Sex Differences in Ranging and Association Patterns in Chimpanzees in Comparison with Bonobos

Comparative analysis of intragroup intermale relationships: a study of wild bonobos (Pan paniscus) in Wamba, Democratic Republic of Congo and chimpanzees (Pan troglodytes) in Kalinzu Forest Reserve, Uganda

Although chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) share a multi-male/multi-female societal organization and form male-philopatric groups, disparities in terms of male aggression and stability of temporary parties are thought to exist among them. However, existing research in bonobos has mainly focused on the high social status, prolonged receptivity, and characteristic sexual behaviors of females, leaving the behaviors of males understudied. Moreover, prior comparative studies on Pan suffer from methodological inconsistencies. This study addresses these gaps by employing a uniform observation method to explore party attendance and aggressive interactions among male bonobos in Wamba and male chimpanzees in Kalinzu. Unlike male chimpanzees, which exhibit dispersion in the absence of receptive females in the group, male bonobos showed a lesser degree of such dispersion. Although the overall frequency of aggressive interactions per observation unit did not significantly differ between the two species, the nature of these interactions varied. Notably, severe aggressive behaviors such as physical confrontations among adult males were absent in bonobos, with most aggression occurring between the sons of the two highest-ranking females. Additionally, in bonobos, females actively engaged in polyadic aggressive behavior as aggressors, while all instances of coalitionary aggression in chimpanzees originated from male aggressors. These findings underscore the substantial impact of female behaviors on the observed distinctions in male aggressive interactions between the two species.

Interindividual differences in crop foraging behavior of chimpanzees (Pan troglodytes schweinfurthii) at a forest–agriculture interface

The expansion of agriculture in equatorial areas is fragmenting and reducing wildlife habitats. For primates, it also increases opportunities to consume crops as high-energy resources, exacerbates conflicts with farmers, and increases exposure to diseases and agrochemicals at the edge of protected areas. In species with sex differences in ranging behavior, individual exposure to such opportunities and threats may vary by sex. Chimpanzees show a great feeding flexibility and are territorial species with varied ranging patterns according to site, sex, or individuals. Within a community whose territory is crossed by a high-traffic road and partially bordered by maize gardens, we tested hypotheses of interindividual differences in access to crops based on age, sex, ranging behavior, and kinship. By analyzing the presence of Sebitoli chimpanzees (Pan troglodytes schweinfurthii, Kibale National Park, Uganda) on video clips from 16 camera traps, we estimated the individual dispersion range across the community’s territory and the crop foraging frequency along maize gardens over 16 months. While all age and sex classes were represented at the forest–garden interface, large intrasex differences were observed: some mature males and females were not observed to participate. The crop foraging frequency of adult females in maize gardens was significantly correlated with the location of their ranging areas. Related individuals revealed similar range patterns within the forest territory without sharing crop foraging habits. However, social learning and energy and risks–benefits trade-offs as potential drivers of crop consumption are not excluded.

Attractiveness of female sexual signaling predicts differences in female grouping patterns between bonobos and chimpanzees

Here we show that sexual signaling affects patterns of female spatial association differently in chimpanzees and bonobos, indicating its relevance in shaping the respective social systems. Generally, spatial association between females often mirrors patterns and strength of social relationships and cooperation within groups. While testing for proposed differences in female-female associations underlying female coalition formation in the species of the genus Pan, we find only limited evidence for a higher female-female gregariousness in bonobos. While bonobo females exhibited a slightly higher average number of females in their parties, there is neither a species difference in the time females spent alone, nor in the number of female party members in the absence of sexually attractive females. We find that the more frequent presence of maximally tumescent females in bonobos is associated with a significantly stronger increase in the number of female party members, independent of variation in a behavioural proxy for food abundance. This indicates the need to look beyond ecology when explaining species differences in female sociality as it refutes the idea that the higher gregariousness among bonobo females is driven by ecological factors alone and highlights that the temporal distribution of female sexual receptivity is an important factor to consider when studying mammalian sociality.

