The black-footed ferret is a small carnivore, similar in length to a domestic cat. Its short legs and long, slim body make it superbly adapted for life in the burrow systems of prairie dogs, which are almost its sole prey. It spends most of its life under ground - remaining in each burrow for up to ten days at a time - only emerging to seek a mate or another burrow to plunder. Prairie dogs do not wait to be hunted down by the ferrets. They often plug their burrow entrances with soil from the inside, but a determined ferret will dig through using its powerful forepaws.

The black-footed ferret's limited diet means its fate is inexorably linked to that of the prairie dog, which was once one of the most abundant mammals in the American West. Up to the 19th century, it lived in vast colonies that extended almost uninterrupted from southern Canada to northern Mexico. But as ranching and agriculture expanded, the prairie dog became a target for extermination. In the early 1900s, the US government even put a bounty on its head. The prairie dog was eradicated from 98 per cent of its original range. Even today, it is still trapped and poisoned, because ranchers believe that this tiny rodent competes with livestock for grass - a belief that belies recent research showing that prairie dogs and cattle feed on different plants.