Zarin Pearl Machanda

Injuries are a significant, recurring stressor across the lifespan. In species where injuries occur primarily from conspecific aggression, injury risk is expected to most affect those who are targeted for aggression or who are physically or socially disadvantaged. We test that assumption by examining patterns and predictors of injury in two longitudinal datasets comprising 955 injuries sustained by wild East African chimpanzees (Pan troglodytes schweinfurthii) in Gombe and Kibale National Parks. Most injuries were open wounds, and most such wounds were inflicted by conspecifics, particularly adult males. Female injury risk was strongly predicted by the rate of contact aggression received and increased for females with sexual swellings. Although socially dominant to females, adult males were injured more often. Receiving contact aggression did not predict male injury risk in either population. In Kibale, high-ranking males and those most aggressive to others received the most injuries, adding to evidence from this species that success in an intensely competitive mating market involves costly and persistent trade-offs with somatic health. Our results demonstrate that injury risk cannot be inferred from patterns of aggression alone. In particular, sexual selection on male physical traits and competitive behaviour escalates risks from male–male conflict.

Objectives: Sex differences in social behavior and status are pervasive across primates and other mammals. In the wild, chimpanzees ( Pan troglodytes ) exhibit many sex‐typed behaviors: adult female chimpanzees exhibit lower aggression, are subordinate to adult males, and also are generally less social than males. This pattern is thought to partly reflect the energetic constraints (e.g., feeding competition) that wild female chimpanzees face. Materials and Methods: We test the drivers of sex differences in chimpanzee behavior by examining a semi‐free‐ranging African chimpanzee sanctuary group where individuals are provisioned, an environment that should relax ecological constraints on socializing. Using two years of focal observations on a group with 45 chimpanzees (17 males, 28 females), we examined sex differences in social proximity, grooming, aggression, coalition formation, and dominance. Results: In contrast to patterns in wild chimpanzees, we found that males and females in this population exhibited comparable rates of affiliative behavior. Males engaged in more aggressive behavior overall than females, as in the wild. However, females were equally likely to aggress either sex, and a high proportion of female aggression involved coalitions. Finally, David's scores revealed that a few of the adult females outranked some of the lower‐status adult males. Discussion: These findings show that sex differences in chimpanzee social behavior are partially flexible, and females may show more affiliation, cooperation, and higher status when ecological conditions are favorable. More generally, some forms of female power can emerge even in a species with strong sex biases in behavior for male affiliation and status.

Across social species, strong social bonds are linked to health benefits, including reduced disease risk and increased survival. Chronic glucocorticoid exposure, frequently associated with negative health outcomes, may mediate this relationship, as bonds may reduce stressor exposure or buffer physiological responses. Among primates, these relationships have mostly been studied in female-bonded species, where kinship shapes cooperation and access to resources. Whether bonds confer simsilar benefits in species with different social structures – especially bonds between non-kin – remains unclear. We tested whether strong affiliative bonds were associated with glucocorticoid production in wild adult chimpanzees using 22 years of behavioural and urinary cortisol data. Bonds were quantified for same-sex adult dyads using an index of party association, proximity, and grooming. Bond strength was measured by averaging each individual’s top three bonds per year. Stronger bonds predicted lower cortisol in females but higher cortisol in males, a pattern that persisted after accounting for dominance rank, aggression, and contextual stressors. In this male-bonded species, stronger male bonds predicted higher physiological costs, while stronger female bonds, though weaker, appeared to attenuate stress. Our results challenge the assumption that social bonds are universally health-promoting, and suggest their physiological consequences vary with sex and social organization.

The ability to follow another's gaze direction is a foundational skill in human cognition that underpins more complex social abilities. Other primate species also exhibit a variety of gaze‐following behaviors, but previous experimental work has focused only on captive populations. Studies of cognition in wild animals are crucial to understanding the biological context of different psychological processes, as well as to characterize developmental change in cognition in animals interacting in natural social groups. We therefore conducted field experiments with wild chimpanzees ( Pan troglodytes schweinfurthii ) ranging from infancy to old age in Kibale National Park, Uganda. Chimpanzees observed a human demonstrator who either looked up on test trials or down on control trials across up to four trials presented on different days. We found that wild chimpanzees reliably followed the actor's gaze, looking up more often during test trials where she looked up compared to control trials where she looked down. We further characterized the development of this skill and found that whereas juveniles and younger adults consistently followed the demonstrator's gaze, infants and older adults did not. Our findings show that even chimpanzees without a history of social interactions with humans will reliably follow gaze, providing the first evidence that wild primates show such co‐orienting responses. This work further provides support for the hypothesis that chimpanzees show distinct developmental trajectories for gaze‐following compared to humans, even in wild social contexts. Summary: Gaze‐following is a foundational social cognitive skill shared with several other primate species, but all studies to date have been conducted with captive populations. We conducted the first test of gaze‐following through field experiments on wild chimpanzees. We found that wild chimpanzees, who do not have a history of social interactions with humans, follow a human demonstrator's gaze even in the absence of prior experience. Robust gaze‐following did not emerge until the juvenile period, providing support for the hypothesis that chimpanzees show developmental patterns of co‐orienting that are distinct from human trajectories.

