Recess

The field has certainly not shed all of its skeptics, however, most of whom cite the lack of data. “There certainly are species that probably play,” says ethologist Judy Diamond, a professor and curator at the University of Nebraska State Museum, who studies play in birds with her husband Alan Bond. But “no systematic work has been done on their play. The question is whether people can come up with a rationale for why this is play and exclude other explanations.”

One of the “other explanations” many play researchers find difficult to rule out is that of exploratory behavior. When an animal plays with an inanimate object, for instance, it’s easy to confuse play with simple exploration, Diamond says. “How you separate object play from exploratory behavior is really quite difficult,” she says. “Of course it could be play, [but] it could be anything.” Even Pigface could have simply been exploring his environment, not playing with a basketball, says Bekoff. “It was one of those situations where you couldn’t really tell what in the world was going on unless you ask[ed] the animal.”

“My guess is that play is an offshoot of exploration,” says Mather, recalling the strong exploratory tendencies of the octopuses she has studied. The distinction, which is not an easy one to make, is that play should have fragments of other nonplay behaviors—the third of Burghardt’s five criteria—whereas exploration is expected to consist of longer bouts of investigation. “It’s the fragmentation you should be looking for in play, which you wouldn’t see in exploration,” she says.

Thus, by conducting studies of behavior that adhere strictly to Burghardt’s criteria, many are warming up to the possibility that play might exist in a wide range of species outside of mammals. “It wouldn’t surprise me,” says Sharpe. “There does appear to be good evidence,” at least in some species.

“To find play outside the realm of mammals is incredibly exciting [and] quite humbling, actually,” says Lewis, a skeptic-turned-believer in nonmammalian play behavior. Furthermore, she adds, “It just challenges our view on the evolution [of play]. If play exists in all of these different [species], either play exists in everything, which we don’t know that it does, or is this something that’s so adaptive it’s evolved repeatedly across lineages?”

While the debate over how wide-ranging the behavior is rages on, the concept of animal play is now generally accepted as a separate and important category of behavior. As such, the question is no longer if animals play, but why. And there’s no easy answer.

For most behaviors, says Sharpe, the function is clear, such as the use of aggression to win resources or mates. With less obvious behaviors, the general experimental approach is to stop the animals from doing the behavior and take a look at the effects. In a recent study of social grooming in meerkats, for example, the researchers treated the animals with insecticides to reduce the time they spent picking off ticks and fleas and found, surprisingly, that they became less aggressive. “But it’s really difficult to stop an animal from playing without impacting other aspects of its behavior,” Sharpe says.

Indeed, “in the 1960s and ’70s before the advent of ethics committees,” she adds, “a number of studies did just that—raising rats and monkeys in social isolation so they couldn’t play-fight, or confining young ungulates to small spaces so they couldn’t gallop and frolic.” While the researchers saw effects, they were “largely meaningless” because of all the other aspects of the animals’ lives that were affected by the experimental conditions, she says. “[So] no one’s been able to come up with any convincing, strong evidence that there’s a benefit at all.”

“To me, that’s the most important question still—why do individuals engage in play?” agrees neuroscientist Anthony Auger of the University of Wisconsin in Madison. “There [are] many theories, but I don’t know which is the best.” Because play exists in such a wide range of animals, social behavior researcher Giada Cordoni of VademECOS in Italy says it is likely “a behavior that is maintained by natural selection.” As such, “we can argue that play has some adaptive function.”

Perhaps play has a delayed benefit, but no immediate function. One leading theory is that play helps animals develop socially and cognitively, and, in highly social species, helps establish dominance hierarchies. Some of the strongest evidence for this hypothesis comes from comparative studies of closely related species. Bekoff’s research on canid species—which include coyotes, wolves, and domestic dogs—found that “the more social canids played more in early life,” he says. Furthermore, “the amount of play in early development is related to the later social organization [and] the complexity of the relationships that the animals form.”

A similar pattern was also seen in bird species in which play has been well documented, including parrots, corvids, and hornbills. In these birds, complex social play, such as the rambunctious wrestling and synchronized wing-flapping and hopping behaviors observed in some parrot species, appeared more likely in species that lived in complex, social groups and maintained relationships after the juveniles fledged from the nest. This suggests that “one function [of play] might be that it ameliorates aggression,” says Diamond. “Play may have evolved as a mechanism to reduce fighting when there are selection pressures that prevent [juvenile] dispersal.”

