The Serious Business of Play

Watch a pair of fox cubs on a spring morning and you will see a kind of joyful chaos. They pounce and roll, grab each other by the scruff, break apart and come together again, their movements exaggerated and loose in a way that an adult fox hunting in earnest would never permit itself. It looks like pure fun, and in a sense it is. But play is also one of the most purposeful activities a young animal engages in, and its apparent frivolity conceals a serious evolutionary function.

Practising for the Real Thing

The most obvious role of play is rehearsal. When a kitten stalks a ball of wool or a lion cub ambushes its sibling, it is running through the motions of hunting without the cost of failure. The stalk, the pounce, the bite, the twist of the body that pins a struggling prey, all are practised in miniature, long before the young animal's life depends on getting them right. Play lets an animal make its mistakes in safety, building the physical coordination and the quick reactions that cannot simply be installed by instinct but must be honed through repetition.

This rehearsal is not confined to hunting. Young ungulates leap and buck and run in sudden bursts, exercising the muscles and reflexes they will one day need to outrun a predator. Play-fighting among young males tests strength and builds the skills of combat that will later settle contests over territory and mates, ideally without the serious injury that a real fight might bring. The behaviours practised depend on the animal, but the principle is constant: play is the apprenticeship of the body.

There is a developmental window to all this. Play peaks in the young and fades with maturity, concentrated in exactly the period when the brain and body are most plastic, most able to be shaped by experience. The young animal's nervous system is laying down connections at a furious rate, and play appears to help sculpt those connections, tuning the circuits of movement, perception and response. An animal deprived of the chance to play in youth may grow into a clumsier, more anxious, less socially capable adult, poorly equipped for the demands that adulthood will place upon it. Studies of young mammals raised without playmates have found exactly this pattern of lasting deficit, a reminder that play is not an optional luxury but a developmental necessity.

Learning to Live Together

For social animals, play does something beyond building physical skill: it builds relationships. Rough-and-tumble play teaches the young the limits of acceptable behaviour, the point at which a nip becomes a bite and the game turns into a quarrel. Crucially, play depends on restraint. A stronger animal that wants the game to continue must hold back, pulling its bites and letting its weaker partner win now and again, and in learning this self-handicapping the young animal learns the social intelligence that group life demands.

Play also seems to carry signals that keep it from tipping into aggression. Many species have specific gestures that announce an invitation to play, the famous bow of a dog with its front legs lowered and rump in the air being the clearest example. These signals say, in effect, that what follows is not to be taken seriously, and they allow animals to engage in behaviour that would otherwise provoke a fight.

Perhaps the most intriguing possibility is that some animals play simply because it feels good. The neural systems that produce pleasure and reward are ancient and widespread, and there is evidence that play activates them. Young rats emit ultrasonic chirps during rough-and-tumble play that researchers have cautiously likened to laughter. If play is partly its own reward, that would not undermine its evolutionary function but explain it, for an activity that feels good is an activity an animal will seek out often enough to reap its benefits. Nature, it seems, has made the hard work of growing up something a young animal is glad to do.