Methylation and merriment: an exploration of epigenetic mechanisms underlying play behaviour.

Species from octopi to humans engage in play. This review examines how epigenetic mechanisms, such as DNA methylation, may regulate play behaviour across taxa. We frame play through historical definitions, categorizing it into object, locomotor, and social forms, and examine how each may be linked to epigenetic shifts, for example in brain-derived neurotrophic factor (BDNF) expression. We then explore the role of domestication in enhancing play via methylation of stress and sociality genes, comparing domesticated chickens, dogs, and foxes to their wild kin. We link the neurobiology of play, spanning the hypothalamic-pituitary-adrenal (HPA) axis and reward circuits, to epigenetic modulation. Assessing the evolutionary fitness advantages of play, we compare adaptive benefits against the surplus resource theory. Despite its presence in many taxa, there remains limited direct evidence for a role of epigenetic mechanisms in play, and we urge research into the developmental and adaptive roles of play across a wider range of species.