. In a series of four experiments relying on the memory confusion protocol, it demonstrates that people spontaneously distinguish between competent and incompetent social partners. This mental categorization is present equally in partner and leader evaluations. These results have interesting implications for partner choice literature and evolutionary leadership theories.     

Chimpanzees coordinate in a negotiation game

A crucially important aspect of human cooperation is the ability to negotiate to cooperative outcomes when interests over resources conflict. Although chimpanzees and other social species may negotiate conflicting interests regarding travel direction or activity timing, very little is known about their ability to negotiate conflicting preferences over food. In the current study, we presented pairs of chimpanzees with a choice between two cooperative tasks—one with equal payoffs (e.g., 5-5) and one with unequal payoffs (higher and lower than in the equal option, e.g., 10-1). This created a conflict of interests between partners with failure to work together on the same cooperative task resulting in no payoff for either partner. The chimpanzee pairs cooperated successfully in as many as 78–94% of the trials across experiments. Even though dominant chimpanzees preferred the unequal option (as they would obtain the largest payoff), subordinate chimpanzees were able to get their way (the equal option) in 22–56% of trials across conditions. Various analyses showed that subjects were both strategic and also cognizant of the strategies used by their partners. These results demonstrate that one of our two closest primate relatives, the chimpanzee, can settle conflicts of interest over resources in mutually satisfying ways—even without the social norms of equity, planned strategies of reciprocity, and the complex communication characteristic of human negotiation.     

The dining etiquette of desert baboons: the roles of social bonds, kinship, and dominance in co-feeding networks

To better understand how individual relationships influence patterns of social foraging in primate groups, we explored networks of co-feeding in wild desert baboons (Papio ursinus). To minimize the risk of aggression and injury associated with contest competition, we expected that individual group members would choose to co-feed with those group-mates that are most likely to show tolerance and a willingness to share food patches. We tested two alternative hypotheses about who those group-mates might be: the "social bonds hypothesis" predicts that preferred foraging partners will be those with whom individuals share strong social bonds, indexed by grooming, whereas the "kinship hypothesis" predicts that preferred foraging partners will be relatives. We also investigated and controlled for the effects of dominance rank, given that competitive ability is known to shape foraging patterns. Social network analyses of over 5,000 foraging events for 14 adults in a single troop revealed that baboon co-feeding was significantly correlated with grooming relationships but not genetic relatedness, and this finding was also true of the female-only co-feeding network. Dominant individuals were also found to be central to the co-feeding network, frequently sharing food patches with multiple group-mates. This polyadic analysis of foraging associations between individuals underlines the importance of dominance and affiliation to patterns of primate social foraging.     

Intermale affiliative behavior in ringtailed lemurs (Lemur catta) at the Beza-Mahafaly reserve, Madagascar

Intermale affiliative behavior was studied in three groups of naturally occurring ringtailed lemurs over a one-year period. The adult males in the sample exhibited affiliative behavior with all other males in their social groups; but affiliative behavior between some male-male dyads occurred markedly more often than between others. These dyads are called “preferred partnerships.” The formation of preferred partnerships did not appear to be influenced by male dominance rank or age-class. Furthermore, these partnerships were of a short-term nature, and none persisted throughout the entire study period. The following factors may explain the absence of rank or age effects, and the brevity of preferred partnerships: (1) frequent fluctuation in the male dominance hierarchy in each study group; (2) the fact that higher-ranking males may not offer specific benefits to lower-ranking males; and (3) the fact that male dispersal affected the male membership of each group over the 12-month study period. The focal males engaged in significantly more affiliative behavior during the lactation period compared with the other reproductive seasons. Migration partners may offer each other predator protection during the transfer process, and although males that transfer together do not engage in alliance or coalition behavior towards resident males, males transferring in pairs or threesomes are in a better position to spot and defend themselves against attack by resident males. Migration partners also provide social contact for each other while in the process of immigration.     

To be or not to be a good social partner?

