Page 134 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 6
great apes, CY may find its roots in a shared underlying socio-cognitive mechanism that was already present in at least the last common ancestor (LCA) of all hominids. Moreover, since CY is also present in some Old-World monkeys (Palagi et al., 2009; Paukner & Anderson, 2006) and non-primate species (e.g., Gallup et al., 2015; Madsen & Persson, 2013; Norscia et al., 2021; Silva et al., 2012), its roots could be much older, or CY is an example of convergent evolution. To date, few data exist to perform comparisons and most interestingly, the picture among the great apes is not yet clear. There is convincing evidence for CY in chimpanzees (Amici et al., 2014; Anderson et al., 2004; Campbell et al., 2009; Campbell & Cox, 2019; Massen et al., 2012). In bonobos, two observational studies (Demuru & Palagi, 2012; Palagi et al., 2014) and an experiment (Tan et al., 2017) show clear evidence for CY, while one experimental study did not (Amici et al., 2014). However, the latter study only tested four individuals, thus making it very likely that CY is, indeed, present in bonobos. Finally, the first comprehensive study on gorillas combining an experimental and naturalistic approach found no evidence for CY (Palagi et al., 2019a). Unfortunately, data on CY in orangutans are scarce, which, considering their semi-solitary lifestyle (Van Schaik, 1999) may be of comparative interest for a social phenomenon like CY. To date, the only existing study involving orangutans failed to find evidence for CY (Amici et al., 2014), yet the sample size was too small to be conclusive. In general, orangutans in the wild roam mostly solitarily: males travel alone, and mothers travel with their offspring (Singleton & Van Schaik, 2002; Te Broekhorst et al., 1990). Due to overlapping home ranges, occasional encounters and affiliation are possible, but generally do not occur frequently (Singleton & Van Schaik, 2002; Te Broekhorst et al., 1990). Consequently, finding out whether CY is present in orangutans will further help elucidate the hypotheses previously discussed.
The current study attempts to clear up the picture of CY in hominids in two ways. First, we aim to find a convincing answer to whether CY is present in orangutans or not via an experimental design involving the presentation of yawning and neutral stimuli of orangutans to 8 orangutans. Second, we also investigate whether this potential yawn contagion is affected by a familiarity bias, i.e., whether CY is stronger between individuals that know each other versus unfamiliar individuals. To this end, we exposed orangutans to videos showing either yawn or control clips of familiar (i.e., conspecifics living in close proximity) and unfamiliar orangutans, as well as a 3D avatar (Kolbrink, 2017) and measured their response (yawns). Additionally, we also measured the occurrence of self-scratching to rule out potential effects of stress on the occurrence of yawning (Troisi, 2002). So far, CY appears to be exclusively present in
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