We've all seen wildlife documentaries showing young animals—lion cubs, perhaps—wrestling, chasing, pouncing on their siblings. Observe household puppies and kittens and you'll see the same behavior: young animals at play.
Play is defined as spontaneous, energetic behavior with no apparent purpose or goal. But whenever there's considerable expenditure of energy, a closer look is warranted. There may not be apparent goals, but the true benefits of play are being recognized by a growing number of disciplines.
Evolutionary biologists have come up with a number of theories about the benefits of play. There's the obvious: play as practice or preparation. Coyote pups chase and pounce and bite as young predators in training. Young deer run and leap and kick as a prey species learning escape tactics. Advances in neuroscience have confirmed that play helps build the many brain cell connections essential to cognitive development.
Another theory relates to stress and the emotions. Survival in the wild has many challenges, and constant vigilance is required either to hunt for food or to avoid being hunted. Play releases existing stress and quite possibly builds resistance to future stress.
Delve into the subject and you'll find a lot of crossover conclusions. All the benefits of play—applied to otters, crows, bear cubs and the like—are also applied to children at play. A youngster at play, undirected by adults, is a youngster exuberantly going about the important business of physical, cognitive and emotional development.