Jungle Gyms: The Evolution of Animal Play
by Alex Hawes
"The Battle of Waterloo was won on the playing fields of Eton."
-attributed to The Duke of Wellington
On the African savanna, a lion cub is wrestling with a young peer, pouncing and pawing, at one time going for the kill, and at another playing the victim. Not too far away, a young zebra is gamboling about by itself, every so often kicking out its hind legs for no apparent reason. Years later, each animal will have matured, and the zebra may find itself desperately fleeing from the pouncing lion, and fiercely kicking back its legs in an attempt to ward off the attack.
Research into animal play has most often focused on the link between juvenile playfulness and serious adult activity. Humans have long recognized this correlation in themselves. Plato, 2,400 years ago, advocated that Greek children be provided with numerous toys and tools in order to develop the skills necessary for their adult lives.
Even to the untrained eye, animal play behavior can be quite obvious. Countless examples pop up in the scientific record. One researcher's hand-raised badger would often break into bouts of somersaulting. A rhino in the Basel Zoo in Switzerland would push and even toss around a 100-pound rubber ball with great bravado--waggling its ears and snorting all the while. Young mountain goats are often seen rearing up on their hind legs, in impressive displays aimed at no one. A primatologist observed two young bushbabies (tiny primates) "hang suspended from a horizontal stick the while beating at each other with their free arms like two inverted pugilists." Another scientist witnessed a rhesus monkey in the wild doing a reverse flip!
Play is not seen only in mammals. Young garden warblers were observed again and again picking up pebbles and marbles from the ground, flying up to a branch and dropping them into a glass, producing a sharp jingling sound. Keas, large parrots from New Zealand, are real clowns--they stand on their heads, turn somersaults on top of branches and even in deep water, land upside down, use branches as swings, and make and push around snowballs. Some have speculated that reptiles may even occasionally show signs of fun and frolic. (However, the only chance you have to witness playful insects or fish is probably in a Disney movie.)
Although we have acknowledged playfulness in animals for hundreds of years, it has only been since the advent of the Darwinian revolution in the last century that adaptive explanations, based on behavioral studies, have been put forth. At the end of the 19th century, the English philosopher Herbert Spencer proposed the "surplus energy theory" to explain animal play. He suggested that such seemingly useless behavior has arisen from the inactivity of the higher animals, who don't need to spend every minute of the day foraging in order to survive. Bored animals play, according to Spencer.
Karl Groos, a German scientist and a contemporary of Spencer, focused on the ultimate benefits of play to individuals. Groos introduced the idea that juvenile play serves as practice for adult behavior. He even went so far as to propose: "Now we see that youth probably exists for the sake of play."
Despite this enthusiastic and bold launch into the study of play, the field has more recently received less attention, and less respect. "Play research is the ugly duckling of behavioral science," writes the University of Alaska's Robert Fagen, an expert in the field. The subject suffers first of all from difficulties in definition. While play may be easily recognizable, it is much harder to define as a concrete category of behavior. Also, much evidence of play comes from anecdotes, and conclusions about its significance often are based on speculation, rather than quantifiable data.
Practice Fields and Play Rehearsals
John Byers, of the University of Idaho, and Marc Bekoff, of the University of Colorado, have done their best to establish a definition: "Play is all motor activity performed postnatally that appears to be purposeless, in which motor patterns from other contexts may often be used in modified forms and altered temporal sequencing." (Kind of takes the fun out of play, doesn't it?) Simply put, play resembles serious behavior in appearance, but not in purpose or outcome.
Animal play can be categorized roughly into locomotor play, predatory play, object play, and social play. These categories share certain underlying characteristics--patterns in which individuals act out behavior similar to adult forms, but which are not completely fulfilled (a lion cub won't seriously bite a playmate it has wrestled to the ground; zebras playing chase won't actually kick each other).
Locomotor play consists of juvenile animals carrying out movements--either of parts of their bodies or their entire bodies--similar to the physical actions of adults, but with no immediate or obvious end goal. Examples of locomotor play include running, leaping, pirouetting, head shaking, heel kicking, and whirling around. As the examples may hint, hoofed mammals (ungulates) are the major participants in this type of play. The verb "caper" stands as testimony to this sort of frisky behavior in young goats; the scientific term for goats, taken from Latin, is Caprinae. Locomotor play appears to function as motor skill training for animals that rely on agility and speed to avoid predation. Not surprisingly, running is the most common aspect of locomotor play observed.