Surbeck and colleagues investigate the proximate drivers of female gregariousness in bonobos and chimpanzees across different observed communities. Their findings indicate that varied levels of sexual signalling in these two species result in different social behaviours regarding female grouping and potentially cooperation.

Comparative social grooming networks in captive chimpanzees and bonobos

Despite similar dispersal patterns, models of Pan sociality emphasize sex differences in social bonding between the two species. Such disparities are attributed to hypothesized differences in environmental selective pressures that structure association patterns. However, recent research documents greater within-species variation in social bonds in both species. Here, we examine grooming networks in captive chimpanzees at the North Carolina Zoo, and captive bonobos at the Columbus Zoo. We hypothesized that male-female grooming relationships would be the strongest in both species, but that males and females of both species would not significantly differ between centrality, strength, or clustering. Via Mantel tests, we found that neither bonobos (t = - 0.070, r = - 0.009, two-tailed p = 0.942) nor chimpanzees (t = - 0.495, r = - 0.0939, two-tailed p = 0.6205) had significant differences in grooming between or within sexes. Neither species had significant sex differences in centrality, strength, or clustering. To account for idiosyncratic factors affecting grooming distribution, we examined the effect of origin, kinship, and group tenure on social network position. We found that wild-born bonobos exhibited greater eigenvector centrality (t = - 2.592, df = 9, p = 0.29) and strength (t = - 2.401; df = 9, p = 0.040), and group tenure was significantly correlated with strength (r = 0.608; N = 11, p - 0 = 0.47). None of these factors varied with social network position in chimpanzees. Our findings suggest that in captive settings, idiosyncratic factors related to individual history play a greater role in structuring social networks. Such variation may point to the behavioral flexibility inherent in fission-fusion networks, and mirror between-site variation found in wild chimpanzees. However, some idiosyncratic factors shaping captive networks may be an artifact of captivity.

Sex-specific association patterns in bonobos and chimpanzees reflect species differences in cooperation

In several group-living species, individuals' social preferences are thought to be influenced by cooperation. For some societies with fission-fusion dynamics, sex-specific association patterns reflect sex differences in cooperation in within- and between-group contexts. In our study, we investigated this hypothesis further by comparing sex-specific association patterns in two closely related species, chimpanzees and bonobos, which differ in the level of between-group competition and in the degree to which sex and kinship influence dyadic cooperation. Here, we used long-term party composition data collected on five chimpanzee and two bonobo communities and assessed, for each individual of 10 years and older, the sex of its top associate and of all conspecifics with whom it associated more frequently than expected by chance. We found clear species differences in association patterns. While in all chimpanzee communities males and females associated more with same-sex partners, in bonobos males and females tended to associate preferentially with females, but the female association preference for other females is lower than in chimpanzees. Our results also show that, for bonobos (but not for chimpanzees), association patterns were predominantly driven by mother-offspring relationships. These species differences in association patterns reflect the high levels of male-male cooperation in chimpanzees and of mother-son cooperation in bonobos. Finally, female chimpanzees showed intense association with a few other females, and male chimpanzees showed more uniform association across males. In bonobos, the most differentiated associations were from males towards females. Chimpanzee male association patterns mirror fundamental human male social traits and, as in humans, may have evolved as a response to strong between-group competition. The lack of such a pattern in a closely related species with a lower degree of between-group competition further supports this notion.

Obstacles and catalysts to peaceful coexistence in chimpanzees and bonobos

As our closest living relatives, comparisons of the social lives and behavioural ecologies of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide relevant insights into the evolutionary constraints of peaceful coexistence in Hominid societies. In this review, we compare and contrast findings from the two Pan species in order to examine some of the obstacles and catalysts for peaceful behaviour in our ape relatives. Through comparing the social structures, behavioural mechanisms and ecological drivers for peaceful behaviours in Pan, we develop hypotheses regarding the evolutionary constraints of peaceful co-existence in hominid societies.