Abstract
In nonhuman primates yawning is often classified as one of three types physiological response (e.g. regulating blood oxygen levels; increasing arousal), a sign of tension or anxiety, or a form of threat. So-called “threat yawns” are described especially in adult male Old World monkeys, who yawn more frequently than adult females. There is a lack of evidence for a similar sex difference in great apes. However, observations of lowland gorillas using open swamp areas in Gabonese rainforest revealed a greater frequency of yawning by adult males, especially silverbacks, than by other age-sex classes. Yawning occurred in relatively cool and rainy weather conditions. During rainy spells the gorillas became immobile and huddled to conserve body temperature. We suggest that yawning by silverbacks in this situation functions to increase arousal, important for group-leaders as they need to remain vigilant and ready to respond to any external threat to their group, such as from extra-group males or potential predators.