Such escape-oriented play is not limited to ungulates. Young in many primate species chase each other, perhaps training for emergency situations when predators threaten them. Adelie penguin juveniles climb on blocks of ice, and then knock each other off, in an Antarctic version of "King of the Hill." This game mirrors the way penguins dart onto ice flows or land at the first sign of a leopard seal attack.
For anyone who has had a pet kitten, the next type of play--predatory play--is undoubtedly familiar. Chasing a string or its own tail, wrestling with a ball of yarn, stalking its owner's legs--these are all examples of a young cat training for a future of hunting prey. Paul Martin, of the University of Cambridge, suggests, though, that animals may need only minimal play practice to become competent predators. One study found no link between the amount coyotes play and their success in hunting. Nonetheless, the tendency to play-hunt persists even after hunting skills have been attained: An adult cat, for example, will play with dead or dying prey, well after the cat has become a successful hunter.
Felids and canids are not the only predatory players. Groups of swallows, which prey on airborne insects, will drop a feather from high off the ground and then swoop down and catch the feather in their beaks again and again as it slowly falls to earth. Falcons and crows exhibit similar playfulness, using a wide variety of objects as toys to drop and retrieve in mid air. Young kingfishers mimic catching fish by diving down and capturing small twigs floating on the surface of streams or ponds.
The third category, object play (which is fairly self-explanatory) overlaps with predatory play considerably, as the previous examples demonstrate, but not entirely. For instance, bird stick-play may serve to develop nest-building skills. Non-predatory object play, however, has been manifested primarily by the primates, and to the greatest extent by the primates most closely related to humans. The increased complexity and dexterity of the hands of these species, as well as longer periods of maturation and an increased capacity for learning, may explain this trend. Chimpanzees' technique of fishing termites out of holes with sticks and vines reflects the importance of object play for developing manual dexterity. Young chimps toy around with sticks, trying to fish. While unsuccessful at first, the youngsters improve with age and practice.
A Friendly Game of...
The examples of play mentioned so far generally relate to an animal's gaining experience in its relationship with its environment, and with its own physical abilities. However, playfulness also allows juveniles to learn how to deal with fellow group members. Social play, the fourth and final category, may facilitate cooperation, develop alliances, and encourage innovation in social behavioral patterns. Animal young wrestle with each other, play games of "tag," and exhibit other behaviors similar in certain ways to competitive and often aggressive adult contests, but without the actual violence.
Young animals from across the mammalian spectrum engage their peers in bouts of wrestling and other physical activities. Elephants slap and wrestle each other with their trunks. Giraffes use their long necks to spar gently, necko a necko. Moose calves may practice their head-butting on unsuspecting trees when a peer is unavailable. Animals will even cross the species boundary to find play partners; juvenile chimps and baboons in the wild occasionally tussle together.
Researchers suggest that social play may safely teach young the skills they will later use in aggressive social competition. However, such play may also simultaneously strengthen social bonds between group members, a process that serves to limit the amount of actual aggression between group members.
If social play can mirror real aggression in appearance, what then lets animals know that their partners are only playing? Animals, it turns out, communicate playful intentions with certain stereotyped signals. The most widespread play signal is the play face, a relaxed, open-mouth expression seen in many mammal species, used virtually from birth. The human smile almost certainly evolved from this ancestral trait. If someone smiles or laughs while hitting you in the arm, you realize that his or her intentions are very different than if he or she is frowning, with a tightly closed mouth. In other animals, a playful expression communicates much the same thing. For great apes, the extension of one's arms, hand contact, and an exaggerated gait similarly relate that "what follows is play."
Animals use specific signals during, as well as before, bouts of play that may serve to keep playful activities going, and prevent them from becoming serious. Marc Bekoff, who has observed dogs, coyotes, and wolves, has noticed that, during playfights, members of each of these canid species "bow" to each other--crouching over and lowering their heads to their feet. Bows may signal to a partner that "'I want to play despite what I am going to do or just did--I still want to play,'" Bekoff writes.
"Play implies contrast," says Donald Symons, of the University of California at Santa Barbara. Yet while a playful bite is not a real bite, that's not to say it is pointless. As Symons notes, although batting practice is not the same as a real game situation, it still helps develop a baseball player's abilities.
