Page 56 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
familiar faces (Ramon & Gobbini, 2018). Here, we have shown that familiarity further interacts with the processing of emotional expressions at a very early stage of visual attention. Moreover, our findings also suggest that how familiarity interacts with the processing of emotional expressions can depend on species-specific characteristics, such as other-regarding tendencies. While humans and bonobos are both social species, humans tend to prefer the social in-group over the out-group (Fehr et al., 2008), whereas bonobos are known to preferentially share food with out-group members (Hare & Kwetuenda, 2010) and peacefully interact with them (Furuichi, 2011). How familiarity modulates emotional attention has not yet been studied in other species, but chimpanzees would be an interesting comparison species as they typically empathize with group members but not with unfamiliar chimpanzees (Campbell & De Waal, 2014; Wilson & Wrangham, 2003) and gaze longer at familiar rather than unfamiliar males (Lewis et al., 2021). As such, one hypothesis could be that chimpanzees have a stronger attentional bias towards emotional expressions of familiar conspecifics than of unfamiliar individuals.
Studies on emotion perception and attention in the other great apes – gorillas and orangutans – are, to the best of our knowledge, rare, but would further provide further evolutionary insights. Gorillas and orangutans have unique social systems, with gorillas living in harem-like societies with one adult male and multiple females and their offspring (Robbins et al., 2004) and orangutans living a semi-solitary life (Singleton et al., 2009). While we did not find an emotion bias in orangutans in another study (Laméris et al., 2021), previous work shows that they look longer at negative facial expressions (Pritsch et al., 2017) and automatically mimic facial expressions (Davila-Ross et al., 2008), but that mimicry is not necessarily affected by familiarity (Van Berlo et al., 2020b). Furthermore, gorillas are known to affiliate less frequently with conspecifics than for instance chimpanzees (Cordoni et al., 2018), but they do appear to mimic facial expressions, specifically the play-face that occurs during playful interactions (Bresciani et al., 2021). We currently do not have clear predictions on how familiarity might modulate emotional attention in these species, but given the existing evidence, an immediate bias towards emotional expressions may only be present in species that have high affiliative tendencies (like bonobos, chimpanzees [but see Kano & Tomonaga (2010) and Kret et al. (2018)], humans, and some monkey species), given that they continuously need to monitor behaviors of others in the group. Again, more research is needed to pin down differences between different species’ attention allocation to emotional expressions, and how these interact with social factors such as familiarity.
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