The Secrets of Animal Play

Scientists gain insights into the function of youthful frolic

By Kim A. McDonald

Consider the puppy. At only three weeks of age, this tiny ball of fur has already begun gnawing, pawing, and tugging at its litter mates. At four to five weeks, its antics rival those of a rambunctious schoolboy -- biting, chasing, stalking, 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 monkeys, whales, and sewer rats, many groups of mammals and even some birds devote a significant fraction