Yawn contagion plasticity in semiwild chimpanzees: females ‘leading’ the way at times

Yawning is pervasive across vertebrates, fulfilling physiological and social functions. In this study, we investigate yawning and its potential contagious nature in 60 semiwild chimpanzees, Pan troglodytes, a species living in a male-bonded society characterized by male philopatry and female dispersal. Through ChimpFACS, unsupervised clustering, and NetFACS analyses, we detected two distinct morphological forms of yawning that vary in duration depending on the yawner's age and sex. Such variability fosters nuances, challenging the traditional view of yawning as a fixed action pattern. Furthermore, our study demonstrated yawn contagion in chimpanzee groups where females cannot freely migrate, thus forming long-lasting associations. Contrary to existing literature attributing yawn contagion in chimpanzees predominantly to males, our data reveal evidence of a sex bias in yawn contagion sensitivity with females: (1) eliciting higher yawn response rates; (2) responding more frequently; and (3) responding rapidly to other females. Such notable divergences from previous literature highlight variability in this phenomenon. We contend that sex influences yawn contagion more than social closeness, challenging conventional interpretations. The current study indicates that both proximate and ultimate factors shaping the adaptive value of contagious yawning remain complex and not yet univocally identified. Studying yawn contagion in different settings allows us to examine a more complex picture of the phenomenon, which in the past has been identified as a fixed action pattern, but the contribution of recent research does not appear to be fixed at all.