First evidence of contagious yawning in a wild lemur

Abstract: Contrary to spontaneous yawning, yawn contagion occurs when yawning in a subject (responder) is elicited by the yawns of others (triggers). Yawn contagion has been associated with inter-individual synchronisation, activity coordination and possibly emotional contagion, based on the perception–action mechanism. We collected data on yawn contagion and grooming and verified—for the first time—yawn contagion presence and modulating factors in a wild strepsirhine. Specifically, we considered the diurnal lemur Indri indri (inhabiting Maromizaha rainforest, eastern Madagascar), which lives in socially cohesive family units. We recorded 613 yawning events involving 28 individuals and found that yawn contagion was present in the indris (with the best predictor for an individual to yawn at a given time of day was observing another group member yawning) and that it was positively influenced by grooming levels (but not by the spatial distance) between trigger and responder. Age and sex had no significant relationship with yawn contagion likelihood. Because yawn contagion has been found in different mammalian species regardless of their phylogenetic closeness, this study, reporting the phenomenon in a lemur species with highly cohesive behavioural pattern and able to emit coordinated vocal displays, adds a valuable piece to the investigation of the pressures that may have favoured yawning as a (possibly emotional) communicative cue during evolution. Significance statement: Yawn contagion is associated with inter-individual synchronisation and activity coordination. While this behaviour is often investigated in apes, its presence in lemurs is debated. Here, we explored presence and modulating factors of yawn contagion in Indri indri, a critically endangered primate living in small family units where individuals show coordinated circadian rhythms and a highly cohesive behavioural pattern. We first demonstrated the presence of yawn contagion in wild indris where, in line with the high degree of behavioural synchrony showed by the individuals within a group, it may possibly indicate a transmission of physiological states. We also demonstrated the association of contagion with grooming rates, but not with the spatial proximity between triggers and responders, nor with their sex and age, pointing at social closeness as the most likely modulating factor.