In 2014, Marius, a perfectly healthy two-year-old giraffe, was killed and fed to the lions in Copenhagen Zoo.

It caused an outcry at the time but, as a new Horizon documentary explains, it made perfect sense – and lifted the lid on something happening in zoos across the world, which they rarely admit to.

Every year, around 3,000 to 5,000 animals are culled in European zoos, and for logical reasons. If you encourage animals to breed in zoos, you can’t order a certain quantity and gender on demand. So you will inevitably end up with surpluses. If no other zoo wants them, it makes sense to feed them to their natural predators, however cute they look.

In Horizon, Liz Bonnin, the presenter of ITV’s Countrywise, who’s studied and worked in zoos, watches a sable antelope being fed to the lions at Copenhagen Zoo. From here she lays out the arguments for and against culling, taking a calm and objective approach to the whole, vexed subject of zoos and animal welfare.