Two Australian magpie fledglings gently peck at each other. One bird flops lazily on its side, while the other continues to peck and then falls over too. For a moment, they poke each other with their feet, as their black-tipped tail feathers flare. Then one hops back up, tackles the other, wrestles the tiny bird to the ground and they roll on the grass like kittens.
It was 34 years ago that John Nelson, PhD, a zoology professor at Monash University in Melbourne, summoned young Sergio Pellis, PhD, to watch a 16 millimeter film of the magpie fledglings, projected onto his office wall.
"What the hell are they doing that for?" Pellis asked Nelson.
"Nobody knows," the professor said.
Thanks to research by Pellis and others, that's no longer the case. Over the last four decades, scientists have observed play among the unlikeliest of animals. Stingrays bat balls around with their flat bodies and fins. Monitor lizards mischievously pick notebooks out of their keepers' pockets. Sea turtles swim back and forth through hoops.
In some cases, play occupies a critical role in an animal's development, and in others it is seemingly useless, says Pellis, a behavioral neuroscientist at the University of Lethbridge, in Alberta, Canada. Researchers are now on the cusp of mapping play along evolutionary lines to determine its role in species' evolution. Research by Pellis and others suggests that play is an important factor in shaping the brain and behavior of a species.
What is play?
According to University of Tennessee psychology professor and animal-play expert Gordon Burghardt, PhD, animal play is, in part, characterized by the fact that it's pointless—for example, behavior that looks like hunting but does not result in killing prey, such as a cat pouncing on a rubber mouse or play fighting. Play must be pleasurable, spontaneous, voluntary or fun, he says. It must be repeated. It must be initiated when an animal is in a relaxed, low-stress state. And it must be different in either structure or time from serious behavior.
Among rats, there are clear differences between rough-and-tumble play and real fighting, Pellis says. When play fighting, rats compete for the nape of the neck, and once contacted, the neck is nuzzled playfully. When the fighting is serious, they go for the rumps and flank, and they bite.
For rats or dogs or primates, it's easy to recognize the behavior as play. But with others, it's harder to tell. Consider insects. Elisabetta Palagi, PhD, a professor at the University of Pisa, in Tuscany, has studied play in wasps and found behavior that meets all of Burghardt's criteria, she says.
When paper wasps form colonies in the spring, they engage in a ritual to establish their hierarchies. The dominant female wasp approaches the subordinate, raises her head over the other wasp's head, rapidly beats her antennae, and often licks, bites and begs for food by putting her mouth close to the other wasp's mouth. What caught Palagi's attention was that months earlier, when wasps gather together in sheltered quarters prior to winter hibernation, they behave in a similar way. And at this time, the behavior appears to have no function, according to her study, published in the Journal of Comparative Psychology (Vol. 120, No. 4).
The behavior could be similar to play fighting in young mammals, Palagi says. "By play fighting, immature animals assess their own strength and acquire social competence, thus attempting to establish dominance hierarchies well before any reproductive conflict could arise," she says.
Not all experts buy into the theory that wasps play. Pellis, for instance, doubts that the behavior has any real function for the insects. "I'm not 100 percent convinced," Pellis says.
The purpose of fun
Researchers have long searched for the purpose of play, but not all instances of play may serve any higher goals. For example, play-like behavior in wasps doesn't seem to help them develop social skills later in life, Palagi says.
Burghardt agrees that some play behavior has little function; the animal equivalent of doodling or twiddling your thumbs. And in some cases, the function is indirect and not so obvious.
"The wasps, for example, may be getting exercise or aiding some general skills by the fall interaction, even if they have nothing to do with the spring competition," Burghardt says.
Other times, play is essential for an animal's survival, he says. That's certainly the case for play fighting, he says. "In this case, play has achieved a function in helping animals learn new skills," says Burghardt.
Studies by Pellis support Burghardt's theory. His research shows that young rats deprived of play fighting have social, cognitive and sexual deficits as adults, including hyper-defensiveness, trouble solving problems and difficulty mating. When rats are prevented from play fighting as pups, they never develop social skills to interact with other rats, to escape when threatened or to adapt to difficult situations. And since much of play fighting mimics sexual behavior, rats without play have been known to mount the wrong part of their mates' body, such as the head.
"Bottom line is, you've got to play to be socially competent and to have a brain able to deal with problems," Pellis says.
Because of such research, many experts believe a tendency to play—and to enjoy play—may serve an evolutionary purpose. Through play, animals develop the set of skills necessary for survival and reproduction, and natural selection over time favors these animals.
To explore that idea, Pellis has created a "cladogram," a map that shows relatedness among a group of primates, along with the presence or absence of play and the complexity of the play behavior. In some lineages, play appears to become more complex as the species evolved, suggesting that it helps a species survive. In others, it doesn't.
When mating opportunities are rare, for example, animals can't afford to learn sexual behavior through play. Unlike its other rodent cousins, the Australian hopping mouse seems to exhibit little or no rough-and-tumble-play behavior. Though little research has been done on the mouse, Pellis speculates that fewer opportunities for mating may have caused sexual behavior to evolve to become more instinctual.
"For the [animals] where the male runs into the female very rarely, you've got to make sure you've got the equipment to jump on and make sure you're the one," he says. "You can't afford to wait for it to get better."
In other cases, nature selects for behavior that must be sharpened by play. For example, animals with complex social hierarchies, such as gorillas, may need play to hone their social skills, which may, in turn, strengthen the functioning of their prefrontal cortex, Pellis says.
"You have a bigger brain, and you can have more complex play, and that feeds back onto making the brain more complex," he says. "And you can imagine a positive escalation going on."
So far, Pellis and his colleagues have collected data on play among 35 species of primates. He's now working to analyze their findings, to see if the evidence supports his theories.
If these researchers have it right, play may occupy a vital role in shaping the brains and behavior of entire species. And beyond rats and primates, it may also shed light onto the evolution of human intelligence.
"Play may be the reason we evolved so quickly, that we have big brains and intelligence," Burghardt says. "Play may have been an important aspect in the rapid evolution of human mentality."
Jenny Marder is a writer in Washington, D.C.