Cooperation across social borders in bonobos

Divergence in bonobo and chimpanzee social life

New research is shedding light on the nuances and complexity of social relationships in our closest relatives, revealing cooperative intergroup relationships in bonobos, in contrast to lethal intergroup violence in chimpanzees. At the same time, intragroup relationships, at least among males, are characterized by higher frequencies of aggression and lower rates of coalitionary cooperation in bonobos than chimpanzees.

Investigating Changes in Social Networks Following Conflict in Zoo-Housed Bonobos (Pan paniscus)

Bonobos (Pan paniscus) are successful at managing conflict and promoting group affiliation, but it is still uncertain how conflict affects the entire social group, particularly in captive populations. We conducted social network analyses using dyadic proximity data to understand the structure of five alternating social group compositions of a population of zoo-housed bonobos, using measures of centrality and social strength. We then compared social network measures between neutral, post-conflict (intergroup and intragroup), and post-single-party outburst (scream, display to the public) contexts to determine how conflict influences social behavior within the entire social network. We found that, across conflict contexts, dominant females have higher social group centralities than other individuals. Dominant females also received social initiations at a higher rate than others, suggesting this centrality is structurally maintained by other individuals. Further, rates of proximity are significantly higher following single-party outbursts than in other contexts, which indicates an important social distinction between this context and others and may be best explained by considering these outbursts as signals of group-level social tension, during which group members may seek social interaction and reassurance. Consistent differences in proximity rates were observed between dominant females and other group members, indicating that the social role of the dominant female in a bonobo social group across conflict contexts is different from that of other bonobos. The results of this study emphasize the flexible nature of bonobo sociality, highlight the distinct and important role of dominant females within the social network, and present social network analysis as a useful methodological tool for studying group-level changes in bonobo sociality and behavior.

Revisiting the spaces of societies and the cooperation that sustains them

We embrace Moffett's call for more rigorous definitions of social organizations but raise two intersecting critiques: (1) The spaces controlled by societies are not exclusively physical, and (2) cooperation is required to maintain control over spaces, physical or otherwise. We discuss examples of non-physical societal spaces across species and highlight the top-down group cooperation challenge that is maintaining them.

Emotions mediate nonlinear phenomena production in the vocalizations of two ape species

Nonlinear phenomena (NLP) are widely observed in mammal vocalizations. One prominent, albeit rarely empirically tested, theory suggests that NLP serve to communicate individual emotional states. Here, we test this 'emotional hypothesis' by assessing NLP production in the vocalizations of chimpanzees and bonobos across various social contexts. These two species are relevant to test this hypothesis since bonobos are more socially opportunistic than chimpanzees. We found that both species produced, albeit at different frequencies, the same five distinct NLP types. Contextual valence influenced NLP production in both species with negative valence being associated with more frequent NLP production than positive and neutral valence. In contrast, using aggression severity and caller role as proxies for arousal, we found that in bonobos, but not in chimpanzees, vocalizations uttered during contact aggression or from victims and females contained more NLP. In contrast, the type of NLP produced was neither influenced by valence nor arousal in either species. Our study supports the emotional hypothesis regarding the occurrence of NLP production in mammals, particularly in opportunistics such as bonobos. This reinforces the hypothesis of an adaptative role of NLP in animal communication and prompts further investigations into their communicative functions.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Bonobos respond aversively to unequal reward distributions

Inequity aversion (IA) is the resistance to unequitable rewards given similar investments. It has been postulated as an important mechanism by which human cooperation thrives. To understand the evolutionary origin of human IA and its distribution across the animal kingdom, many species have been tested on IA, with mixed results. Whereas chimpanzees were originally found to show IA, more recent studies showed that their IA response could be explained by social disappointment. We conducted two studies on IA in bonobos using established paradigms: a token-exchange task and the social disappointment task. Bonobos could exchange tokens for equal or less-preferred food rewards than their partners (Study 1) and were tested with humans and machines to control for social disappointment effects (Study 2). We found that bonobos responded aversively to unequal food distributions in both studies, which was reflected by more refusals to participate when disadvantaged. Notably, and contrary to chimpanzees, this effect could not be explained by social disappointment, although Study 2 was only partially consistent with an IA explanation. Overall, our findings indicate that bonobos possess the sensitivity to recognize and respond to unfair treatment, which supports the notion that IA may have coevolved as a stabilizing mechanism for cooperation.

The natural history of social bonds

This paper reviews the evolutionary processes that shape the evolution of sociality in mammalian species in an effort to understand the importance of sociality in the lives of modern humans. A body of theory and empirical evidence compiled by behavioral ecologists helps us to understand why (some) other animals live in groups, why group-living animals form differentiated social bonds, how animals benefit from their social connections, and why some individuals are more social than others in their groups. Together, the answers to these questions help us to understand why humans are such social creatures, and why our social connections play such an important role in our lives.

