On a cool summer night in Wyoming, Dean Biggins waits patiently in his truck. Suddenly, Biggins, a wildlife biologist, spots a pair of eyes in the distance. Baggins can't believe his luck: He's just caught sight of a black-footed ferret  --one of the world's rarest animals. Just over a century ago, hundreds of thousands of black-footed ferrets lived on the grasslands in North America known as the prairie. But the animals died off as prairie dog disappeared. Black-footed ferret depend on prairie dogs for both food and shelter. After a ferret has eaten a prairie dog, it moves into the animal's burrow . When cattlemen started setting up ranches  on the prairie in the early 1900s, they thought prairie dogs, which eat grass, might compete with their cattle and sheep for food. To make sure that didn't happen, they killed off billions of prairie dogs. As prairie dog populations dropped, black-footed ferrets disappeared. By 1979, researchers couldn't find any ferrets and declared the species extinct. Then, two years later, a small group was discovered. Although scientists were excited about the discovery, it soon became clear that the ferrets were in danger. A disease called sylvatic plague  was killing the ferrets. Soon, just 18 ferrets remained. "To help the species survive, the researchers put all the ferrets in cages in 1986 and began raising them," says Randy Matchett, a wildlife biologist. To help the black-footed ferrets learn survival skills, wildlife biologists put them through a process called preconditioning. Researchers build a fence around old prairie dog burrows and put the ferrets in them when they are a couple of months old. While the animals get used to their new home, researchers feed them live prairie dogs so they can practice hunting. "Now about 1,000 black-footed ferrets are back in the wild. Despite their amazing comeback, black-footed ferrets aren't out of the woods yet," says Matchett. Sylvatic plague is still a threat, and ranchers still don't want too many prairie dogs on their land. "We have come a long way, and we've learned a lot," says Matchett. "I think if we keep working, we'll be able to help this species recover." How do scientists help the ferrets?
They take measures to stop the disease spreading. They call on people to stop hunting the animals. They set ferrets free when they are two months old. They train the animals so they can survive in the wild.