Page 145 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Evidence for yawn contagion in orangutans
be a promising way to study social cognition (Wilson et al., 2020). Future studies will have to verify whether the lack of evidence for CY using an avatar in our study is because the effect is truly absent, for instance by looking specifically at how similarity with another individual (on a physical level) affects CY. In humans, there is ample evidence that the more similar that individuals are in terms of physical characteristics, but also personal convictions and views, the more likely they are to automatically mimic behavior (Chartrand & Bargh, 1999).
Future studies can improve on the current study design in several ways. First, we only used orangutan males as stimuli. In previous studies with chimpanzees (Massen et al., 2012) and bonobos (Demuru & Palagi, 2012), the sex of the triggering yawner affected the occurrence of CY; i.e., in chimpanzees, male yawns were more contagious whereas in bonobos, female yawns were more contagious. In gelada baboons, CY is more prevalent among females, especially when they are closely bonded (Palagi et al., 2009). It is possible that these results can be explained by emotional closeness between individuals, as in chimpanzees males typically form strong social relationships (Mitani et al., 2000), and in bonobos and gelada baboons it is mostly females that bond (Dunbar & Dunbar, 1975; Furuichi, 1989).
Alternatively, results could be explained by the differences in hierarchy with
chimpanzees being male dominant (Goldberg & Wrangham, 1997) and bonobos 6 female dominant (Furuichi, 1989), and by the strong matrilineal bonds between gelada
baboons (Silk et al., 2004). Investigating whether there is an interaction between sex
of the stimulus and of the responder in orangutans could help elucidate the roots
of the observed sex effects in CY in some species. The restricted selection of stimuli
and the low sample size did unfortunately not allow us to perform such analyses. It
is noteworthy, however, that the males in our study yawned more frequently than
the females (i.e., the total yawning rate of males was 74, whereas females yawned
only 9 times. See Table S2A). Yawns occur more frequently in males of species with
canine polymorphism, and also during aggressive contexts (Leone et al., 2014). Given
that all our stimuli were male, perhaps there is a role for dominance or rivalry in
the occurrence of CY in orangutans (Moyaho et al., 2015). Nevertheless, one could
argue that this leads to tense situations, thus leading to more self-scratching when
observing yawns of others, which is not what we found.
Additionally, all of our videos contained flanged males. Flanged adult males are often preferred over unflanged males by receptive female orangutans (Knott et al., 2010), and can be viewed as threatening by unflanged males (Delgado & Van Schaik, 2000). As such, in addition to interactions between the different sexes and CY, it may
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