Play stimulation modifies brain and behaviour in chickens - effects of domestication.

Play is ubiquitous among vertebrates, but there is limited knowledge about the function of this costly behaviour. Young domesticated chickens play more than their wild ancestors, the Red Junglefowl, which opens for comparative studies that may enlighten possible functions. We compared social and explorative behaviour and brain morphology of domesticated White Leghorn chicks with that of Red Junglefowl chicks after half of the birds of both breeds had received extra opportunities to play during a five week period. Irrespective of play stimulation, Red Junglefowl played less but were more explorative than White Leghorn, weighed less, had a larger brain relative to body size and a smaller cerebellum relative to total brain size. Play stimulation significantly increased later exploratory behaviours compared to the control chicks irrespective of breed, and increased sociality mainly in White Leghorns. Play-stimulated chickens had a lower body weight than controls, but higher relative brain mass, most obvious in WL (although absolute brain weight was not affected by play treatment). There was a tendency that play-stimulated chicks had a larger cerebellum mass, and in Red Junglefowl, play-stimulated chicks had a larger cerebellum relative to total brain mass than controls, which was not the case in the White Leghorn. There were no effects of play stimulation on neuron or overall cell density in cerebellum. In conclusion, play stimulation in young chicks modify later explorative and social behaviour as well as brain size and composition, and the effects are different in domesticated and ancestral chickens. However, the brain variables are relatively weak, so further investigation is warranted on these effects. Overall, the findings suggests that play may serve different functions depending on evolutionary history and ecological niche.