Many scientists are also looking closely at the neuroanatomy of play behavior for clues to its role. “We tend to associate play, particularly the complex cognitive aspects of play like social play, with animals that are smart,” says Lewis. “So there’s this ongoing assumption that there should be this relationship between brain size and play behavior.” But when a comparative study of mammals showed that “absolute brain size tells us nothing,” she says, researchers started to look at particular regions of the brain that might affect the frequency and complexity of play behavior.

Looking at primates, researchers tied social play to the size of particular brain regions important in social cognition, finding that species with a larger cerebellum, neocortex, hypothalamus, and amygdala all engage in more social play. The findings corroborate the idea that animals are using social play to learn social rules, Lewis says. The amygdala and hypothalamus, for example, are “completely bound up in emotion and the socioemotive aspects of [social development],” she says. “Primates, being social, are naturally very keyed in to the points of view of others in their group, and they need to be able to do that to be successful as primates.”

Similarly, the cerebellum, involved in social learning, and the neocortex, the “thinking part of the brain,” seem important for learning how to respond in socially appropriate ways, Lewis says. “If you fail to learn how to react appropriately to things, you’re not going to make a good adult. Young animals have to play to make their adult brains better.”

In fact, studies using rats showed that one-third of genes in the neocortex were modified by play. “That means play is having a big effect on our higher brain,” says neuroscientist Jaak Panksepp of the College of Veterinary Medicine at Washington State University. “But we know that [the] higher brain is not needed for play,” he adds, “because we’ve taken it away and animals [still] play normally.”

Another popular theory says that play helps juveniles develop motor skills. “In [the] ’70s and ’80s, everyone thought [play’s function] was practicing physical skills—hunting in predatory animals or fighting in [species] like baboons and social animals,” says Sharpe.

In juvenile ground squirrels, for example, individuals who played more often and with many partners appeared to have improved motor skills. Because juvenile American kestrels prefer to play with objects that resemble prey, researchers have suggested the behavior helps hone their hunting skills in adulthood. Similarly, in populations of herring gulls that drop prey items to break them open for consumption, juveniles sometimes drop and re-catch prey before they hit the ground. Researchers have classified this as play behavior, possibly serving to improve the young birds’ foraging ability by strengthening muscles or solidifying neural connections important for dropping behavior.

But direct support for this hypothesis is lacking, and recently, “there’s been more of an emphasis [on how play] might be priming [animals’] more physiological [systems], like their stress response,” Sharpe says. “When animals play they use the same neurochemical receptors that are used in [dealing with stress],” she explains. “When young animals prime these systems, they respond to stress differently—they don’t get as stressed or anxious, and they recover quicker.”

This idea was formally conceived in 2001 and dubbed “training for the unexpected.” Because “the behavior of animals that play is more flexible,” says Cordoni, “play seems to be useful to manage the uncertain situations.”

“Training for the unexpected seems to be a fairly global reason why animals play,” agrees Bekoff. “Across species, it seems that play really prepares animals to deal with unexpected circumstances in their social and physical environment because it gives them the opportunity to practice different variations on behavior.”

But while all of these hypotheses—social, cognitive, and motor development, and preparing for unexpected situations—are likely to play a role in why play behavior evolved, the evidence for these benefits remains sparse. Furthermore, biologists are now beginning to realize that play may also serve an immediate function, as play has recently started to be documented in adults as well. “It’s difficult to think that adult play may work for developing motor skills,” Palagi says.

Instead, Palagi and her colleagues found evidence that adult play in chimpanzees served to help them to cope with their competitive tendencies just before feeding time. Similarly, a study of adult Japanese macaques that continue to play with stones as juveniles do suggested that the behavior is psychologically relaxing, and may help maintain neural pathways and slow the cognitive degeneration associated with aging.

“The context is very important for definition of the possible roles of play,” Cordoni says. “I’m quite sure that play has got various functions depending on the species, the sex, the age, the social context and environment. It’s a very complex behavior, and because it’s complex, it’s very interesting.”