Animals constantly 'work out'. If you've ever had an indoor cat they seen as generally one of the lazier creatures but they will every once in a while just spaz out and start bouncing off the walls. It's not just for fun - it's an instinct that helps keep them in shape. It's the same for all animals. They will constantly work out. It looks like play, but it serves a valuable purpose -- somewhat like how young predators will regularly play fight with each other. That's not just for fun.

Evolution and life is primarily an optimization game. And our bodies are product of that. When we do things, we tend to become better at them. When we don't do things, we tend to lose our ability to do them well, hopefully in lieu of new focuses. Even the brain works this way - as becomes readily apparent when you age and can see in your acquaintances who continues to work with their brain, and who instead spends their time watching television and imbibing.