Gender issues in primate social signals: the case of yawning

Contagion yawning (yawning elicited by someone else’s yawn) and spontaneous yawning are two distinct phenomena; nevertheless both can have signalling functions and be influenced by sex differences. Spontaneous yawning is possibly associated with behavioural state changes and synchronization between group members. Sex differences in yawning frequencies emerge in dimorphic species, in which males have an active role in inter-group territorial defence. In contrast, sex differences disappear in species characterized by low levels of sexual dimorphism, such as lemurs, chimpanzees and bonobos. The probability that a subject (responder) will be infected by the yawns of another subject (trigger) can be influenced by social correlates, such as the social bond between yawner and potential responder (e.g. in humans and bonobos) and rank (e.g. in chimpanzees). Hence, depending on society features (male/female dominance, type of phylopatry, presence of matrilineal subgroups, etc.), the quality and frequency of yawn contagion can differ between males and females. For example, in geladas a full matching between trigger’s and responder’s yawn (teeth uncovered, full open mouth, etc.) is observed only in contagion between females. In bonobos, females’ yawns elicit more yawning responses than males’ yawns, whereas in chimpanzees, male yawn signals trigger the highest number of responses, thus being probably more relevant to the rest of the group than female signals. In humans, it has been hypothesized that gender differences in empathic abilities may be reflected in sex differences in yawn contagion, higher between closely bonded individuals. In this work, we analyse sex differences in both spontaneous and contagious yawning across primates, with a special focus on humans.