Determinants of social preferences in female chimpanzees

Because of their low rates of affiliative interactions, there has been a long-standing debate about whether or not female chimpanzees have social bonds similar to those found in male chimpanzees and females of many other primate taxa. In addition, transient partner associations in a fission-fusion context combined with variable space use overlaps between individuals pose unique challenges for identifying true social preferences based on spatiotemporal association patterns. In recent work, I resolved these challenges by applying a new statistical approach to the largest available dataset on female chimpanzee associations. My findings provide the strongest evidence to date for the existence of active social preferences and affiliative bonds among unrelated female chimpanzees (Foerster et al., 2015). But what mediated these social choices? I found that female associations were influenced by their offspring; in particular, mothers of juvenile sons were more likely to associate with each other compared to mothers of daughters. It is likely that such preferences facilitate early social bonding among males, which may increase cooperation and future reproductive success. Further, low-ranking females appeared to bond preferentially with each other; I am currently testing whether these preferred relationships increase access to high-quality feeding habitats or reduce aggression directed at females.

Ecological constraints on chimpanzee social behavior

The distribution and abundance of food resources are among the most important factors that influence animal behavior, but also among the most difficult ones to accurately quantify. For this reason, much uncertainty still exists about how primate sociality adapts to resource constraints. Over the last two years I collaborated with Ying Zhong (Duke University) to develop models of feeding habitat quality at small spatial scales. This new method combines maximum entropy modeling of food species distributions with data on diet composition to estimate the relative importance of locations as feeding habitat (Foerster et al, in review). This method now allows me to test whether social bonds increase access to better quality habitats using long-term data on recorded feeding locations during focal animal observations. In addition to studies of small-scale determinants of social behavior within social groups, some of the most powerful insights into selective pressures on behavior can be gained by comparative studies across populations. In collaboration with Zarin Machanda (Harvard University), I aim to identify population differences in constraints on female social relationships and their potential ecological correlates. Results will greatly advance our understanding of how ecological selective pressures mediate the evolution of primate social bonds and friendships.

Health consequences of chimpanzee social bonds

Social bonds can come with potential benefits such as increased access to resources or improved health and longevity. However, they can also increase the risk of pathogen transmission. The outcome of this tradeoff might critically influence the evolution of sociality, yet remains poorly understood. Collaborating with Thomas Gillespie (Emory University) and colleagues I tested whether chimpanzee social contact facilitates the transmission of gastrointestinal parasites, as suggested by recent studies on other species. Results show that more gregarious individuals (those who spend more time in larger subgroups) have more parasites and that, all else being equal, individuals with stronger grooming bonds have fewer parasites (Foerster et al., in prep.). These findings suggest that there is a measurable health benefit to social bonds and that an optimal social strategy may consist of minimizing gregariousness while maintaining strong grooming bonds. I found further support for the beneficial effects of social contact through a collaboration with Andrew Moeller (Yale University) and colleagues, which indicated that stronger social connections contribute to the maintenance of shared gut microbial communities, and possibly the evolution of adaptive microbiome composition within and between chimpanzee host generations (Moeller, Foerster, et al., accepted). Given research in other study systems on the interaction between gut microbiota and intestinal parasites, future work could test whether the sharing of adaptive microbial communities through social contact helps limit parasitic infections.