Multi-level societies: different tasks at different social levels

Multi-level vertebrate societies, characterized by nested social units, allow individuals to perform a wide range of tasks in cooperation with others beyond their core social unit. In these societies, individuals can selectively interact with specific partners from higher social levels to cooperatively perform distinct tasks. Alternatively, social units of the same level can merge to form higher-level associations, enabling individuals to benefit from large social units without always maintaining a large core social unit. The reasons why multi-level sociality evolves in some systems but not in others are not well understood. We propose that this is partly due to a lack of data, especially regarding the fitness consequences of cooperation at different social levels. First, we argue that in multi-level societies individual fitness benefits should increase when performing tasks in cooperation with associates from higher social levels. Second, as more multi-level societies are documented across taxa, we will continue to find similar cooperative tasks performed at each of the different social levels. By providing compelling species examples, from dolphins to fairy-wrens, we underscore that despite the diversity of multi-level social organization, convergence in task performance across social levels will become clearer as more data accumulates. Finally, we highlight the role of multi-level sociality in buffering fluctuating environmental conditions by enabling flexible social associations to emerge according to need.This article is part of the theme issue 'Division of labour as key driver of social evolution'.

Gut microbiota alterations in golden snub-nosed monkeys during food shortage and parturition-nursing periods

Adopting unique survival strategies during spring food shortages and simultaneous parturition and nursing is crucial for golden snub-nosed monkeys. Social behaviors, such as altruism within one-male units (OMUs), are decisive for family health, but the role of microbiota in regulating these behaviors remains unknown. We conducted the gut microbiota from members of 10 OMUs using 16S RNA sequencing technology. We found that in adult males, gut microbiota diversity significantly decreased in food shortages and parturition-nursing period. Meanwhile, there was a notable reduction in 12 metabolism-related pathways, including those related to carbohydrates, amino acids, and lipid. The gut microbiota of adult male monkeys shifts from being enriched with the genera Akkermansia in winter to the genera norank Muribaculaceae in spring. This transition alters the pathways for nutrient acquisition, thereby reducing the consumption of stored energy. In contrast, other OMU members (adult females and subadults) did not experience adverse effects on the metabolic functions of their gut microbiota during the food-scarce spring, which is also a critical period for parturition and lactation in adult females. This study elucidates the co-evolution of altruistic behavior and gut microbiota in Sichuan snub-nosed monkeys, insights into the interaction mechanisms between mammalian microbiota and survival strategies.

Free mobility across group boundaries promotes intergroup cooperation

Group cooperation is a cornerstone of human society, enabling achievements that surpass individual capabilities. However, groups also define and restrict who benefits from cooperative actions and who does not, raising the question of how to foster cooperation across group boundaries. This study investigates the impact of voluntary mobility across group boundaries on intergroup cooperation. Participants, organized into two groups, decided whether to create benefits for themselves, group members, or everyone. In each round, they were paired with another participant and could reward the other's actions during an 'enforcement stage', allowing for indirect reciprocity. In line with our preregistered hypothesis, when participants interacted only with in-group members, indirect reciprocity enforced group cooperation, while intergroup cooperation declined. Conversely, higher intergroup cooperation emerged when participants were forced to interact solely with out-group members. Crucially, in the free-mobility treatment - where participants could choose whether to meet an in-group or an out-group member in the enforcement stage - intergroup cooperation was significantly higher than when participants were forced to interact only with in-group members, even though most participants endogenously chose to interact with in-group members. A few 'mobile individuals' were sufficient to enforce intergroup cooperation by selectively choosing out-group members, enabling indirect reciprocity to transcend group boundaries. These findings highlight the importance of free intergroup mobility for overcoming the limitations of group cooperation.

The extent of western lowland gorilla social relationships within and between groups

The nature of western lowland gorilla social relationships within and between groups is largely understudied, partly due to the challenges of monitoring associations between individuals who live in neighboring groups. In this study, we examined the social relationships of four western lowland gorilla groups in the Ndoki landscape of northern Republic of Congo. To do so, we compiled all-occurrence social interaction and silverback nearest neighbor social networks from data collected during daily group follows conducted over several years. We observed a total of 5,923 dyadic all-occurrence social interactions (1,350 ± 489 per group, 138 intergroup interactions) and 54,989 dyadic silverback nearest neighbor associations (13,747 ± 3,963 observations per group, 105 nearest neighbor observations of intergroup partners during group scans). For all groups, we found that males were more social than females, younger individuals were more social than older gorillas, and slightly greater rates of social behaviors were observed during periods of higher fruit availability. While there was a considerable amount of interindividual variation in social behavior, the network of social interactions demonstrated a large extent of social relationships within and between groups. Additionally, we performed simulated network removals to assess the impact on social dynamics. Across all groups and the total population, the removal of blackback and immature individuals markedly decreased the number of intra- and intergroup relationships (>60% decrease). The documented extent of western lowland gorilla social relationships has direct implications for the conservation of species with multi-level social dynamics. Gaining clarity on the ways in which western lowland gorilla groups naturally occur in the wild, not only provides a greater understanding for their conservation, but also offers insights for managing their social dynamics within captive environments.

