Page 43 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotional attention is modulated by familiarity
of bonobos developed to selectively attend to emotional signals from potentially interesting unfamiliar social partners.
At the same time, it is interesting that there is no effect of emotion in the
familiar condition. A recent eye-tracking study by Lewis et al. (2021) showed that 2 bonobos attended longer to familiar group members rather than unfamiliar bonobos,
indicating that seeing familiar individuals somehow interests the bonobos. It is
possible that when viewing familiar individuals, the effect of emotional expressions
on attention is further affected by pre-existing knowledge about those individuals.
Other research indeed suggests that social characteristics of the observer in relation
to the observed individual(s) may play a role in how emotions are processed. For
instance, attention has been shown to be modulated by e.g., sex (Schino et al., 2020),
social bond (Kutsukake, 2006; Whitehouse et al., 2016), rank (Lewis et al., 2021;
Micheletta et al., 2015; Schino & Sciarretta, 2016), and kinship (Schino & Sciarretta,
2016). The current study sample did not allow us to disentangle potential effects of
social characteristics on an attentional bias towards emotions. However, inspection of
the two bars representing the familiar and unfamiliar condition in the top left plot of
Figure 4 suggests that the inter-individual variance was comparable between these
two conditions. Another possibility for why an attentional bias towards the emotional
expressions of familiar conspecifics was not observed may be related to the fact that
familiar and unfamiliar conspecifics were shown within the same experiment (and
not within the same trial). The emotional expressions of unfamiliar conspecifics may
be of such high relevance for this species, that it rendered biases towards expressions
of close associates and kin insignificant. We cannot test this in our data, but future
work could try to zoom in on how attention to emotions is modulated by specific characteristics of familiar individuals (e.g., age, relationship, rank).
An alternative explanation for our findings is that results are driven by heightened novelty of the unfamiliar stimuli (Bradley, 2009). However, we could rule this out, because bonobos on average responded as fast to stimuli of unfamiliar (novel) as of familiar individuals. A worthwhile follow-up experiment is to directly compare familiar and unfamiliar individuals (emotional and neutral) within trials in order to disentangle effects of emotion and familiarity. In addition, studying an attentional bias towards emotions of familiar and unfamiliar individuals in chimpanzees could be a fruitful next step. While chimpanzees and bonobos are very closely related to each other and equally closely related to humans, differences in social organization (with females being dominant in bonobos, and males in chimpanzees), and social tolerance (chimpanzees are highly territorial) may also differentially affect where attention is
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