In an outdoor enclosure, two juvenile black-footed ferrets pounce and somersault, tumbling over each other in a playful fight. The loser breaks free and rushes into a tunnel lined with a plastic tube.

In the early 1900s, the United States was likely home to more than 5 million ferrets. But ferrets, which hunt prairie dogs for food and live in their burrows, were almost wiped out early in the 20th ...