The play fever: individual and group factors influence the contagion of play behaviour within social hominid groups.

Via the perception-action model and mirror neuron system, individuals replicate others' behaviour (behavioural/social contagion) and reduce prediction errors on other behaviours. Among contagious behaviours, play offers a safe context for cooperation, competition and tolerance. Thus, play contagion is interesting to better understand the evolution of social dynamics in many non-primates and primates, including humans. To our knowledge, no study has examined play contagion across social hominids nor analysed individual/group features to identify common modulators. In 2022-2025, we gathered all-occurrences data on play, affiliative and agonistic patterns in social hominids (two groups of captive chimpanzees, bonobos and gorillas; and two toddler classes of ≤ 3 years and their educators). In the 3 min period, we checked if individuals not involved in play would start playing either after the onset of a trigger play session (post-play (PP)) or in absence of any previous play session (matched-control). Play occurrence was highest during PP, informing play contagion. Contagion increased with higher male-to-female, immature-to-adult and affiliative-to-agonistic interaction ratios. Males, youngsters and individuals that were in trigger proximity (≤ 10 m) showed the highest contagion. Shared across social hominids, play contagion and its common modulators might trace back to their last common ancestor, underlying the transmission of behaviour, and possible underlying emotions, within groups. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.