Page 81 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Attention towards humans’ and bonobos’ emotion
observed in how people evaluated human compared to bonobo scenes. Specifically,
while yawning was considered the least arousing emotion category of the human
scenes, it was the most arousing one for the bonobo scenes. Possibly, the display
of the relatively large canines of bonobos played a role here. When humans yawn,
they typically do not show their teeth and the fact that they often cover their
mouth with their hands, shows that it is not a socially well-accepted expression. The
canines provide a likely interpretation given that a similar finding was observed for
the distress category where the canines are visible as well. At the same time, such 3 differences make it problematic for the interpretation of the current findings. Is the
effect indeed due to the canines? Or is it that the opening of the mouth of a bonobo raises a greater risk of being bitten because it is more difficult to predict subsequent actions of another species that has a completely different body and that one has hardly any experience with? One species might be better at expressing a certain emotion thanks to certain physical characteristics. For example, bonobos have more bodily hair than humans and during dominance displays, these hairs my bristle, called pilorection, which makes them appear larger and perhaps get the message across better. Whether or to what extent physical differences in the face and body of different species translate into perceptual differences of their expressions is a research topic that has remained unexplored. This needs to be confirmed in a comparative study with bonobos and humans. Importantly, differences in facial musculature between bonobos and humans are negligible (Diogo, 2018).
The literature is torn on the intensity between differentially valenced stimuli. Which one would be more intense: sex or aggression? Interestingly, aggressive and sexual images were, on average, perceived as most intense in our study. In contrast to earlier literature showing that men perceive sexual images as more intense compared to women (Bradley et al., 2001), we observed no such difference. However, in Bradley et al. (2001), the greatest difference was found in the valence ratings and the intensity ratings of the sex scenes differed only mildly. In the current study, a gender difference did occur in the human distress category. Females indicated to perceive these scenes as more arousing than males, which is also in line with earlier work (Bradley et al., 2001). Females also perceived some of the bonobo scenes as more arousing then men, including scenes showing yawns, grooming and play. However, a gender difference was also observed in the neutral bonobo scenes so this might also just reflect a more general gender difference in self-report. Indeed, in general, women gave higher intensity ratings than men, as demonstrated by a main effect (see also Bradley et al., 2001; for a review, see Kret & De Gelder, 2012a).
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