Under the endangered species act, buying or selling an endangered animal requires a permit. The permits are hard to get — even for zoos and aquariums.
But there's a loophole.
"If I donate or loan an endangered species to you, I need no permit," says Kris Vehrs of the Association of Zoos and Aquariums.
So a barter system has sprung up among zoos and aquariums to trade animals without using money. They even do it with species that aren't endangered. But barter can be complicated.