PLAY'S the THING

Natural History, July, 1999 by John A. Byers

Some young animals spend hours running, leaping, boxing, and wrestling, while others seem to have mush less fun. Could it be a case of mind over metabolism?

I'll never forget my first look at a koala brain. I was spending a sabbatical year in Australia and at the time was visiting a study site northwest of Melbourne, where colleagues Tony Lee, Kath Handasyde, and Roger Martin had radio-collared several koalas. When one of the animals died, my biologist friends invited me to attend the necropsy. As the veterinarian lifted off the top of the koala's skull, I was amazed to see that the brain did not fill the space inside. The smooth cerebral hemispheres--each about as large and thick as the peel from a quarter of an orange--were so small that they did not meet at the midline, and when the veterinarian removed the brain and placed it on the table, the feeble hemispheres flopped apart, revealing the midbrain.

At one point in the species' evolution, the koala brain undoubtedly filled its skull, and the current mismatch suggests that the reduction in brain size was rapid. But why did it happen at all? The answer, most probably, is diet. Koalas eat little other than nutrient-poor eucalyptus leaves, and they have the low metabolic rate and slothlike habits appropriate for animals with a leaf-eating lifestyle. Very few fossil koalas exist, and next to nothing is known about how these creatures came to be strict leaf eaters, but biologists agree that their ancestors must have had a more diverse diet. Reduction of the brain--a metabolically expensive organ to maintain--may have been part of the koala's adaptation to a low-energy diet. (Metabolic rate--the rate at which an animal breaks down glucose into usable energy--is measured by the volume of oxygen consumed per gram of tissue per hour. Across species, it declines predictably as body size increases: a mouse has a relatively high rate, an elephant has a much lower rate, and a deer has one that is in between. In general, the metabolic rates of marsupials are about 70 percent those of placental mammals of the same body mass.)

My primary interest in the koala's brain was not its dietary connection, however. I had come to Australia to find out which marsupial species, if any, play. The young of many animals engage in playful behavior--loosely defined as vigorous, frequently complex movements that are performed (often repeatedly) for no apparent reason or immediate benefit and that generally have counterparts in serious adult behavior. Young pronghorn antelopes, which I have studied for years in Montana, sprint in long loops away from and then back to their mothers, practicing the sorts of moves needed to escape predators. Fox kits stalk one another, pouncing and biting with apparent ferocity, rolling over and over in the dirt until--their attention diverted perhaps to a passing butterfly--they instantly stop "fighting" and give chase.

In most species, the rate of play drops to near zero at about the time of weaning, although some animals never seem to go through a playful period. As soon as it emerges from its egg, a young lizard, for example, acts like a miniature adult, feeding itself and finding and defending a territory. Having long been interested in why some animals play and others do not, I decided in 1985 that the time had come to take a look at marsupials--a large and diverse group of mammals including kangaroos, wombats, and koalas and characterized by the young completing their development in an external pouch. My own exposure to marsupials had been pretty much limited to the sole North American species, the opossum, and I was delighted to have an excuse to head Down Under, which I did in 1986 and then again for a sabbatical in 1993-94. Specifically, I was interested in the relationship between degree of playfulness, brain size, and metabolic rate.

Generally low, the metabolic rates of marsupials are also what scientists term invariant, meaning that they are close to what one would predict based on body mass (the larger the body, the lower the rate). Though brain size follows a generally similar pattern (the greater the body mass, the larger the brain), it is harder to predict in marsupials. The wombat's brain, for instance, is larger than would be expected from its body mass. It fills the skull, and the surfaces of the cerebral hemispheres are strongly folded. (Folding occurs in the brains of many mammals, including humans, and is a way to increase the surface area of the brain and thus pack in additional nerve cells.)

It was just this combination of low, invariant metabolic rate and variable brain size that had brought me to Australia. Play--running, wrestling, leaping, twisting in midair--uses up a lot of energy, and the lower an animal's metabolic rate, the more of its total energy supply is consumed by play. Below a certain rate, I reasoned, it might not make metabolic sense to play: an animal might need all its energy to satisfy life's basic needs, such as finding food and shelter. If metabolic rate was of prime importance in the evolution of play, then all marsupials--with their invariant rates--should play (or not play) about the same amount.