Page 146 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 6
also be interesting to study potential effects of the two different morphs of orangutan males on CY.
Furthermore, due to power issues, we could not reliably test effects of age on CY. In humans, while spontaneous yawns can occur already before birth (De Vries et al., 1982), CY does not seem to appear until the age of four to five (Anderson & Meno, 2003; Millen & Anderson, 2011), although when children of 3 years old are specifically told to look at the eyes of the stimulus they show CY as well (Hoogenhout, 2013). Similar developmental trajectories of CY have been reported in other animals (Anderson et al., 2004; Madsen et al., 2013; Madsen & Persson, 2013; Palagi et al., 2009). In our study, there were only two individuals younger than 5; one 15 months (Indah) and one three-year old (Baju). We observed one yawn occurrence in Indah (in the yawn condition), in Baju we observed six events (four in the yawn and two in the control condition). We decided to include these individuals in our study because while it is true that CY shows a relatively slow developmental pattern in humans, orangutans are born more precocial, and developmental rates in nonhuman primates are much faster compared to humans (Clancy et al., 2007). Therefore, CY may possibly also occur earlier in development in orangutans, but with only anecdotal evidence we cannot verify this in our study.
Third, while we tested effects of familiarity in our study by including both familiar and unfamiliar yawners, the fact that we only had yawns from the two adult males to use as stimuli restricted any potential investigation of the potential link between social closeness of the responders and the familiar individuals on the stimuli. The positive effect of social closeness on the occurrence of CY is well established in humans (Provine & Hamernik, 1986), chimpanzees (Campbell & de Waal, 2014, but see Massen et al., 2012), and bonobos (Demuru & Palagi, 2012), but is strongly debated in other species such as dogs (Neilands et al., 2020) and budgerigars (Miller et al., 2012b). For dogs, it should be noted that CY is interspecific, and that domestication might have had influential effects on how CY is modulated. Inverse effects have also been reported. For instance, a large study in rats has shown a familiarity bias in the opposite direction with rats being more likely to yawn in response to unfamiliar yawns (Moyaho et al., 2015). Similarly, a recent study investigating scratch contagion in orangutans found that during tense situations, orangutans are more likely to take over self-scratching from individuals with whom they have a weak bond (Laméris et al., 2020), indicating a (negative) correlation between social closeness and the contagiousness of a behavior or motor pattern. Thus, it remains possible that social modulation of CY is present in orangutans, at least in those living with conspecifics
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