A systematic review of play invitation signals in great apes: ontogenetic rather than phylogenetic differences structure variation in ludic communicative displays

Play invitation signals mark interactions as non-threatening and playful, serving as a key mechanism for coordinating social play among great apes. Within the framework of gestural origins theories, particularly the contrast between genetic codification and ontogenetic ritualization, this systematic review-conducted following PRISMA guidelines-examined empirical evidence on play invitation signals in Pan troglodytes, Pan paniscus, Gorilla gorilla, Pongo pygmaeus, and Pongo abelii. Studies were included if they provided developmental categorization or detailed age information and an ethogram explicitly describing play invitations. A total of 29 studies met these criteria. Results revealed marked ontogenetic variation across species, with play invitation signals peaking during infancy and juvenility, declining in subadults, and showing a secondary rise among adult females. These findings highlight the importance of developmental trajectories for understanding cognitive and communicative flexibility in great apes. They suggest that the structure of interspecific variation in play invitation repertoires does not align consistently with phylogenetic relationships, supporting a model in which socio-developmental factors play a substantial role in shaping ludic communicative displays.