Abstract
Accessing animal minds has remained a challenge since the beginnings of modern science. Here, we used a little-tried method, functional infrared thermal imaging, with wild chimpanzees during common social interactions. After removing confounds, we found that chimpanzees involved in competitive events had lower nose skin temperatures whereas those involved in cooperative events had higher temperatures, the latter more so in high- than low-ranking males. Temperatures associated with grooming were akin to those of cooperative events, except when males interacted with a non-reciprocating alpha male. In addition, we found multiple audience effects. Notably, the alpha male’s presence reduced positive effects associated with cooperation, whereas female presence buffered negative effects associated with competition. Copulation was perceived as competitive, especially during furtive mating when other males were absent. Overall, patterns suggest that chimpanzees categorise ordinary social events as cooperative or competitive and that these perceptions are moderated by specific audiences.
Highlights
Accessing animal minds has remained a challenge since the beginnings of modern science
The pictures were taken during 13 different ordinary social event types (n = 1003 events, Table 1) from which we extracted nose temperatures (Supplementary Table S3)
For cooperative events, we found that the best model to explain nose temperature variation contained two variables: the presence of the alpha male and the subject’s own rank (‘cooperation model’, likelihood ratio test (LRT): χ2 (2) = 9.82, p = 0.007; Supplementary Table S4)
Summary
Accessing animal minds has remained a challenge since the beginnings of modern science. Individuals are more likely to warn ignorant rather than knowledgeable audiences[16], while females are less likely to reveal their sexual activity in the presence of other females than m ales[17] and less likely to greet other males in the presence of the alpha m ale[18] Each of these studies demonstrated the relatively complex ways in which individuals interacted socially, suggesting a cognitive apparatus capable of rapidly assessing challenging social constellations. Given the documented high-levels of social awareness in chimpanzees, we predicted temperature differences depending on audience compositions, whether it would be favourable (e.g., bond partners or kin) or hostile (e.g., dominant males), an effect which would be mediated by the social rank of the focal individual To test these predictions, we collected and analysed facial thermal data of adult male chimpanzees engaged in a variety of ordinary social activities, including grooming and copulating, with different audience compositions
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