Yawning is a common, species-typical behaviour in vertebrates, generally associated with transitions from sleep to wakefulness. In primates, yawning occurs within a group in a variety of social contexts, suggesting that it has a communicative role, a hypothesis examined in this study. Yawning has rarely been studied. Quantitative studies have been concerned only with age, sex and social-status differences in yawning frequency.

The causation and function of yawning were examined in a quantitative, 796-h ethological analysis of observations of two groups of Cercocebus albigena and one group of Macaca fascicularis. In both species, yawns occurred in two main contexts: during transitions from rest to activity (the ‘rest yawn’) and following social interactions (the ‘emotion yawn’). The rest yawn represents 90 % of yawns and is common to every age-sex group. By contrast, the emotion yawn (10 %) is more frequent in adult males. Regardless of context, yawning was more frequent in males than females. Within males yawning frequency increased with age. Thus, sex hormones may modulate the physiological processes involved. Yawns were usually performed by individuals in seated, lying or standing positions. Yawns were frequently followed by short periods of rest, so that, when triggered by a social interaction, they occurred at its end. The morphology of the yawning act did not vary in regard to context age and/or sex of the yawner.