Page 183 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

that are preserved in the hominid lineage. What can the results on emotion-biased attention and mimicry tell us about the most recent common ancestor of humans, bonobos, and chimpanzees? Likely, this ape-like creature was sensitive to the emotions of others, having brain mechanisms in place that guided their attention towards emotional expressions, and that elicited automatic mimicry of facial and bodily expressions. Nevertheless, findings on an attentional bias towards emotions are mixed for chimpanzees, complicating this conclusion. For instance, some studies find no evidence for an immediate bias towards emotions in chimpanzees (Kret et al., 2018; Wilson & Tomonaga, 2018), whereas another study reports that chimpanzees preferentially look at agonistic scenes (Kano & Tomonaga, 2010a). Evidence from behavioral observations, however, corroborate the idea that chimpanzees and bonobos are both sensitive to the needs of others. For instance, both species are known to console each other after stressful situations (e.g., de Waal & van Roosmalen, 1979; Goldsborough et al., 2019; Palagi & Norscia, 2013). In addition, there is evidence for contagious yawning in bonobos and chimpanzees (e.g., Campbell & de Waal, 2014; Demuru & Palagi, 2012), as well as for facial mimicry (Palagi et al., 2019b, 2020a), again suggesting that the basic mechanisms for emotion perception (De Waal & Preston, 2017) were already present in the last common ancestor of bonobos, chimpanzees, and humans.
If we look further down the phylogenetic tree to the common ancestor of
humans and orangutans (Figure 2 in the introduction), we still see some evidence
for sensitivity to the emotions of others. Recently, we tested orangutans’ attentional
bias towards emotional scenes versus neutral scenes, and our preliminary findings
indicated a lack of an attentional bias towards emotions in these great apes. The only
other study to date that has looked at emotion-biased attention in orangutans found
that they preferentially look at the silent bared-teeth face compared to the bulging- 8 lip face (shown during aggressive encounters), but a direct comparison between the
silent bared-teeth face and a neutral face was not significant (Pritsch et al., 2017).
Clearly, more research is needed to understand how and to what extent orangutans
perceive the emotional expressions of others. Importantly, we find evidence for yawn
and self-scratch contagion in orangutans, and one other study has found mimicry of
the play face in this species as well (Davila-Ross et al., 2008). Albeit limited, there is
also evidence for facial mimicry in gorillas (Palagi et al., 2019b). Whether yawn or
self-scratch contagion reflect true emotion contagion is still highly debated (see
e.g., Gallup, 2021; Massen & Gallup, 2017; Palagi, Celeghin, et al., 2020), but given
their presence in orangutans as well as evidence for facial mimicry in gorillas, it is
General discussion
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