Cooperation based in mutual benefit provides a perfect scenario to start selfish behaviors aimed to obtain greater benefit at the expense of the partner. Here we investigate if mutual benefit cooperation can be stable between individuals that cooperate with kindness (good partners) or if they will be displaced by other individuals that try to obtain more benefit with less cost (bad partners). Our model assumes an asymmetry between partners in such a way that one of them (actor) proposes the cooperation whereas the other (receiver) always accepts the offer. It also assumes that actors can choose the partner on the basis of their past experiences with the potential partners. With the help of a simple two-locus mathematical model we show that not only the gene that conditions the actor preference to choose good partners can increase in frequency but also the gene responsible of the good partner behavior.     

Competition for grooming partners and interference in affiliation among female mandrills

Grooming is the most common primate affiliative behaviour, and primates compete for accessing grooming partners. We studied a captive group of mandrills (Mandrillus sphinx) to evaluate the role of different types of competitive interactions in shaping the distribution of grooming among females. Mandrill females preferentially groomed high‐ranking individuals, but low‐ranking females were less able to do so. Interference in others’ grooming and a (consequent) reluctance of low‐ranking females to access dominant group mates occurred frequently and contributed to the observed pattern of grooming distribution, while takeovers of grooming partners was relatively rare. Interference in others’ affiliation was possibly used to prevent the formation of revolutionary alliances. Difficulties in accessing individuals already engaged in grooming exerted a strong but rank‐independent effect on grooming interactions. These results highlight the role of competition in determining access to preferred social partners.     

It's not just lunch: extra-pair commensality can trigger sexual jealousy

Do people believe that sharing food might involve sharing more than just food? To investigate this, participants were asked to rate how jealous they (Study 1)--or their best friend (Study 2)--would be if their current romantic partner were contacted by an ex-romantic partner and subsequently engaged in an array of food- and drink-based activities. We consistently find--across both men and women--that meals elicit more jealousy than face-to-face interactions that do not involve eating, such as having coffee. These findings suggest that people generally presume that sharing a meal enhances cooperation. In the context of romantic pairs, we find that participants are attuned to relationship risks that extra-pair commensality can present. For romantic partners left out of a meal, we find a common view that lunch, for example, is not "just lunch."     

Partner choice creates fairness in humans

Many studies demonstrate that partner choice has played an important role in the evolution of human cooperation, but little work has tested its impact on the evolution of human fairness. In experiments involving divisions of money, people become either over-generous or over-selfish when they are in competition to be chosen as cooperative partners. Hence, it is difficult to see how partner choice could result in the evolution of fair, equal divisions. Here, we show that this puzzle can be solved if we consider the outside options on which partner choice operates. We conduct a behavioural experiment, run agent-based simulations and analyse a game-theoretic model to understand how outside options affect partner choice and fairness. All support the conclusion that partner choice leads to fairness only when individuals have equal outside options. We discuss how this condition has been met in our evolutionary history, and the implications of these findings for our understanding of other aspects of fairness less specific than preferences for equal divisions of resources.     

Anticipation of conflict by chimpanzees

Captive chimpanzees appear to anticipate the occurrence of conflict during feeding by grooming and being in proximity at increased rates during the hour prior to feeding. The effect is more marked when food is clumped than when it is dispersed, suggesting that the proximate cause is the anticipation of increased levels of competition. Chimpanzees did not choose high ranking individuals more often as prefeed grooming partners; rather, they preferred to associate with their normal grooming partners (as reflected in post-feed grooming preferences) and close kin. A strong correlation between prefeed association patterns and spatial proximity during clumped feeding sessions suggests that their main concern is to be allowed to feed near individuals who are able to monopolize food sources.     

Partner switching can favour cooperation in a biological market

Intraspecific cooperation and interspecific mutualisms can be promoted by mechanisms that reduce the frequency with which cooperative organisms are exploited by unhelpful partners. One such mechanism consists of changing partners after interacting with an uncooperative individual. I used McNamara et al.'s (Nature, 451, 2008, 189) partner switching model as a framework to examine whether this mechanism can select for increased cooperative investment by house sparrows (Passer domesticus) collaborating to rear offspring; previous research on this species has shown that substantial cooperative investments by both pair members are required to achieve high pay‐offs from collaborating. I found that the poorer the outcome of a breeding attempt relative to the number of eggs the female invested, the greater the likelihood of partner switching. The incidence of partner switching changed seasonally, with peak switching coinciding with an increase in the number of alternative partners available to females. After females switched partners, their breeding outcomes rose to match those of females that remained with the same partner; this was not the case for males that switched partners. Consistent with the model's prediction, males in stable partnerships achieved over 25% higher than average reproductive success, which was attributable to both persistently good breeding outcomes and their older partners' high fecundity. These results provide empirical support for the hypothesis that partner switching favours increased cooperative investment levels, and they demonstrate that variation in the relative value of by‐product benefits can enhance that process.     

