Consider the puppy. At only three weeks of age, this tiny ball of fur had already begun gnawing, pawing, and tugging at its littermates. At four to five weeks, its antics rival those of a rambunctious child, chasing and wrestling with its siblings at all hours of the day and night.

Such behavior is not unusual among social mammals. From human children to whales to sewer rats, many groups of mammals and even some birds play for a significant fraction of their youth. Brown bear cubs like puppies and kittens, stalk and wrestle with one another in imaginary battles. Deer play tag, chasing and fleeing from one another. Wolves play solitary games with rocks and sticks. Chimpanzees tickle one another.

However fascinating these displays of youthful exuberance may be, play among animals was ignored by scientists for most of the century. Biologists assumed that this seemingly purposeless activity had little effect on animal development, was not a distinct form of behavior, and was too nebulous a concept either to define or to study. Even the term "play" caused problems for researchers, because it suggests that watching animals goof off is not an activity for serious scientists.

But a steady accumulation of evidence over the past two decades now suggests that play is a distinct form of behavior with an important role in the social, physical, and mental development of many animals. In one study, kittens, mice, and rats were found to play the most at ages when permanent changes were occurring in the muscle fiber and the parts of their brains regulating movement. Kittens were most playful between 4 and 20 weeks of age; rats, from 12 to 50 days; and mice, from 15 to 29 days. Development at those ages is comparable to that of a two-year old human infant. At these precise times in the development of these animals, muscle fibers differentiate and the connections to areas of the brain regulating movements are made. Such changes apparently are not unique to kittens, mice, and rats, but apply to mammals in general.

Thus, research on play has given biologists an important tool with which to probe the development of the brain and motor system of animals. The study on rats, kittens, and mice may, for instance, provide a physiological explanation for why infant animals employ in their play the same kinds of play that they will later use as adults. By stalking and capturing imaginary prey over and over again, a kitten builds its muscle and brain connections in a way that allows it to perform those actions later in life.

Play may also provide insight into the social development of animals. When the rough-and-tumble of play ends traumatically with a yelp or a shriek, young animals may be learning the limits of their strength and how to control themselves among others. Those are essential lessons for an animal living in a close-knit group. Perhaps, some scientists guess, as mammals gathered into social groups, play took on the function of socializing members of the group. Not everyone agrees with this theory, though. Another explanation is that play may not have evolved to confer any advantage but is simply a consequence of higher cognitive abilities or an abundance of nutrition and parental care.