Page 142 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 6
Looking at self-scratching, we first investigated the likelihood of self-scratching when viewing yawning and control videos, and found that the occurrence of self-scratching did not differ between conditions (b=-0.17, SE=0.17, p=0.319). Similarly, self- scratching rates were not significantly higher in the yawn versus control condition (b=0.10, SE=0.09, p=0.301). Moreover, both models did not deviate significantly from their null-model (c2(1)=1.06, p=0.303). Hence, it is unlikely that orangutans were more aroused viewing yawn videos compared to viewing control videos, at least when measured via self-scratching. Additionally, we also included self-scratching in our original models on yawning as a covariate, and found it to not significantly explain the likelihood of yawning, nor to influence the found effects of condition and the lack thereof in the avatar treatment (see supplements for these analyses).
Discussion
Here we find that orangutans yawn contagiously in response to conspecifics yawning, independent of whether the conspecific is a familiar or unfamiliar individual. Furthermore, orangutans were not susceptible to yawns of an avatar. Additionally, the videos used in our experiment appeared to be similarly arousing. That is, there was no difference in self-scratching (an indicator of stress) between the conditions. We here discuss the consequences of our findings for the different proximate and ultimate hypotheses that currently exist.
CY has thus far been observed in highly social species (Anderson et al., 2004; Demuru & Palagi, 2012; Joly-Mascheroni et al., 2008; Palagi et al., 2009; Romero et al., 2014; Wojczulanis-Jakubas et al., 2019a; Yonezawa et al., 2017, but see: Baenninger, 1987; Deputte, 1978; Malavasi, 2014; Massen et al., 2016; Reddy et al., 2016). Orangutans have meaningful social interactions that occur more often than is expected by chance alone (Roth et al., 2020), but these interactions occur at a much lower frequency compared to bonobos and chimpanzees (Te Broekhorst et al., 1990; Van Schaik, 1999). Interestingly, our results show that orangutans exhibit CY, suggesting that a high degree of affiliation within a species is not necessary for CY to occur. This also indicates that more studies are needed that investigate the presence or, importantly, absence of CY in a variety of species that differ on their social organization and affiliative tendencies. At the same time, it has to be noted that our sample consists of zoo-housed orangutans that were also born in captivity. In captivity, frequencies of affiliation can exceed those observed in the wild (Edwards & Snowdon, 1980), thus
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