Who plays may be just as important as how one plays. Young animals play much more than older ones, which makes sense if play has a role in an individual's development. Juvenile males more often display social play than their female peers. Particularly in polgynous species, where dominance may improve males' access to mates, adult males generally compete with each other more actively and aggressively than females. Symons found that immature male macaques play more roughly than their female counterparts, and male-male playfights more often end in real violence. In one study of young mule deer, males and females both demonstrated locomotor play (after all, both sexes need to develop anti-predator defenses), but only juvenile males incorporated head-butting, a component of male rivalry, into their play repertoire.
In many animal species, females and males play in essentially the same way, though males may play more often. However, in other species, particularly primates, females may play in a qualitatively different way than males. While male juveniles prefer rough-and-tumble play, young females in such species show more of an interest in grooming and caretaking--especially when infants are involved.
Jane Lancaster, of Rutgers University, studied "play-mothering" in female vervet monkey juveniles. Outside of the birthing season, young female vervets would spend as much time with peers as males would. However, when infants were born into the group, juvenile females gravitated to these newborns and their mothers. "It is common to see each new mother acquire an entourage of juvenile females who follow her about during the day waiting for a chance to touch the infant," Lancaster writes. The young females would cuddle, touch, carry, and groom any infant given the chance, often to the growing annoyance of the infants' mothers. No such behavior was witnessed on the part of male vervet juveniles.
Fun Evolution
Piecing together an evolutionary explanation of play from the hundreds of examples in the scientific record has proven challenging. Playing seems to take time and energy--expensive commodities in an evolutionary economy--from individuals' budgets. Such behavior must then benefit players in some substantial way in order to offset its costs.
While certain of the more active varieties of locomotor play may be immediately beneficial for one's physical conditioning and strength, most play does not confer such instant rewards. Rather, the purpose of play--if one can assign it a single, underlying function--seems to be the rehearsing and honing of behavior important for adults. Juvenile cubs obviously can't practice hunting and killing prey upon each other without moderating aspects of hunting techniques. Social play must differ from real dominance competition, or else it would escalate into actual aggression. Ungulates can't practice escape with real predators, for obvious reasons. Play therefore seems to serve as relatively risk-free motor training for behaviors normally carried out only in life-threatening or otherwise serious occasions.
These days, research is focusing on the specific ways juvenile play best serves as effective practice. National Zoo biologist Katerina Thompson is examining questions such as what causes play to start and end, and how individuals choose with whom to play, in the National Zoo's population of sable antelope at the Conservation and Research Center (CRC) in Front Royal, Virginia. She is trying to get a firmer grasp on the vague concept of practice. One theory Thompson has is that peers act as "yardsticks of development," whereby juveniles can compare their motor skills with the skills of others their age, and learn to avoid partners too strong or aggressive.
Not Just Child's Play
The evolution of play behavior may correspond somewhat with the evolution of intelligence. The animals regarded as most intelligent--birds and mammals--play a lot, whereas the animals deemed less intelligent--fish, reptiles, and amphibians--show little or no apparent evidence of play (though how could we tell if a cod were playing?). The propensity to play also seems to be linked to the length of the juvenile period--animals that take longer to reach maturity have longer to learn and practice new behavior, unfettered by the pressures of adult life.
Human children, with the longest juvenile periods and highest intelligence in the animal kingdom, certainly show no lack of playfulness. Their play demonstrates more symbolism and imagination in form and content than that of other animals. However, who are we to say that other animals at play don't draw upon their imagination, similar to that of a child's make-believe? If one accepts certain continuities between our own behavior and that of our animal relatives, then such similarities should not be altogether surprising.
Looking at it the other way, we should see aspects of animal play in children's behavior. Play seems to offer many animals valuable practice for later life. So might the play of human children. As the Duke of Wellington suggested, something as seemingly innocent as sports or other forms of play may reflect an instinctive rehearsal for adult behavior that is far more serious.
So the next time you see animals at the Zoo--or children in the playground--tussling or bounding about in joyful abandon, consider how such frivolity might develop the motor skills needed for their adult lives. In the end, it's not all just fun and games.
(ZooGoer 25(1) 1996 Copyright 1996 Alex Hawes. All rights reserved.)
