Groos's answer to the question about the biological purpose of play allows us to make sense of the patterns of play that we see throughout the animal world. For starters, it explains why young animals play more than do older ones of the same species; they play more because they have more to learn. It also explains why mammals play more than do other classes of animals. Insects, reptiles, amphibians and fishes come into the world with rather fixed instincts; they don't need to learn much in order to survive, given their ways of life, and there is little evidence in them of play. Mammals, on the other hand, have more flexible instincts, which must be supplemented and shaped through learning and practice provided by play.

Groos's theory also explains the differences in playfulness found among different orders and species of animals. Among mammals, primates (monkeys and apes) are the most flexible and adaptable order, with the most to learn, and they are the most playful of all animal orders. Among primates, human beings, chimpanzees, and bonobos (a species of ape closely related to chimpanzees and to humans) have the most to learn, and they are the most playful species. Also among mammals, carnivores (including the dog-like and cat-like species) are generally more playful than herbivores, probably because success in hunting requires more learning than does success in grazing. Aside from mammals, the only other animal class in which play has been regularly observed is that of birds. The most playful birds are the corvids (crows, magpies, and ravens), raptors (hawks and their relatives), and parrots. These are all long-lived birds, with larger brain to body weight ratios than other birds, which exhibit much flexibility and cleverness in their social lives and ways of obtaining food.

The idea that play's purpose is to promote skill learning helps us to understand species differences in types of play as well as in amounts of play. To a considerable degree, you can predict what an animal will play at by knowing what skills it must develop in order to survive and reproduce. Lion cubs and the young of other predators play at stalking and chasing; zebra colts, young gazelles, and other animals that are preyed upon by lions and such, play at fleeing and dodging (see post on chasing games and sports); young monkeys play at swinging from branch to branch in trees. Among species in which males fight one another for access to females, young males engage in more play fighting than do young females. And, at least among some species of primates, young females, but not young males, engage in much playful care of infants.

Human children practice all sorts of skills through play, including skills specific to their culture.

In The Play of Man, Groos extended his insights about animal play to humans. He pointed out that human beings, much more so than any other species, must learn different skills depending on the society in which they develop. Therefore, he argued, natural selection led to a strong drive, in human children, to observe the activities of their elders and incorporate those activities into their play. Children in every culture play at the general categories of activities that are essential to people everywhere, but their specific forms of play, within each category, are shaped by the kinds of activities they see around them. When children are free, they play far more, and in a far greater variety of ways, than do the young of any other species because they have far more to learn.

Consistent with Groos's theory, children play in ways that promote the full range of skills that human beings everywhere must develop:

• We, like all mammals, are physical beings who must develop strong bodies and learn to move in coordinated ways, and so we have physical play, which includes chasing and rough-and-tumble games that are quite similar to the ways that other mammals play. In many other respects, however, we are unique, and our play reflects that uniqueness.

• We are the linguistic animal, and so we have language play, which teaches us to talk.

• We are Homo sapiens, the wise animal, and so we have exploratory play, which combines curiosity with playfulness to teach us about the world around us.

• We are the animal that survives by building things--including shelters, tools, devices to help us communicate, and devices to help us move from place to place--and so we have constructive play, which teaches us to build.

• We are an intensely social species, requiring cooperation with others in order to survive, and so we have many forms of social play, which teach us to cooperate and to restrain our impulses in ways that make us socially acceptable.

• We are the imaginative animal, able to think about things that are not immediately present, and so we have fantasy play, which builds and exercises our capacity for imagination and provides a foundation for what we call intelligence.

These terms, which I have put in italics, do not refer to mutually exclusive categories of play, but rather to various functions that play can serve. Any given instance of play may serve more than one of these functions. A lively outdoor group game may be physical play, language play, exploratory play, constructive play, social play, and fantasy play all at once. Play, in all its forms combined, works to build us into fully functioning, effective human beings. (For an expansion of these ideas, see post on how the varieties of play match the requirements of human existence.)

Also consistent with Groos's theory, cross-cultural studies of play have shown that children play especially at the kinds of activities that are most valued by their culture. Children in hunting and gathering cultures play at hunting and gathering, using the kinds of tools that adults in those cultures use. Children in farming communities play at animal tending and plant cultivation. Children in modern western cultures play at games that involve reading and numbers, if they grow up in settings where these are valued, and they play with computers and other modern forms of technology, the tools of today.