In all mammalian species, the period of a young mammal' s life in which it is most frequently playful coincides with the period of most rapid growth of the neural connections in the mammal' s brain that give rise to various complex patterns of movement, posture, and social response
Indeed, the neural connections created by frequent play during this period later become indispensable for the mammal' s survival and well-being as an adult
The young of nonmammalian species such as fish, reptiles, and birds do not normally engage in playful behavior that serves the function served by play in the development of young mammals.