Zoos breed their animals or acquire them from other zoos. Babies are great crowd-pleasers, but when the babies grow up, they don’t attract the same number of people, so zoos often sell them off in order to make room for younger animals. The unwanted adult animals are sometimes sold to “game” farms where hunters pay to kill them; some are killed for their meat and/or hides. Other “surplus” animals may be sold to smaller, more poorly run zoos or, worse, to laboratories for experiments.
For more information, see PETA’s factsheet about zoos.