KNOXVILLE, Tenn., Oct. 25 (UPI) -- A dog or cat at play is not an unusual sight, but a U.S. researcher says many animals apparently need some play time.
And Gordon Burghardt, a psychology professor at the University of Tennessee, Knoxville, says by more accurately defining play and watching it in the animal kingdom, humans may better understand themselves, ScienceDaily.com reports.
Burghardt has defined "play" both in people and also in species not previously thought capable of play, such as fish, reptiles and invertebrates.
Burghardt says the definition has five criteria, which can be summed up in one sentence: "Play is repeated behavior that is incompletely functional in the context or at the age in which it is performed and is initiated voluntarily when the animal or person is in a relaxed or low-stress setting."
"I studied the behavior of baby and juvenile reptiles for many years and never saw anything that I thought was play," Burghardt says. "Then I had an epiphany when I saw Pigface, a Nile soft-shell turtle, batting around a basketball at the National Zoo in Washington, D.C. I realized reptiles play, too."
Play is embedded in species' biology, including in the brain, Burghard says, and like the rest of an animal's psychology -- emotions, motivations, perceptions and intellect -- is part of their evolutionary history and not just random, meaningless behavior.
"Play is an integral part of life and may make a life worth living," he says.