Whipsnade

The zoo’s two female greater one-horned rhinos, 29-year-olds Behan and Beluki, were found to have tumours and cysts in their reproductive organs.

The antelope went extinct in the wild, but has been brought back from the brink through conservation efforts.

The snails became endangered in the 1990s after a on native species was introduced. So far the programme has returned more than 24,000 of the creatures.