The serious business of animal play.

Daybreak is playtime at the Brookfield Zoo outside Chicago. Baby male giraffes stretch their long, strong necks, swinging and clashing them like swords. Furry nestled lion cubs turn into a furious tangle of chasing, pouncing, ear gnawing, and tail chomping. Kangaroo young, called joeys, bound away from their mothers, using their powerful legs to leap and twist in the air. Young ibex kids race up steep rocky inclines and hurtle over ridgetops, performing feats that would shame the most steel-nerved skateboarders.

For several summers, John Byers, a biologist from the University of Idaho, has observed and recorded these playful scenes. Fun, as it seems, he says, play among young animals is more than just amusement: It's a serious business, an adaptation essential to survival.

ANIMAL WORKOUTS

Only a relatively small group of animals--mostly young and almost exclusively birds and mammals--are known to play, says Byers.

Biologists believe a prime purpose of this play is exercise. Puppies racing across a field, for example, are building sturdy bones and increasing the mass and strength of their muscles. Sprinting also increases the animals' endurance and strengthens their hearts.

But rough-and-tumble play does more than just make an animal fit. Biologists believe it also enhances neuromuscular development--the development of the parts of the brain that tell muscles what to do.

As a mountain-climbing ibex kid dashes over rocky terrain, nerve cells in its brain are wiring together a network. This network coordinates the actions of many of the muscles that allow the ibex kid to, say, leap safely from one tiny ledge to another.

Such play is, in a sense, practice. Just as you might practice batting a ball over and over again, a young Rhesus monkey at play learns how to coordinate its muscles to race through a tangle of branches without a slip.

In addition, "play usually mimics the sort of activities the animals perform as adults," says Byers. A young kitten pouncing on a rolling ball is not trying to be cute; it's probably mastering instinctive hunting skills.

PLAY AND STAY TOGETHER

Animal play is not usually a solitary affair. Young primates "organize" games not unlike those you'd see on a human playground. "Chimps and gorillas play games like hide-and-seek, follow the leader, and king of the mountain," says Frank Poirier, an anthropologist at The Ohio State University.

Biologists believe such play builds strong social bonds among animals that must cooperate to survive. For example, wolf cubs that play together now will someday hunt together in a pack.

Animals also learn other lessons in their group play. "All primates learn what their status is among the group," says Poirier. "They learn which animals are stronger and which are weaker." Much the same is true of young hippos, which often bite and rain one another in playful bouts, testing their strength.

During "play fights," animals are careful not to hurt one another. Bear cubs angle their paws when boxing so that they don't tear each other with their claws. Fox pups wrestling and fighting each other do bite, but not hard enough to cause wounds.

Play bouts among mammals often start with a formal invitation or "play signal." Like a karate master before a match, a puppy invites play with a "play bow," by kneeling forward on its forepaws. A young colt will suddenly spring. A chimpanzee will grin. A panda will turn a somersault.

"The purpose of play signal," says Poirier, "is simply to let the other animals know where you are coming from--|I want to play [not to fight]."

REAL LIFE

Most animals outgrow play after a few years or so, depending on their rate of development or maturation, says Poirier. "Female animals generally develop faster than males," he says. "So play among females usually ends at an earlier age. You see this in many human societies."

Yet some animals remain playful throughout their lives. Dolphins in captivity play with balls, toy rings, or just about any object thrown in their tanks. Adult river otters have been observed to wrestle and frolic year-round. Full-grown male gorillas will play with their young for hours at a time. Young ibex mothers may occasionally kick up their hooves to encourage lazy offspring.

But animal play is still mainly kid stuff. See for yourself this summer at your local wildlife conservation park.