Page 94 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 4
emotions (see .e.g., Adolphs, 2002), we used a combination of expressions as well as emotional scenes. Emotional scenes can convey more contextual information, as they contain whole-body expressions that can communicate emotions as well as action intentions (De Gelder et al., 2010). Furthermore, previous studies have shown that emotional scenes modulate attention in a similar way as facial expressions (e.g., Kret et al., 2016; Kret & van Berlo, 2021; van Berlo et al., 2021), therefore indicating that they provide sufficient emotional information to the participant.
Emotional scenes involved individuals engaged in socially relevant behavior and/or having an emotionally relevant facial or bodily expression. Though it can be argued that we do not exactly know what bonobo emotions are, we do know the social relevance of certain facial expressions (such as the fear-grin, the relaxed open-mouth play face and yawning) and socio-emotional behaviors (sex, grooming) (De Waal, 1988). The fear-grin is often expressed during stressful situations and agonistic interactions, while the relaxed open-mouth face (or play face) is expressed during playful interactions (De Waal, 1988). Yawning is a widespread behavior in vertebrates and it is highly contagious (Demuru & Palagi, 2012; Massen et al., 2015; Palagi et al., 2014; Van Berlo et al., 2020b). Its contagiousness is linked to social closeness, and yawning could therefore serve a social function (Casetta et al., 2021; Norscia et al., 2020). Furthermore, yawns capture immediate attention in bonobos (Kret et al., 2016). Other socio-emotional behaviors that are relevant to bonobo society are sex and grooming. Bonobos use sex to prevent or resolve conflicts and reduce stress levels (De Waal, 1988). Grooming is an important social behavior used to form and strengthen social bonds between individuals (Dunbar, 1991). As such, emotional scenes in our task consisted of one or more bonobos playing, having sex or displaying an erection (male) or a large swelling (female), grooming, displaying distress, and yawning. Neutral scenes consisted of one or more bonobos lying down, sitting or walking with a neutral facial expression (see Tables S1 and S2 in supplements).
To make direct comparisons between bonobos and humans possible, we selected emotional scenes of humans that were equivalent to or an approximation of the emotional bonobo scenes. The stimuli consisted of humans playing, having sex (specifically: engaged in a romantic embrace), embracing (“grooming”), displaying distress (crying), and yawning. As there is no clear human equivalent for grooming in humans, we opted to use embracing as it is a reflection of social closeness and involves physical contact, just like grooming (Forsell & Åström, 2012). Neutral scenes of humans depicted one or more
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