Primate Behavior and the Importance of Comparative Studies in Biological Anthropology

Biological anthropology seeks to understand humans from an evolutionary perspective. Namely, what makes humans different from other animals, and how did we get this way? Many relevant traits are physical, but many others are behavioral. For example, when and why did our species develop complex cognition, enduring bonds, and intense cooperation? Given the importance of behavior, biological anthropologists have a long history of turning to our primate relatives to generate hypotheses about the evolutionary processes shaping humans. Indeed, primate behavior is foundational to our field. But not all biological anthropologists appreciate the value of primate behavior for understanding human evolution. Beyond lip service in introductory paragraphs and grant proposals, many primatologists do not make explicit how their work is relevant to human evolution. In this review, we have three main goals: (1) emphasize how comparative studies of primate behavior are crucial to biological anthropology; (2) outline how primatologists and biological anthropologists can improve their work by avoiding common problems that arise when making such comparisons; and (3) provide a primer on the concepts and methods underlying comparative analyses of traits. We provide examples to highlight these points related to cognition, sociality, and diet. We conclude with several recommendations including (1) detailed, high-quality studies of behavior that allow for appropriate comparisons within and across species; (2) using primates as a "gateway clade" and expanding our research to any relevant taxa; and (3) careful attention to the ethical implications of making comparisons to other primates given racist tropes and a history of eugenics.

Group-level signatures in bonobo sociality

Humans show remarkable differences in social behaviour between families, groups, communities and cultures, whereas such group-level within-species variation in socio-behavioural propensities is typically overlooked in other species. Studies on intraspecific variation in animal social structures are needed to inform an evolutionary account of human sociality. Here, we study multiple independent bonobo populations (n = 6) in zoological settings to investigate if and how bonobos (n = 70) show group-specific signatures in sociality. By applying tailored Bayesian statistical methods, we find that beyond individual and dyadic variation, the groups substantially differ from each other in core dimensions of great ape sociality: social proximity, grooming and play. Moreover, the groups' network structures are distinct regarding cohesiveness and clustering, with some groups forming cohesive wholes, while others showcasing high levels of sub-grouping. Overall, while there is consistent evidence of differences in sociality between the groups, the patterns of cohesiveness and clustering are not consistent across the networks. This suggests that rather than groups having different levels of sociality, different patterns of sociality exist in each group. These findings warrant caution with characterising bonobos' behavioural phenotype at the species level, and identify an essential source of variation that needs to be integrated in phylogenetic analyses.

Beware! Different methods lead to divergent results on yawn contagion modulation in bonobos

Contagious yawning (CY)-linked to physiological synchronization and possibly emotional contagion-occurs when one individual's yawn induces yawning in others. CY was investigated over different time windows (minutes from the triggering stimulus) via naturalistic or experimental studies (using real and video yawns, respectively) with contrasting results, especially in bonobos. We verified whether in bonobos result divergences may derive from different methods. We gathered yawning data on 13 bonobos at Twycross Zoo (UK) via a naturalistic (all-occurrences observations) and experimental approach (by showing yawn/control video stimuli). Based on literature, we used 1- and 3-min windows to detect CY. Due to fission-fusion management, individuals could form permanent or non-permanent associations (more/less familiar subjects under naturalistic setting). Video yawn stimuli may come from group mates/stranger models (more/less familiar subjects under the experimental setting). Stimulus type and time window affected CY modulating factors but not CY detection. Familiarity and age effect on CY showed opposite trends in 3-min trials and 1-min observations. CY was highest in oldest, non-permanently (rather than permanently) associated subjects in the naturalistic setting, but in the youngest subjects and with ingroup (rather than outgroup) models in trials. The age effect differences on CY might be due to decontextualized yawns and immature subject curiosity toward videos. The reversed familiarity effect suggests CY's context-dependent function in promoting social synchronization with socially distant group mates, as failing to coordinate as a group may lead to social disruption. Complementary methods are needed to fully understand motor replication phenomena.