The social intelligence hypothesis proposes that the demands of social life shape the evolution of cognition, but different aspects of social interactions may be relevant. To test how competitive versus cooperative interactions shape social cognition, we assessed multiple metrics of social cognition in Barbary macaques ( Macaca sylvanus , n = 40) and rhesus macaques ( Macaca mulatta , n = 60). These closely related species have similar social organization, but diverge in social styles: Barbary macaques are more tolerant, whereas rhesus macaques are more despotic. Monkeys completed a battery of experimental tasks measuring gaze-following (co-orienting with others), knowledge attribution (representing others’ underlying knowledge states), goal attribution (interpreting others’ actions in terms of underlying intentional goals) and temperament (boldness in response to exploring novelty). While the rhesus macaques were more willing to approach a novel object than were Barbary macaques, both species showed similar success in each social task. However, individual Barbary macaques were more likely to show greater overall proficiency across all social measures combined than were individual rhesus monkeys. Overall, these results indicate that similar social cognitive capacities may evolve in distinct social contexts, and suggest socio-cognitive skills may be relevant for both competitive and cooperative interactions in primates. This article is part of the Theo Murphy meeting issue ‘Selection shapes diverse animal minds’.

The use of tools to drink water is well‐documented in wild chimpanzees, but the specific function of this behavior is unclear. Here we use a large data set of drinking behaviors spanning 14 years of observation from the Kanyawara community of chimpanzees living in Kibale National Park, Uganda, to test two possible functions of leaf‐sponges and other drinking tools. On the one hand, chimpanzees may use tools to access water that is hard to reach, which predicts that chimpanzees will preferentially use tools to drink at tree holes and crevices compared to all other locations. Conversely, chimpanzees may use these tools to filter stagnant water, in which case they would use tools more often at holes and puddles compared to running water sources (e.g., streams). We compared both likelihood of using a tool to drink at different locations, as well as overall rates of drinking, and found chimpanzees in this community most often drink from streams without tools. However, when they do use tools, they preferentially do so to drink at tree holes. Given known age and sex effects on tool use in chimpanzees, we also examined demographic variation in drinking tool use to understand the emergence of this behavior. While females use tools more often than males overall—in part driven by differences in drinking rates at different locations—both males and females use tools more frequently at tree holes than other locations when they do drink there. Finally, comparisons by age indicate that this selectivity strengthens over development with older chimpanzees showing a more pronounced effect of using tools more often at tree holes, suggesting that younger chimpanzees may exhibit exploratory tool use behavior. These results pinpoint the specific function of tool use during drinking and further suggest that even simple tools may require learning for use in appropriate contexts.

Environmental education research methods often focus on measuring changes in people’s attitudes toward conservation. While attitudes are an important indicator of change, it is critical to target incentivised behaviour because conservation efforts often involve behavioural changes that are costly to one’s self (e.g., reducing water use, purchasing pricier alternatives). In the present study, we measured both attitudes and behaviours in a short-term conservation education program near the primate-rich Kibale National Forest in Western Uganda. Adolescents () participated in a three-hour curriculum over three weeks integrated into an after-school wildlife club. After almost 30 years of successful primate conservation education in this region, we chose a novel taxonomic group (bats) for this study. Our pre-registered study had two conditions: an experimental condition, in which students learned about bats as an important local pollinator species, and a control condition, in which students learned about the moon. Only the experimental condition emphasised conservation’s delayed benefits. We compared attitudes, knowledge, and behaviours pre- and post- curriculum. To capture behaviour, we employed an investment task in which participants could invest their own resources in a ‘fund’ to support work relating to bats or the moon, depending on condition. Across both conditions, participants showed positive shifts in attitudes and knowledge from pre- to post-measures and showed increased investments in bat- and moon-related funds. These findings reveal that short-term conservation education influences adolescents’ investment decisions as well as their attitudes and knowledge. Our method can easily be edited for conservation education of any species including endangered primates.

Human communication is strikingly multi-modal, relying on vocal utterances combined with visual gestures, facial expressions and more. Recent efforts to describe multi-modal signal production in our ape relatives have shed important light on the evolutionary trajectory of this core hallmark of human language. However, whilst promising, a systematic quantification of primate signal production which filters out random combinations produced across modalities is currently lacking. Here, through recording the communicative behaviour of wild chimpanzees from the Kibale forest, Uganda we address this issue and generate the first repertoire of non-random combined vocal and visual components. Using collocation analysis, we identify more than 100 vocal-visual combinations which occur more frequently than expected by chance. We also probe how multi-modal production varies in the population, finding no differences between individuals as a function of age, sex or rank. The number of visual components exhibited alongside vocalizations was, however, associated with vocalization type and duration. We demonstrate that chimpanzees produce a vast array of combined vocal and visual components, exhibiting a hitherto underappreciated level of combinatorial complexity. We conclude that a multi-modal approach is crucial to accurately representing the communicative abilities of non-human primates.