Structural comparison of stress behavior in human's two closest living primate relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

Abstract

Despite their close genetic relationship, chimpanzees (Pan troglodytes) bonobos (Pan paniscus) exhibit significant differences in their socio-behavioral patterns. We can wonder if these might be reflected in their response to stress. However, observational studies comparing rates of stress in the two species are lacking and therefore limiting our understanding of what influences stress in chimpanzees and bonobos. Here we used focal and all occurrences behavioral observations to quantify and qualify the rates of four displacement behaviors – rough autoscratching, gentle autoscratching, yawning and nose wiping – that are known for being stress-related behaviors in primates. We observed that despite what was expected, bonobos and chimpanzees do not differ in their rates of displacement behaviors (with the exception of yawning) and that differences are actually higher when comparing within-species groups. Since correlations between the four displacement behaviors were low—except for the correlation between rough and gentle autoscratching in chimpanzees—it suggests that stress may be perceived in varied ways and can reflect different levels of arousal. Individual factors, such as sex, age, and aggression received influence displacement behaviors in chimpanzees and bonobos. Female chimpanzees were found to engage in higher levels of both rough and gentle autoscratching compared to males, while male bonobos exhibited more gentle autoscratching than females, reflecting the species-specific social structures. Older individuals showed fewer displacement behaviors, suggesting improved stress regulation with age. Interestingly, aggression received was linked to a decrease in gentle autoscratching, underscoring the role of context in interpreting displacement behaviors. These findings underscore the need for a broader approach when studying displacement activities in the future.