We humans are often guilty of anthropomorphism. We apply our own human emotions and characteristics to the animals around us. We know our dogs are happy because they smile and wag their tails, and we know they are sad when they hang their head or lay around all day. These examples seem to make sense to us, but what about applying human characteristics of play behavior to animals. Would it make sense to say, "That aquatic turtle is working for the weekend?" Could one say that a reptile might be simply finding food and shelter so that it can later engage in play behavior?

Dr. Gordon Burghardt of the University of Tennessee is a pioneer in the field of ethology, the naturalistic study of behavior from an evolutionary perspective. He has studied play behavior in reptiles, mammals, birds and even animals that humans have a hard time anthropomorphizing, such as fish.

To define play behavior, Burghardt has created five criteria to recognize something as play behavior. A one-sentence definition written by Burghardt reads, "Play is repeated, intrinsically rewarding, but incompletely functional (not functional at the time it is being performed), behavior differing from more serious versions structurally (how it looks), contextually (when it is performed), or ontogenetically (youth playing more than adults), and initiated when the animal is in a relaxed or low stress setting."

"Don't let the name trivialize it - play is important." - Gordon Burghardt 

This definition of play behavior in animals helped Burghardt develop what he calls his Surplus Resource Theory.

"Play is going to be much more prominent, even within the same species, when the animal has resources available," Burghardt said. "Nutritional resources which give it the metabolic ability to engage in behavior that if they're starving or sick they're body isn't up to engaging in. And they can be adequately fed, but if they're in an environment with predators all over, or if they're stressed for some other reason, they're not going to play much. So they need that protection - that relaxed side. They also need the time resources. If an animal spends all their time trying to find food, like a shrew that eats close to their weight in food in a day they're not going to have much time left for doing other things."

So these characteristics of animals that have the surplus resources to engage in play behavior might likely point to the process of natural selection. The animals with more playful behavior will survive and procreate. But Burghardt is careful to not to thrust that assumption on the entire animal kingdom. There are many other factors involved in animal play behavior besides surplus resources.
   
"Animals that have more parental care or are what we call more altricial, you find more play in those animals than even closely related species that are born ready to go," Burghardt said. "These animals that must care for themselves from birth don't really have the luxury of playing around." This is why we see much more obvious play behavior in young animals that are still under parental care. We also see these behaviors more obviously in mammals rather that reptiles.
   
"Reptiles are cold-blooded and we know that they are not as capable of extensive motor activity as are mammals," Burghardt said. "You could put a big lizard in the middle of a football field and quickly chase it down. Even though they could go real fast, they tire quickly. If you try to do that with a dog you would be running around for hours."
   
This is because of the different uses of metabolism used by reptiles and mammals. Mammals use aerobic while reptiles use anaerobic. Mammals take oxygen in and use that to create energy for activity. Reptiles use stored energy from fat and other resources to convert into oxygen to use for activity. Reptiles' oxygen carrying capacity in the blood is 10 percent that of mammals. This is why play behavior in reptiles appears less often than in mammals.
   
However, in a reptile such as an aquatic turtle, play behavior is much more prominent. Burghardt explained that this is because of the environment. Since these turtles move in water, a much less costly environment for locomotion, their bodies are capable of more activity.
   
Burghardt has questioned, and debunked, many older theories about animal play behavior such as the idea that highly social animals are more playful, big-brained animals are more playful, or that some animals aren't really playing, but they are practicing adult skills. Many of these theories dealt with delayed benefits - animals engaging in a behavior in play that will benefit it later in life. This didn't always make sense to Burghardt, especially with reptiles.
   
"We have snakes that give birth to 50 babies in a litter and very few of those are going to survive," Burghardt said. "So the main issue for a young snake is to survive, not to practice things that are going to help it in adulthood if the chances of it surviving until then are very low to begin with."
   
Burghardt believes that although play does not seem essential to survival, it is a very important aspect of life, and that, in fact, many species (especially humans) spend a lot of time collecting the resources to allow for play.
   
"Don't let the name trivialize it," Burghardt says. "Play is important."