From puppies romping through a field or rat pups wrestling and nipping at each other to an infant shaking a rattle, nearly all social animals play in some form. Myriad species spend some of their time in activities that have no obvious function other than fun.

Why would a behavior develop across multiple species if it doesn't have some ulterior function? The most common theory is that juveniles play at the skills they will need as adults. But some newer thinking proposes it's more than that. In fact, play seems to have some immediate perks, such as aerobic conditioning, as well as long-term benefits that include preparing animals for the unexpected and giving them a sense of morality.

Research suggests that play likely serves multiple developmental needs. And, says psychologist Peter Smith, PhD, of Goldsmiths College, University of London, the reasons may have changed for different species, and almost certainly vary depending on the type of play. For physical activity play and rough-and-tumble play, researchers can attempt some comparisons from animals--especially dogs, cats and monkeys--to human children. For object and especially pretend play, there are probably different functions, and these kinds of play are more specifically human.

On an immediate level, physical play seems to help juveniles of many species become physically strong by providing aerobic exercise and honing motor skills, says University of Colorado psychologist Marc Bekoff, PhD, who studies the antics of wolves, dogs and other canines. It also serves as social training, teaching children and other juveniles the rules of social behavior and helping them understand where they fit into the social hierarchy of their community.

Morality play

Bekoff's most recent thinking on this aspect of play is that it teaches animals a sense of morality. Through roughhousing, animals form social bonds, acquire different dominance ranks and learn what behaviors are acceptable: how hard they can bite, how roughly they can interact and how to resolve conflicts. They then generalize these codes of conduct to other situations.

"They learn what's 'right' or 'wrong,' what's acceptable to others--the result of which is the development and maintenance of a well-oiled social group," writes Bekoff in the January 2001 issue of the Journal of Consciousness Studies.

Human children learn similar lessons in their play as they interact with peers and learn which behaviors gain them friends and social status and which do not, say researchers. And play among juveniles of all species may be practice for later life: Males play-fight to prepare for fighting for territory or mates, females play at caring for young.

"It seems as though play affords juveniles opportunities to learn and practice new skills," says Anthony Pellegrini, PhD, of the University of Minnesota, who studies play in children. "The more complex the organism and the corresponding skills to be learned, the longer the juvenile period is extended and the more important play seems to be."

Although Bekoff agrees, he thinks there's even more at "play" than simple rote practicing of adultlike skills. Instead, he prefers to think of play as a way to prepare animals for the unexpected. In fact, juveniles tend to mix skills rather than performing those needed for a single adult action such as predation, mating or aggression.

"Animals put actions together in a hodgepodge sequence--they might bite, mount and give a hip-check, for example," says Bekoff. "You would never see that pattern if they were truly mating or hunting. I see this variable sequencing as training for the unexpected--keeping animals from getting stuck in one behavioral pattern."

No play is no good

What happens if animals--human or other--are deprived of play? It's difficult to design a study that can separate play from other forms of behavior, says Pellegrini. For example, if you deprive an organism of social play, you likely deprive it of most types of social interaction.

With that said, his research clearly shows that providing children with breaks during the school day maximizes their attention to cognitive tasks. This is particularly the case for boys, and there's some indication that, at least for primary school children, it's important for these breaks to involve play.

The work of Jaak Panksepp, PhD, who studies play in rats at Bowling Green University, suggests that depriving young animals of play might delay or disrupt brain maturation. In particular, his research finds that play reduces the impulsivity normally seen in rats with damage to their brains' frontal lobes--a type of damage thought to model the human disorder attention deficit hyperactivity disorder (ADHD) because it affects executive functions such as self-control.

He and his student Nikki Gordon have also found some evidence that play increases gene expression in the frontal lobe for a protein thought to be involved with brain maturation. Without play, Panksepp suggests, self-control and other executive functions may not mature properly.

This research has led him to propose a connection between rough-and-tumble play and ADHD. In fact, based on their findings that "abundant access to rough-and-tumble play" reduces the hyperplayfulness and impulsivity of rats with frontal lobe damage, he and his colleagues propose that a regimen of social rough-and-tumble play might be one way to help children with ADHD control impulsivity. Aside from any clinical problems that might be associated with play, "children, or other juvenile animals, that do not have the opportunity to play may not develop the social skills required to interact successfully with their peers, at any age," says David Bjorklund, PhD, of Florida Atlantic University.

Even though children can perform pretend play--which has been linked to both social (see article on page 46) and cognitive development--on their own, they're more apt to engage in it when interacting with other children or adults. This suggests that isolation from other children may hamper aspects of children's cognitive, as well as social, development, says Bjorklund.

The idea that all types of play--from fantasy to rough-housing--have an important, if not critical, role in child development has not gotten much attention from mainstream child development researchers, says Pellegrini. Even so, the evidence is mounting that play has immediate as well as long-term consequences. And such findings are taking on particularly important meaning as some school districts toy with the idea of eliminating opportunities to play from their primary school curricula.

"Our research has clearly shown that this is misguided, and may actually do harm," he says.

Beth Azar is a writer in Portland, Ore.