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. (PsycINFO Database Record

Yawning, although easily recognized, is difficult to explain. Traditional explanations stressed physiological mechanisms, but more recently, behavioral processes have received increasing attention. This is the first study to test a range of hypotheses on yawning in wild primate populations. We studied two sympatric strepsirrhine species, Lemur catta, and Propithecus verreauxi, of the Ankoba forest (24.99°S, 46.29°E, Berenty reserve) in southern Madagascar. Sexual dimorphism is lacking in both species. However, their differences in ecological and behavioral characteristics facilitate comparative tests of hypotheses on yawning. Our results show that within each species males and females yawned with similar frequencies supporting the Dimorphism Hypothesis, which predicts that low sexual dimorphism leads to little inter-sexual differences in yawning. In support of the State Changing Hypothesis yawning frequencies was linked to the sleep-wake cycle and punctuated transitions from one behavior to another. Accordingly, yawning frequencies were significantly higher in L. catta than in P. verreauxi, because L. catta has a higher basal level of activity and consequently a higher number of behavioral transitions. In agreement with the Anxiety Hypothesis, yawning increased significantly in the 10 min following predatory attacks or aggression. Our findings provide the first empirical evidence of a direct connection between anxiety and yawning in lemurs. Our results show that yawning in these two strepsirrhines occurs in different contexts, but more research will be necessary to determine if yawns are a single, unitary behavior.

Here we show for the first time that the plasticity in morphology and duration of yawning in Macaca tonkeana can be associated with different functional contexts.Macaca tonkeana is classified as a tolerant macaque species characterized by social interactions minimally constrained by dominance rank or kinship. Tonkean macaques, as other egalitarian species, rely on a complex facial communicative system.We found that the degree of mouth opening (ranging from covered to uncovered tooth yawns) and the duration of yawning were not strictly dependent. The shortest uncovered tooth yawns were associated with an intense locomotor/physical activity and peaked immediately after stressful social events thus indicating an increase in arousal. In contrast, longer yawns, independently from teeth exposure, were primarily associated with a relaxed state of the subject. In conclusion, our study suggests that to explore the potential different functions of yawning, it is necessary to focus on the variability of its expression both in terms of morphology and duration, because not all yawns tell the same story.