PLAY IN ANIMALS. Play is illustrated by kittens with their ball, puppies and their sham-hunt, lambs and their races, monkeys and their "follow-my-leader." There are often sham fights among birds which seem to be entirely playful, besides exhibitions of flying powers that have no direct usefulness. For it is one of the criteria of play that it is not directly useful. Play is not work, though it may be as strenuous and may lead to exhaustion ; it is not mere exercise, though, perhaps, it exercises best ; it has no deliberate (or, in animals, perceived) end, for the sake of which it is played, yet it may be almost indispensable if the animal is to attain to the full use of its powers. Play is not necessarily social, for many a kitten plays alone; and it is not necessarily competitive, though rivalry may give it zest. Its key note is its anticipation of modes of activity characteristic of adult life.
Play is well illustrated by many young carnivores, such as cats, dogs, foxes, otters and bears; by many young ungulates, such as lambs, kids, calves and foals; by most monkeys, and by less familiar cases like young squirrels and water-shrews. Yet it can not be said to be a general feature in the youthful life of mammals. It is not common among birds; it is only hinted at in reptiles, amphibians and fishes; and it is at most incipient among back boneless animals. This raises the question why a playing period should be interpolated in the life-history of only a small minority. There must be some particular biological advantage in play, yet one which only certain types have been able to secure.
Uses of Play.—The poet Schiller suggested that animal play is an expression of overflowing energy. But while this theory has its grain of truth, it is far too simple. Thus there are many young animals with abundant vigour that never play ; and it is well known that a thoroughly tired animal, such as a dog, may turn in a moment from fatigue to play, as children often do. Moreover, half the problem is that different types of mammals play in characteristic or specific ways.
Schiller's theory of play was re-expressed by Herbert Spencer with the important additional suggestion that imitation accounts for the particular form that the playing takes. The physiological condition of play is superfluous energy, but imitation defines the channel of expression. Young creatures mimic in play what they see their seniors doing in earnest. Here again there is some truth,
and corroboration may be found in the imitativeness of certain forms of playing in children. But Spencer's theory will not cover the facts. Thus an isolated young animal, such as a kitten, will play, and will play true to type, provided that an appropriate liber ating stimulus, apart from imitation, is supplied at an appropriate time. But if a kitten reaches a certain age—usually about two or three months—without having had any experience of mice, it will not afterwards show any "mousing instinct," nor any capacity for playing with a mouse.
A third idea has some relevance, namely the close correlation between pleasant emotions and bodily movements. It is a familiar fact of experience, elaborately studied by the physiologists and psychologists, that pleasant feelings reverberate in various parts of the body, such as the heart, lungs, larynx, food-canal and blad der. The correlation of emotional excitement and activity of the suprarenal bodies is well known. But to the internal movements there may be added movements of the body as a whole, and these will be naturally specific for different types. The child dances with joy; the otter cub gambols exuberantly. This simple movement play may be a useful safety-valve, but it is also a natural expres sion of overflowing joie-de-vivre.
To Karl Groos we owe the illuminating suggestion that play is important as an irresponsible apprenticeship to the subsequent business of life. It is the young form of work, and this accounts for its specificity. The young carnivore has its sham hunt, the young ungulate its amateurish race, neither involving serious re sponsibilities. Under the shelter of parental or communal care the playing animal educates powers essential in after-life, and is afforded opportunities without the serious consequences involved whenever the struggle for existence sets in keenly. As Groos puts it, animals do not play because they are young; they continue young in order that they may play. No doubt non-playing young animals also educate their capacities, but the point is that the interpolation of the play-period is an additional advantage which some plastic and well-endowed creatures have been able to secure for themselves. It is interesting that most of the mammals man has succeeded in domesticating are playing mammals.