Page 124 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 5
Discussion
The contagion of behaviors such as yawning and self-scratching and their possible social function remain poorly understood. The current study aimed to investigate whether yawning and self-scratching are contagious in the orangutan and whether the contagion of these behaviors is linked to the context in which these behaviors occur and the quality of the bond between individuals. Orangutans showed increased self-scratch rates after a group-member scratched, indicating mimicry. This effect was visible within the first 90 s after the triggering scratch. Furthermore, when the relationship quality between the expresser and observer was low, and the observer had seen the triggering scratch, scratch contagion was more likely to occur during tense situations.
Our observation that scratch contagion is stronger in a tense context between weakly bonded individuals is novel, as most other studies report increased mimicry between individuals with a high relationship quality (Campbell & de Waal, 2011; Demuru & Palagi, 2012; Massen et al., 2012; Palagi et al., 2009, 2014). Yet, these studies predominantly looked at yawn contagion for which the social function and emotional load is debated and for which it is unknown how others perceive this behavior (Gallup, 2011; Massen & Gallup, 2017; Palagi, Celeghin, et al., 2020). Self-scratching, on the other hand, is often associated with physiological and psychological stress (Maestripieri et al., 1992; Schino et al., 1996; Troisi et al., 1991) although there is growing evidence that self-scratching also increases during positive arousing events, such as during play bouts (Neal & Caine, 2016). Without further measures (e.g., changes in emotional valence with cognitive bias testing as done by Adriaense et al. (2019) and Saito et al. (2016), we cannot conclude which emotions underlie self-scratching and if scratch contagion is truly linked to emotional contagion. Nonetheless, emotional contagion consists of simpler processes such as behavioral and physiological contagion (Edgar & Nicol, 2018) and the reported link between self-scratching and emotional arousal may suggest that the observed contagious effect of self-scratching in this study is a behavioral manifestation of emotional contagion.
If self-scratching is indeed an expression of emotional arousal, then this behavior could serve as a social cue for others (Laidre & Johnstone, 2013). Some other studies have reported on the potential signaling function of self-scratching. For instance, recent studies show that self-scratching can be used as a signal to coordinate joint travel, for example, between a mother and infant (Fröhlich et al., 2016, 2019a; Hobaiter & Byrne, 2014), and may be used to initiate grooming (Hobaiter & Byrne,
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