Jake S. Brooker’s research while affiliated with Durham University and other places

Sexual behaviour during tense social situations is extensively documented in various animals. Bonobos, our closest living relatives alongside chimpanzees, habitually perform genital contacts during social tension, which is thought to enhance cooperation and conflict management. While chimpanzees also engage in genital contacts during these contexts, the two sister species have yet to be compared systematically, which may have led to inaccurate assumptions. To address this, we directly compared genital and non-genital affiliation among sanctuary-living bonobos and chimpanzees during two socially tense contexts—post-conflict and pre-feeding. Following conflicts, we observed triadic affiliation between bystander–victim pairs and reconciliation between aggressor–victim pairs. Additionally, we experimentally induced a pre-feeding context to examine affiliative contact between group members. During post-conflict contexts, bonobos used genital contacts more than chimpanzees. However, both species used genital contacts comparably during pre-feeding affiliation, although female bonobos and male chimpanzees were most likely to initiate them. In addition, we found group-level variation indicating an influence of demographic factors. Our results indicate that chimpanzees and bonobos overlap significantly in their use of genital contacts during periods of social tension. Given similar evidence in humans, our results support the notion that this was a trait probably also present in our last common ancestor.

Early maternal loss can have lasting detrimental effects on primate social development. While many rehabilitation settings provide enriching environments to buffer against such effects in orphans, previous research indicates that young bonobo (Pan paniscus) orphans exhibit striking deficiencies in socio-emotional competence compared to their mother-reared peers. However, such studies are generally cross-sectional, without accounting for changes across the lifespan. We conducted longitudinal observations in bonobos living in an accredited African ape sanctuary to examine how rearing background, sex and age predict social tendencies including affiliation, consolation and aggression risk. Affiliative tendencies increased in females and decreased in males with age but were overall lower in orphans compared to mother-reared bonobos. Consolation tendencies decreased with age in mother-reared bonobos, while orphans showed consistently lower consolation (akin to levels of older mother-reared individuals). Young and male bonobos were more likely to receive aggression, while mother-reared and older females were more likely aggressors. Our study highlights the potential that ape sanctuaries like this can have by demonstrating that orphans exhibit decreased affiliative tendencies yet show social functioning ranging within patterns of their mother-reared peers. We discuss these results in the context of bonobos’ natural social ecology and ongoing rehabilitation efforts in this species.

Behavioural contagion—the onset of a species-typical behaviour soon after witnessing it in a conspecific—forms the foundation of behavioural synchrony and cohesive group living in social animals. Although past research has mostly focused on negative emotions or neutral contexts, the sharing of positive emotions in particular may be key for social affiliation. We investigated the contagion of two socially affiliative interactive behaviours, grooming and play, in chimpanzees. We collected naturalistic observations of N = 41 sanctuary-living chimpanzees at Chimfunshi Wildlife Orphanage, conducting focal follows of individuals following observations of a grooming or play bout, compared with matched controls. We then tested whether the presence and latency of behavioural contagion was influenced by age, sex, rank, and social closeness. Our results offer evidence for the presence of grooming and play contagion in sanctuary-living chimpanzees. Grooming contagion appeared to be influenced by social closeness, whilst play contagion was more pronounced in younger individuals. These findings emphasise that contagion is not restricted to negatively valenced or self-directed behaviours, and that the predictors of contagious behaviour are highly specific to the behaviour and species in question. Examining the factors that influence this foundational social process contributes to theories of affective state matching and is key for understanding social bonding and group dynamics.

Behavioural contagion—the onset of a species-typical behaviour soon after witnessing it in a conspecific—forms the foundation of behavioural synchrony and cohesive group living in social animals. Although past research has mostly focused on negative emotions or neutral contexts, the sharing of positive emotions in particular may be key for social affiliation. We investigated the contagion of two socially affiliative interactive behaviours, grooming and play, in chimpanzees. We collected naturalistic observations of N = 41 sanctuary-living chimpanzees at Chimfunshi Wildlife Orphanage, conducting focal follows of individuals following observations of a grooming or play bout, compared with matched controls. We then tested whether the presence and latency of behavioural contagion was influenced by age, sex, rank, and social closeness. Our results offer evidence for the presence of grooming and play contagion in sanctuary-living chimpanzees. Grooming contagion appeared to be influenced by social closeness, whilst play contagion was more pronounced in younger individuals. These findings emphasise that contagion is not restricted to negatively valenced or self-directed behaviours, and that the predictors of contagious behaviour are highly specific to the behaviour and species in question. Examining the factors that influence this foundational social process contributes to theories of affective state matching and is key for understanding social bonding and group dynamics.

Studying animal culture has been crucial for understanding the complexities of knowledge transmission and tracing human culture's evolutionary origins. Defined as the use of tools to provide clear practical benefits to individuals, well-documented examples of material culture include nut-cracking and termite fishing in chimpanzees. Additionally, there is growing interest in animal social traditions, which appear crucial for social interaction and group cohesion. We have previously documented such a tradition, in which chimpanzees copied inserting blades of grass in their ears from one persistent inventor. Now, over a decade later, we have observed an unrelated group of chimpanzees where 5/8 individuals began wearing grass in their ears and 6/8 from their rectums. As of 2024, one newly introduced chimpanzee has adopted the grass-in-ear behavior. Given that the behaviors were not observed in seven other groups in the same sanctuary ( N =148), we conclude that social learning of arbitrary behavior occurred and discuss our findings considering the larger scope of animal culture.

