Cooperation across social borders in bonobos

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.

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.

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.

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.