Behavioural contagion in wild spider monkeys (Ateles geoffroyi)

Behavioural contagion is a widespread phenomenon in animal species, which is thought to promote coordination and group cohesion. Among non-human primates, however, evidence of behavioural contagion is currently limited to few Catarrhine species. Here, we investigated whether behavioural contagion is also present in Platyrrhines (i.e. primates from South and Central America), by assessing yawning and scratching contagion in a wild group (N = 49) of Geoffroy’s spider monkeys ( Ateles geoffroyi ). We conducted focal samples to examine whether individuals observing the triggering event (i.e. a naturally occurring yawning or scratching event in the group) would be more likely to yawn or scratch in the following three minutes, as compared to individuals who did not observe the triggering event. We ran generalized linear mixed models using a Bayesian approach, and found that the probability of yawning and scratching was higher for individuals observing others yawning and scratching, respectively, as compared to individuals who did not observe such an event. However, behavioural contagion did not vary depending on the observer’s sex, kinship or relationship quality with the individual performing the triggering event. These findings provide the first evidence for yawning and scratching contagion in a wild group of spider monkeys, and importantly contribute to the debate about the evolutionary origins of behavioural contagion in primates, which is likely to have emerged before the evolutionary split between Platyrrhines and Catarrhines.