Empathy and its subcomponents are well documented throughout the animal kingdom, indicating the deep evolutionary origins of this socioemotional capacity. A key behavioural marker of empathy is consolation, or unsolicited bystander affiliation directed towards distressed others. Consolation has been observed in our closest living relatives, bonobos (Pan paniscus) and chimpanzees (P. troglodytes). However, systematic comparisons are absent, despite potential for interspecific differences. Bonobos are often considered less aggressive, more emotionally sensitive, and more socially tolerant than chimpanzees—key characteristics purported to drive consolation. Furthermore, social and individual factors also appear to drive intraspecific variation in empathy. To address within- and between-species variability in Pan consolation, we systematically tested the consolatory tendencies of large sanctuary-living groups of both species. Bonobos and chimpanzees exhibited similar consolation tendencies; however, within-species analyses revealed further similarities and variation. Bonobo consolation was most often directed towards and received by younger individuals, while chimpanzee consolation was most often directed towards close social partners. In addition, whilst males and females of both species showed decreased consolation with age, young chimpanzee males may console more than young females. These findings suggest that within-species variation in Pan socio-emotional abilities is greater than between-species differences, highlighting the presence of striking behavioural diversity across our two closest cousins.

Empathy is a complex, multi‐dimensional capacity that facilitates the sharing and understanding of others' emotions. As our closest living relatives, bonobos ( Pan paniscus ) and chimpanzees ( P. troglodytes ) provide an opportunity to explore the origins of hominin social cognition, including empathy. Despite certain assumptions that bonobos and chimpanzees may differ empathically, these species appear to overlap considerably in certain socio‐emotional responses related to empathy. However, few studies have systematically tested for species variation in Pan empathic or socio‐emotional tendencies. To address this, we synthesise the growing literature on Pan empathy to inform our understanding of the selection pressures that may underlie the evolution of hominin empathy, and its expression in our last common ancestor. As bonobos and chimpanzees show overlaps in their expression of complex socio‐emotional phenomena such as empathy, we propose that group comparisons may be as or more meaningful than species comparisons when it comes to understanding the evolutionary pressures for such behaviour. Furthermore, key differences, such as how humans and Pan communicate, appear to distinguish how we experience empathy compared to our closest living relatives.

Bonobos are typically portrayed as more socially tolerant than chimpanzees, yet the current evidence supporting such a species-level categorization is equivocal. Here, we used validated group-level co-feeding assays to systematically test expressions of social tolerance in sixteen groups of zoo- and sanctuary-housed bonobos and chimpanzees. We found that co-feeding tolerance substantially overlaps between the species, thus precluding categorical inference at the species level. Instead, marked differences were observed between groups, with some bonobo communities exhibiting higher social tolerance than chimpanzee communities, and vice versa. Moreover, considerable intergroup variation was found within species living in the same environment, which attests to Pan’s behavioral flexibility. Lastly, chimpanzees showed more tolerance in male-skewed communities, whereas bonobos responded less pronounced to sex-ratio variation. We conclude that the pervasive dichotomy between the tolerant bonobo and the belligerent chimpanzee requires quantitative nuance, and that accurate phylogenetic tracing of (human) social behavior warrants estimations of intraspecific group variation.

Researchers have studied non-human primate cognition along different paths, including social cognition, planning and causal knowledge, spatial cognition and memory, and gestural communication, as well as comparative studies with humans. This volume describes how primate cognition is studied in labs, zoos, sanctuaries, and in the field, bringing together researchers examining similar issues in all of these settings and showing how each benefits from the others. Readers will discover how lab-based concepts play out in the real world of free primates. This book tackles pressing issues such as replicability, research ethics, and open science. With contributors from a broad range of comparative, cognitive, neuroscience, developmental, ecological, and ethological perspectives, the volume provides a state-of-the-art review pointing to new avenues for integrative research.

Empathy, the capacity to share and understand others’ states, is crucial for facilitating enduring social relationships and managing ingroup and outgroup dynamics. Despite being at the center of much scrutiny and debate in human research, the evolutionary foundations of empathy remain relatively opaque. Moreover, inconsistencies remain regarding definitions and theoretical models, leading to discrepancies in how to systematically represent and address empathy and understand its evolutionary basis. As a complex, multi-dimensional phenomenon, certain components of empathy are likely to be evolutionarily ancient whereas others may be more derived. As our closest living relatives, nonhuman primates provide an opportunity to explore the evolutionary origins of empathy and its subcomponents. Due to the rich diversity of primate societies, we can comparatively study evidence of affective responding and empathic behaviour within the context of different social dynamics and organisation. Although studies have been conducted on individual primate species, especially the great apes, direct species comparisons are rare. Here we examine the literature investigating evidence for empathy among primates focussing on its underlying affective and cognitive components. In reviewing the literature, we also highlight species that need more coverage to enhance our overall understanding of how empathy has evolved within the primate order.