Preferential learning from non-affiliated individuals in jackdaws (Corvus monedula)

It has been suggested that affiliated social relations may facilitate information transfer between individuals. We here tested this rarely examined hypothesis with juvenile and adult jackdaws (Corvus monedula) in three stimulus enhancement tasks, both in a non-food context (experiment 1) and in a food context (experiments 2 and 3). We first show that siblings and pair partners maintain stronger bonded social relations than do non-siblings and non-pair partners. We therefore tested individuals in sibling and non-sibling dyads and, later in ontogeny, in pair and non-pair dyads. Jackdaws either did not learn from any other conspecific (experiment 1), or they learned from non-affiliated individuals (non-siblings, non-pair partners in experiments 2 and 3). This may be related to two main characteristics of jackdaws' affiliated relationships. First, affiliates share food at a high rate and may rely on their knowledgeable partners to secure food rather than learning from them. Second, affiliates spend most time in close spatial proximity to each other which increases the probability that they simultaneously experience occurrences in their environment. Hence, spatially more distant individuals, which are more likely to be non-affiliated, face different foraging situations and may therefore provide more relevant information which may lead to selective social learning.     

Intranasal oxytocin increases social grooming and food sharing in the common vampire bat Desmodus rotundus

Intranasal oxytocin (OT) delivery has been used to non-invasively manipulate mammalian cooperative behavior. Such manipulations can potentially provide insight into both shared and species-specific mechanisms underlying cooperation. Vampire bats are remarkable for their high rates of allogrooming and the presence of regurgitated food sharing among adults. We administered intranasal OT to highly familiar captive vampire bats of varying relatedness to test for an effect on allogrooming and food sharing. We found that intranasal OT did not have a detectable effect on food-sharing occurrence, but it did increase the size of regurgitated food donations when controlling for dyad and amount of allogrooming. Intranasal OT in females increased the amount of allogrooming per partner and across all partners per trial, but not the number of partners. We also found that the peak effect of OT treatments occurred 30–50 min after administration, which is consistent with the reported latency for intranasal OT to affect relevant brain areas in rats and mice. Our results suggest that intranasal OT is a potential tool for influencing dyadic cooperative investments, but measuring prior social relationships may be necessary to interpret the results of hormonal manipulations of cooperative behavior and it may be difficult to alter partner choice in vampire bats using intranasal OT alone.     

Territorial cooperation and social monogamy: factors affecting intersexual behaviours in pair-living snapping shrimp

Among the factors that may contribute to the evolution of social monogamy are selection for extended mate guarding of females and selection for territorial ‘cooperation’. Many socially monogamous taxa are also territorial, with ‘partners’ sharing a single territory, suggesting that one or both partners may benefit by sharing territorial maintenance. Snapping shrimp (genus Alpheus) are socially monogamous and territorial, living in excavated burrows or with host organisms, with females performing all parental care. The territorial cooperation hypothesis predicts that male and female partners share (1) territorial defence, resulting in a reduction in the risk of eviction from the burrow, (2) burrow construction duties, such that individuals in pairs spend less time in burrow construction relative to solitary individuals, and/or (3) foraging duties, by returning food to the burrow, where it is consumed by both partners. UsingA. angulatus as a model species, a territorial defence experiment revealed that females in pairs were significantly less likely than solitary females to be evicted by female intruders, but males in pairs were not significantly less likely than solitary males to be evicted by male intruders. A subsequent experiment revealed that paired males were significantly less likely to be evicted by an intruding male if paired with sexually receptive females than if paired with nonreceptive females. Another experiment revealed that (1) paired females spent significantly more time in burrow construction than paired males, and (2) both males and females consistently returned food items to the burrow, perhaps incidentally provisioning their mates. These data suggest that social monogamy may have been selected for in part because of the advantages of territorial cooperation, as both males and females are likely to benefit by dividing the labour of territorial defence and maintenance. These tests of the territorial cooperation hypothesis are synthesized with data from tests of the extended mate-guarding hypothesis to place snapping shrimp pairing behaviour into a larger construct incorporating both the influence of ecological pressures (territoriality) and mating interactions between the sexes. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Partner choice in human evolution: The role of cooperation,foraging ability,and culture in Hadza campmate preferences

