Page 125 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Self-scratch contagion observed in orangutans
2014). Another possible communicative function of self-scratching is to signal social distress, which in turn reduces the likelihood of receiving aggression (Whitehouse et al., 2017). In our study, it is possible that orangutans use self-scratching in others as a marker of arousal and that the automatic contagion of such information from weakly bonded individuals during tension has an adaptive value. There was no difference in the probability of scratch contagion between contexts and relationship quality when the orangutan had not seen the triggering scratch, and hence only had auditory cues of this behavior. This can be explained by the fact that the observer had no information about the identity of the initial scratcher which may further highlight a possible link between contagious self-scratching and a social function.
If self-scratching indeed serves as a social signal (Fröhlich et al., 2016, 2019a;
Hobaiter & Byrne, 2014), it is likely intended to change the behavior of the observer
with the ultimate goal to benefit the expresser (Bradshaw, 1993; Laidre & Johnstone,
2013). A similar function of self-scratching is observed during agonistic interactions,
where self-scratching rhesus macaques are less likely to receive aggression 5 (Whitehouse et al., 2017). Because stressed individuals often behave unpredictably
(McEwen & Sapolsky, 1995) they can become a potential social stressor (Aureli et
al., 1992), especially when they are nonfriends or non-kin (Whitehouse et al., 2017)
. Hence, increased awareness of such individuals through mimicry may be beneficial
for observers and adaptive within group dynamics. Although such an adaptive
function of mimicry warrants further investigation, we speculate that orangutans can
benefit from increased self-scratch contagion, and potentially contagion of arousal,
of weakly bonded individuals during tense contexts, as it may help individuals to
prepare for potential unpredictable behaviors of the expresser. This way, scratch
contagion becomes adaptive for both the expresser and observer by increasing social
cohesion through reducing possible aggression (Rauchbauer et al., 2016). While we
could not test such aggression-reducing hypothesis of scratch contagion, this would
be interesting to explore in more detail.
It is important to recognize that increased scratch rates have often been observed during tense situations in general, independent of the identity of the individual providing the triggering scratch (Castles & Whiten, 1998; Kaburu et al., 2012; Palagi & Norscia, 2011; Peignot et al., 2004), although there are a number of studies that actually do not find increased scratch rates during anxiety-provoking circumstances (Aureli & De Waal, 1997; Duboscq et al., 2014; Judge et al., 2006; Pearson et al., 2015). Hence, it is essential to rule out that the heightened scratch contagion between weakly bonded individuals during tense contexts is not merely a
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