Page 143 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Evidence for yawn contagion in orangutans
potentially increasing the likelihood of CY to occur. Nevertheless, our results do show the presence of CY in orangutans and the few generations of zoo-living individuals cannot inform us about any selection pressures that have resulted in this tendency in orangutans. Our results must therefore be discussed in light of the orangutans” natural behavior and social environment.
In our study, we did not find an effect of familiarity on CY, suggesting that at least
in orangutans, social modulation of CY may not be present. While presence of social
modulation of CY is often used as confirmation of CY and emotion contagion sharing
the same underlying perception–action mechanism (Campbell & de Waal, 2011;
Demuru & Palagi, 2012; Norscia & Palagi, 2011a; Palagi et al., 2014; Romero et al.,
2013), its absence in our data makes it more difficult to interpret the emotional bias
hypothesis. Orangutans do have some preferences when it comes to their interaction
partners, thus one could expect social modulation of CY under the emotional bias
hypothesis. For instance, related female orangutans are known to associate more
often than unrelated females (Van Noordwijk et al., 2012), and prefer the long-calls of
dominant males (Setia & van Schaik, 2007). Additionally, in a recent study, orangutans
were shown to scratch contagiously in response to conspecifics self-scratching,
suggesting a potential case of emotion contagion (Laméris et al., 2020). Interestingly,
scratch contagion was stronger between weakly bonded individuals during tense 6 situations, which shows a social closeness bias in the opposite direction. This suggests
that a familiarity bias may be more flexible depending on the situation individuals are
in (e.g., relaxed versus stressful contexts) and the nature of the behavior that is copied
(e.g., self-scratching as an expression of tension).
At the same time, there are other studies on highly social species that do not show a familiarity bias (e.g., chimpanzees: Massen et al., 2012, dogs: Neilands et al., 2020, macaques: Deputte, 1978, and marmosets: Massen et al., 2016). As such, there may be (currently unknown) species-specific traits that determine whether a familiarity bias occurs or not. The exact (social) function of CY remains unclear and thus alternative explanations that do not involve the PAM that is underlying empathy may still be possible (e.g., spreading of vigilance). As has been pointed out by others, solving this issue requires a more systematic study of CY that includes a bigger variety of animals, including solitary animals such as reptiles and amphibians (Massen & Gallup, 2017).
From an evolutionary perspective, our results pose an interesting conundrum: while we found CY in orangutans, it is not present in gorillas, even though the split between orangutans and other hominids is evolutionarily older than the split
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