The ability to choose the partners we interact with is thought to have been an important driver in the evolution of human social behavior, and in particular, our propensity to cooperate. Studies showing that humans prefer to interact with cooperative others is often cited as support for partner choice driving the evolution of cooperation. However, these studies are largely drawn from Western samples, where conditions for partner choice to operate may be especially favorable. Here, we investigate qualities associated with being a preferred partner (i.e., campmate) in Hadza hunter-gatherers of Tanzania in 2016 and 2019. A total of 156 Hadza participants from 17 camps ranked their campmates on generosity, foraging ability, and their preference for them as future campmates. In 2016, Hadza preferred more generous people and better hunters as campmates, with evidence suggesting a stronger preference for better hunters; however, the relationship between generosity and being a preferred campmate was greater in 2019 than in 2016, such that the preference for generous people was stronger than the preference for better foragers, suggesting that campmate preferences are changing. These new findings contrast with reports on data from nearly a decade ago, suggesting that the Hadza do not prefer more cooperative campmates. Further, in 2019, there was anecdotal evidence that Hadza with greater exposure to outside cultural institutions (e.g., schooling, having a job, or living in a village) had a stronger preference for generous campmates than those with less exposure. Taken together, the results suggest that preferences for social partners may, in part, be culturally shaped.     

Food access in captive Ammotragus: the role played by hierarchy and mother–infant interactions

An analysis of individuals' behavior when accessing a restricted food source (troughs) was carried out in a captive population of aoudad (Ammotragus lervia). Access to the troughs followed a strict hierarchical order, as higher‐ranking individuals fed before lower‐ranking ones. Unweaned male and female calves made use of the troughs from the ages of 2 and 3 months, respectively. Both fed from the troughs more frequently and for longer periods when their mother was present, which allowed them to make use of the troughs while skipping the hierarchical order. Calves received fewer threats when in proximity to their mothers, particularly in high‐ranking families. Mothers defended their calves from other herdmates more frequently when at the feeding area than in other areas of the herd. However, only sated mothers let their calves feed freely from the troughs; unsated mothers showed an aggressive behavior even toward their calves. I conclude that a maternal presence is necessary for aoudad calves to successfully feed from troughs, and that families of higher social rank benefit by getting access to this food source earlier in the day and are disturbed less than low‐ranking families. Zoo Biol 21:597–605, 2002. © 2002 Wiley‐Liss, Inc.     

Why hunt? Why gather? Why share? Hadza assessments of foraging and food-sharing motive

Over the last half century, anthropologists have vigorously debated the adaptive motivations underlying food acquisition choices and food-sharing among hunter-gatherer groups. Numerous explanations have been proposed to account for high-levels of generosity in food-sharing, including self- and family-provisioning, reciprocity, tolerated theft and pro-social- or skill-signaling. However, few studies have asked foragers directly and systematically about the motivations underlying their foraging and sharing decisions. We recruited 110 Hadza participants and employed a combination of free-response, yes/no, ranking and forced-choice questions to do just this. In free-response answers, respondents typically gave outcome-oriented accounts of foraging motive (e.g., to get food) and moralistic accounts of sharing motive (e.g., I have a good heart). In ranking tasks, participants gave precedence to reciprocity as a motive for sharing food beyond the household. We found small but clear gender differences in foraging motive, in line with previous predictions: women were more likely than men to rank family-provisioning highly whereas men were more likely than women to rank skill-signaling highly. However, despite these gender differences, the relative importance of different motivations was similar across genders and skill-signaling, sharing and family-provisioning were the most important motivators of foraging activity for both men and women. Contrary to the expectations of tolerated theft, peer complaints and requests for food ranked very low. There are several compelling reasons that evolutionary thinkers, typically interested in ultimate-level adaptive processes, have traditionally eschewed direct and explicit investigations of motive. However, these data may yet provide important insights.