Page 32 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
visual perception, attention is attuned to emotional expressions (Öhman et al., 2001b; Vuilleumier, 2005). Specifically, both threatening and positive signals in the environment can rapidly capture attention (Pool et al., 2016), and this attentional attunement is driven by both arousal-eliciting characteristics of the signal as well as its significance to the observer (Brosch et al., 2008; Frijda, 2017). Interestingly, a similar capacity has been observed in bonobos (Kret et al., 2016). In an experimental setting, bonobos showed an attentional bias towards emotional scenes depicting unfamiliar conspecifics, especially when these scenes were emotionally intense. Moreover, a recent study showed that emotional expressions interfere with attention allocation in bonobos in an emotional Stroop task (Laméris et al., 2022). These findings suggest that the attentional mechanisms that guide social perception have an evolutionarily old foundation, and were likely already present in the last common ancestor of Pan and Homo.
Aside from being attuned to emotional expressions, the brain systems that facilitate the social bond between individuals have also evolved to prioritize the processing of familiar, socially close others. Human studies have shown that faces of friends and family are detected faster than faces of strangers (Ramon & Gobbini, 2018), and that these familiar faces recruit a broader network of brain areas involved in face, emotional, and social processing (Gobbini et al., 2004). Similarly, a recent study with chimpanzees and bonobos showed that they gaze longer at familiar faces than at unfamiliar faces (Lewis et al., 2021). Familiarity can also affect the expressions of emotions. For example, work on the automatic mimicry of emotional expressions shows that individuals are more likely to mimic expressions of familiar others compared to strangers (Palagi et al., 2020b; Prochazkova & Kret, 2017). As attention gates which signals from the environment are preferentially processed, it is therefore plausible that evolution fine-tuned this mechanism to quickly differentiate not only between emotional and neutral cues, but also between expressions of familiar, socially close group members and unfamiliar others.
Compared to the other great apes and humans, bonobos are strikingly xenophilic. Intergroup encounters in the wild proceed relatively peacefully, and neighboring groups have been observed foraging together (Fruth & Hohmann, 2018). Remarkably, two wild female bonobos have recently been observed adopting an infant from a different social group (Tokuyama et al., 2021). Furthermore, in experimental settings bonobos show a prosocial preference for unfamiliar individuals rather than group members (Tan & Hare, 2013). In contrast, humans tend to prioritize their own group members over unfamiliar individuals when it comes to sharing resources (Fehr et
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