Page 133 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Evidence for yawn contagion in orangutans
and friends while controlling for the potential effects of increased attention to socially
close others using a non-visual stimuli (Norscia & Palagi, 2011a). Still, in quite some
social species, the linkage between CY and social closeness or familiarity is not found
(Madsen & Persson, 2013; Massen et al., 2012; Miller et al., 2012b; Neilands et al., 2020;
O’Hara & Reeve, 2011). For example, a recent study analyzing a large dataset on CY in
dogs shows CY is present in dogs, but is not affected by familiarity or other potential
mediators such as sex or prosociality (Neilands et al., 2020). It therefore remains
possible that mechanisms other than the same PAM that underlies emotion contagion
or empathy are mediating CY. For instance, CY may result from stress induced by a
common stressor in the environment (Buttner & Strasser, 2014; Paukner & Anderson,
2006). Thus, rather than being mediated by seeing others yawn, yawning occurs as a
response to the stressor. Individuals that are stressed are known to show higher rates
of self-directed behaviors, of which yawning and self-scratching are examples (Troisi,
2002), and indeed, in one study involving stumptail macaques, monkeys yawned
more frequently in response to a video clip of yawns as compared to a control, but
also scratched more (Paukner & Anderson, 2006). The authors concluded that tension
was most likely mediating the occurrence of yawning in the yawn condition. In short,
while it is likely that CY is a social phenomenon, its exact mechanisms remain an
active field of investigation. 6
Notwithstanding the debate on proximate mechanisms, little attention has been given to more ultimate explanations for CY. One of the few hypotheses out there is that CY is an adaptive mechanism that helps with social coordination (Miller et al., 2012a). Accumulating evidence suggests that yawning itself serves to cool the brain as to maintain homeostasis (Eguibar et al., 2017; Gallup & Eldakar, 2012; Gallup & Gallup, 2007, 2008; Gallup & Gallup, 2010; Massen et al., 2014) and consequently may increase alertness and aid in vigilance. Within this social coordination hypothesis, CY, in turn, may help to spread vigilance within the group, for instance to remain alert for potential predators (Gallup & Gallup, 2007; Miller, Gallup, Vogel, & Clark, 2012). Specifically, it may be adaptive to match the state of a vigilant conspecific as it may have sensed a predator, which the individual itself did not yet sense. To date, however, the social coordination hypothesis remains untested, and the thermoregulatory function of yawning is still debated (e.g., Elo, 2011; Guggisberg et al., 2011, but see Gallup & Eldakar, 2012, for a response to the critique).
Another fruitful way to explore evolutionary hypotheses is through phylogenetic comparisons. Palagi et al. (2019) proposed the common trait among hominids hypothesis which states that, given the shared phylogeny between humans and
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