Yawning and Social Styles: Different Functions in Tolerant and Despotic Macaques (Macaca tonkeana and Macaca fuscata)
Yawning is a multifunctional behavior with a role in social communication. In Old World monkeys, the “tension yawn” is often used as a threat, allowing individuals to completely expose their canines. To explore the role of this phenomenon, we selected 2 closely related species—Japanese macaques (Macaca fuscata) and Tonkean macaques (M. tonkeana)—which differ primarily in terms of their tolerance levels. Japanese macaques are classified as despotic; Tonkean macaques are classified as tolerant. Both species live in multimale–multifemale societies, show a high level of sexual dimorphism, and have comparable yawning repertoires that include displaying a covered teeth yawn and an uncovered gums yawn. We found comparable baseline frequencies of the 2 yawning types and a similar distribution of these behaviors according to sex (males yawned more frequently than females). This morphological homogeneity permitted us to evaluate potential differences in the meaning of yawning as a function of social tension, aggressive contexts, and dominance hierarchy. Divergent social styles determine a functional dichotomy in the use of the covered teeth yawn and the uncovered gums yawn. The covered teeth yawn is not susceptible to social and environmental stimuli and seems to be a form of yawning mostly linked to the physiology of the sleep–wake cycle. However, the uncovered gums yawn is modulated according to different social contexts, and its use could be favored by natural selection, especially in tolerant species, which apparently require more elaborate forms of social communication.