Testing the information centre hypothesis in a multilevel society

In various animal species conspecifics aggregate at sleeping sites. Such aggregations can act as information centres where individuals acquire up-to-date knowledge about their environment. In some species, communal sleeping sites comprise individuals from multiple groups, where each group maintains stable membership over time. We used GPS tracking to simultaneously record group movement in a population of wild vulturine guineafowl (Acryllium vulturinum) to investigate whether communal sleeping sites can facilitate the transfer of information among individuals across distinct groups. These birds live in large and stable groups that move both together and apart, often forming communal roosts containing up to five groups. We first test whether roosts provide the opportunity for individuals to acquire information from members of other groups by examining the spatial organization at roosts. The GPS data reveal that groups intermix, thereby providing an opportunity for individuals to acquire out-group information. We next conduct a field experiment to test whether naïve groups can locate novel food patches when co-roosting with knowledgeable groups. We find that co-roosting substantially increases the chances for the members of a naïve group to discover a patch known to individuals from other groups at the shared roost. Further, we find that the discovery of food patches by naïve groups subsequently shapes their space use and inter-group associations. We also draw on our long-term tracking to provide examples that demonstrate natural cases where communal roosting has preceded large-scale multi-group collective movements that extend into areas beyond the groups' normal ranges. Our findings support the extension of the information centre hypothesis to communal sleeping sites that consist of distinct social groups.

Selective and prolonged attention to emotional scenes in humans and bonobos

Perceiving emotions in others is at the foundation of higher-order social cognition. The importance of emotions is evidenced by the fact that they receive prioritized attention at early stages of processing the environment in humans and some other primates. Nevertheless, we do not fully understand how emotion modulates attention over longer durations in primates, particularly in great apes. Bonobos, one of our closest relatives, stand out in emotion processing and regulation among great apes. This makes them an interesting comparison species and a valuable model for studying the evolution of emotion perception in hominids. We investigated how bonobos and humans spontaneously attend to emotionally valent scenes in a preferential looking task using eye-tracking. With Bayesian mixed modelling, we found that bonobos and humans generally looked longer at emotional scenes, mainly of conspecifics. Moreover, while bonobos did not have a bias toward emotional human scenes, humans sustained their attention toward bonobos playing, grooming and having sex. Furthermore, when exploring an immediate bias for emotions, humans showed a bias toward affiliative human scenes, and bonobos showed a bias away from bonobos-in-distress scenes. These findings suggest that emotions modulate attention at early and later attentional stages in bonobos, similar to humans.

Animal behavior: A tale of two apes

A new paper shows that rates of aggression are higher, and rates of coalition formation are lower, among male bonobos than among male chimpanzees. These findings are noteworthy because they challenge the view that female bonobos' preferences for less aggressive males favored a reduction in male aggression and an increase in social tolerance.

Janus faced: The co-evolution of war and peace in the human species

The human species presents a paradox. No other species possesses the propensity to carry out coalitionary lethal attacks on adult conspecifics coupled with the inclination to establish peaceful relations with genetically unrelated groups. What explains this seemingly contradictory feature? Existing perspectives, the "deep roots" and "shallow roots" of war theses, fail to capture the plasticity of human intergroup behaviors, spanning from peaceful cooperation to warfare. By contrast, this article argues that peace and war have both deep roots, and they co-evolved through an incremental process over several million years. On the one hand, humans inherited the propensity for coalitionary lethal violence from their chimpanzee-like ancestor. Specifically, having first inherited the skills to engage in cooperative hunting, they gradually repurposed such capacity to execute coalitionary killings of adult conspecifics and subsequently enhanced it through technological innovations like the use of weapons. On the other hand, they underwent a process of cumulative cultural evolution and, subsequently, of self-domestication which led to heightened cooperative communication and increased prosocial behavior within and between groups. The combination of these two biocultural evolutionary processes-coupled with feedback loop effects between self-domestication and Pleistocene environmental variability-considerably broadened the human intergroup behavioral repertoire, thereby producing the distinctive combination of conflictual and peaceful intergroup relations that characterizes our species. To substantiate this argument, the article synthesizes and integrates the findings from a variety of disciplines, leveraging evidence from evolutionary anthropology, primatology, archeology, paleo-genetics, and paleo-climatology.

Orangutan males make increased use of social learning opportunities, when resource availability is high

Humans' colonization of diverse habitats relied on our ancestors' abilities to innovate and share innovations with others. While ecological impacts on innovations are well studied, their effect on social learning remains poorly understood. We examined how food availability affects social learning in migrant orangutan unflanged males, who may learn from local orangutans through peering (i.e., observational social learning). We analyzed 1,384 dyadic associations, including 360 peering events, among 46 wild Sumatran orangutan and 25 Bornean orangutan males, collected over 18 years. Migrants' peering rates significantly increased with higher food availability and time spent in proximity to others. Furthermore, migrants in the more sociable Sumatran population exhibited significantly higher peering rates compared to the Borneans, suggesting intrinsic and/or developmental effects of food availability on social learning. These findings emphasize the importance of investigating ecological effects on social learning on the immediate, developmental, and intrinsic levels for our understanding of cultural evolution.

Between-group cooperation in bonobos

Bonobos provide insight into the origins of partner-specific cooperation in human groups.