Liz Bonnin presents a controversial and provocative episode of Horizon, exploring how and why zoos keep animals, and whether they need to change to keep up with modern science, or ultimately be consigned to history. Should zoos cull their animals to manage populations? Liz travels to Copenhagen Zoo to witness their culling process first hand. They think it is a natural part of zoo keeping that is often swept under the carpet. Should some animals never be kept in captivity? In a world exclusive, Liz visits SeaWorld in Florida and asks if captivity drove one of their orcas to kill his trainer. But could zoos be the answer to conserving endangered species? Liz meets one of the last surviving northern white rhinos and discovers the future of this species now lies in a multi-million dollar programme to engineer them for stem cells. Veteran conservation scientist Dr Sarah Bexel tells Liz the science of captive breeding is giving humanity false hope.