Elephants, bears, tigers, and other animals do not voluntarily ride bicycles, stand on their heads, balance on balls, or jump through rings of fire. They don't perform these and other difficult tricks because they want to; they perform them because they're afraid of what will happen if they don't.
For animals in circuses, there is no such thing as "positive reinforcement"—only varying degrees of punishment and deprivation. To force them to perform these meaningless and physically uncomfortable tricks, trainers use whips, tight collars, muzzles, electric prods, bullhooks, blowtorches, and other painful tools of the trade.
http://www.change.org/petitions/ban-the-use-of-animals-in-the-circus-urge-the-usda-to-seize-the-animals-and-send-them-to-sanctuaries
