Abstract
Reading emotional states of the interacting partner is fundamental for social communication. This ability of inferring others’ emotions is specialised for within-species communication, but is known to extend to cross-species interactions. Previous studies have suggested both morphological similarity and familiarity with the expressing species play a role in the success in cross-species emotion communication. To investigate the relative contribution of these factors in cross-species emotion perception of closely related species, humans and bonobos, we asked human participants with varying degrees of experience with bonobos to assign emotion labels to images of bonobo emotional expressions, and rate them on valence and intensity. Moreover, we investigated how the channel (face vs. body) and emotional valence (negative vs. positive) of bonobo expressions modulate the perception. The results show that experts agreed more on the labels assigned to positive and neutral faces and bodies than novices or intermediates, while negative bodies were perceived similarly by all three groups. Interestingly, novices showed a higher agreement score than experts and intermediates to label negative facial expressions. The effect of expert superiority for positive and neutral images was attenuated in valence ratings, and the ratings on negative faces remained difficult even for experts. Similar to the results of the emotional labels, novices agreed specifically well on the interpretation of the negative faces. For intensity ratings, expert superiority remained the same for facial expressions with negative facial expressions yielding the highest agreement scores in general. Our results indicate a mixed effect of similarity and familiarity: while novices predominantly use anthropomorphic strategies, experts drew upon their extensive knowledge to evaluate the emotional states from bonobo images. Bodily expressions showed similar effects of expert superiority, though not as strongly as facial expressions. Overall, experience plays a predominant role in cross-species emotion recognition.