Lowland Gorillas (Gorilla gorilla gorilla) Failed to Respond to Others' Yawn: Experimental and Naturalistic Evidence

Yawn contagion, possibly a form of emotional contagion, occurs when a subject yawns in response to others' yawns. Yawn contagion has been reported in humans, chimpanzees, bonobos, geladas, wolves, and dogs. In these species, individuals form strong, long-term relationships and yawn contagion is highest between closely bonded individuals. This study focuses on the possible expression of yawn contagion in western lowland gorillas (Gorilla gorilla gorilla). Gorillas share with geladas a similar basic social structure (one dominant male and several adult females with offspring) and differ from bonobos and chimpanzees, which live in multimale-multifemale societies. Gorillas stand out because they are spatially aggregated but show especially low levels of social affiliation. If the expression of yawn contagion is linked to the investment of animals in establishing long-term social relationships, the phenomenon should not be detected in gorillas (social relationship hypothesis). For the first time, we applied to the same subjects the naturalistic approach typically used in ethology (all occurrences behavioral sampling) and the experimental approach typically used in psychology (response to video stimuli). During the video demonstration (avatar yawn/control; unfamiliar gorilla yawn/control), we checked for the attentional state of the subjects. Anxiety-related self-directed behaviors were recorded in all conditions and settings. We failed to detect yawn contagion in both naturalistic and experimental settings, with yawning being possibly associated with anxiety during video shows (revealed by the increased frequency of self-directed behaviors). In conclusion, yawn contagion may be a socially modulated phenomenon that remains largely unexpressed when individuals share weak social affiliation.