Of interest are those cases where playing or something like play ing is continued long after youth is past. This is familiar in domestic dogs, but is also exhibited in natural conditions, for instance by the otter. This extension of play may be sometimes associated with the mother's habit of playing with successive lit ters of young ones year after year. But this interpretation does not apply to all cases, for instance to the communal playing of full grown penguins on the sea-ice. Thus Murray Levick has described the diving play in which the succession of birds may be so rapid "as to have the appearance of a lot of shot poured out of a bottle into the water." A favourite activity was to board an ice-floe till it would hold no more, and get carried by the tide to the lower end of the rookery, where every bird would suddenly jump off and swim back against the stream to catch a fresh floe and get another ride down. An adult snake-bird (Anhinga) has been seen playing catch with twigs, an activity obviously correlated with its dexterity of head and neck in catching fish. Many birds have flight games; the "shooting" and tumbling of rooks and herons, and the turning upside-down of ravens may be specially mentioned. It seems im possible to restrict the idea of play to youth.
But in general play is a mode of behaviour characterizing the youthful period of certain well-endowed animals, a precocious exhibition of activities more or less anticipatory of those charac terizing adult life, but not in themselves of direct utility. Its biological significance is partly as a safety-valve for overflowing energy, partly as an early expression of imitativeness, partly as a correlate of pleasant feelings, but mainly as an irresponsible ap prenticeship to adult activities and an opportunity for testing new departures, especially in habit.
(2) It is undoubtedly difficult to draw the line, but it seems useful to try to exclude from typical play all activities bound up with sex-display or courtship. For while these resemble play in being artistic and spontaneous expressions of individuality, they have an immediate outcome : they serve to arouse sex interest and sex desire, whereas typical play never has any immediate reward. If it seem impossible to draw a line between play and display, it might conduce to clearness if the word, sex, were used as a prefix. Thus one might use some phrase such as courting
dance for many of the extraordinary displays that birds make at the breeding season (see BIRD, Reproductive Habits; COURTSHIP OF ANIMALS). W. H. Hudson portrays the dance of the cock-of the-rock (Rupicola) of tropical South America : "A mossy level spot of earth surrounded by bushes is selected for a dancing place, and kept well-cleared of sticks and stones; round this area the birds assemble, when a cock-bird, with vivid orange-scarlet crest and plumage, steps into it and, with spreading wings and tail, begins a series of movements as if dancing a minuet ; finally, carried away with excitement, he leaps and gyrates in the most astonishing manner, until, becoming exhausted, he retires and another bird takes his place." This strikes a note quite different from that sounded in the races of lambs and kids, wild foals ant asses, or "tag" and "follow my leader" among monkeys.
(3) It is part of the essence of play that it is not directly usefui, but has a prospective value in educating efficiency. But not a few animals with abundant spare energy and initiative are known to indulge in occasional adventures which, though they can hardly be called other than playful, have no prospective meaning. Some of the experiments of apes, to which we have already referred, may illustrate this kind of behaviour, and should perhaps be called tricks rather than play. True play is characteristic of a species and is neither occasional nor individual. A naturalist re lates that on one occasion, when botanizing on the Alps, his dog ceased to follow him on the graduated path, and was seen to choose a more direct slope of hard snow. There he lay down on his back, folded his legs and slid down like a toboggan. At the foot he looked up at his astonished master and wagged his tail! No conclusion can be based on single instances, however well documented, but we cite this case as an instance of probable mis interpretation. The observer supposed that the dog had thought out a short cut—an unnecessarily generous view; others have called it a piece of play. But the probability is that it was a casual adventure, such as may be reasonably put to the credit of many a well-endowed animal. Similar instances are known at much lower levels of intelligence. The ecological concept of animal play is most useful when employed in the strictest sense, as already defined. (See ANIMAL BEHAVIOUR; PSYCHOLOGY, COMPARATIVE; SEXUAL SELECTION.) BIBLIOGRAPHY.—Karl Groos, Play of Animals (1898) ; J. Arthur Thomson, Biology of the Seasons (191I) ; The Minds of Animals (1927); P. Chalmers Mitchell, The Childhood of Animals (1912) ; W. P. Pycraft, The Infancy of Animals (1912) ; Murray Levick, Natural History of the Adelie Penguin (1915) ; W. Kohler, The Mentality of Apes (1925) ; Frances Pitt, Animal Mind (1927).