Animals seek out play relentlessly, and when a potential partner doesn't respond to a play invitation, they often turn to another individual. If all potential partners refuse their invitation, individuals will play with objects or chase their own tails. The play mood is also contagious; just seeing animals playing can stimulate play in others. Animals seek out play because it is fun. Elk run across a snow field, jumping in the air and twisting their bodies while in flight, stop, catch their breath, and do it again and again. Bison, following one another, playfully run onto and slide across ice, excitedly bellowing "Gwaaa" as they do so. Chimpanzees may twirl around and turn somersaults when they play with friends or by themselves.

Jane Goodall, The Ten Trusts