Page 144 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 6
between gorillas and other hominids (Schwartz, 1987). It is possible that the number of trials in the study by Palagi et al. (2019) were not sufficient to detect CY, as in our study, even with a large number of trials, we only detected yawns in 11.9% of all cases. Nevertheless, studies with chimpanzees that have few trials were able to establish CY in the past, albeit with a relatively large number of subjects (Amici et al., 2014; Campbell et al., 2009; Massen et al., 2012), and there was also no evidence for CY in naturalistic observations in gorillas (Palagi et al., 2019a).
Interestingly, it has been argued that in the past, orangutans may have been more social, but that due to long periods of low food availability, orangutan gregariousness was no longer viable (Harrison & Chivers, 2007). This may suggest that the ancestor of all apes already possessed the mechanism underlying CY. However, based on observational and relatedness data, it has been suggested that this hominid lived in a group with gorilla-like structure in which one male could monopolize multiple females (Harrison & Chivers, 2007). In this sense, it is difficult to explain why, given a similar social structure, gorillas do not show CY and orangutans do. It is possible that CY was somehow lost in the gorilla lineage, or that CY evolved multiple times over the course of evolution. The loss of CY is theoretically possible, given that CY has been found in some, but not all primates (Baenninger, 1987; Palagi et al., 2019a). Here, there is a role for the type of social system that characterizes a species in the loss (or occurrence) of CY (Palagi et al., 2019a). There is, however, not yet enough variation in data on CY in different species of primates to draw clear conclusions. Furthermore, it is possible that the measures to detect CY in certain species are simply not sensitive enough. All these explanations can be true, given that the occurrence of CY is highly variable in primates in general. It is clear that more studies are needed in order to draw robust conclusions about the evolution of CY.
In our study orangutans did not significantly respond to the avatar, which contrasts with findings in chimpanzees (Campbell et al., 2009). Potentially, orangutans experienced the uncanny valley phenomenon in which the avatar looks very realistic, yet fails to behave like a real orangutan, therefore violating natural expectations of orangutan behavior. Indeed, previous research on monkeys showed that they preferentially looked at real or completely unrealistic 3D model monkeys compared to very realistic 3D models (Steckenfinger & Ghazanfar, 2009). Nevertheless, this would likely have increased self-scratching when viewing the avatar, which was not evident in our study. Furthermore, a recent study investigating the uncanny effect in macaques showed that looking times did not differ between the Primatar (3D monkey head) and real or unrealistic images, indicating that the use of virtual stimuli can still
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