Ferrets gone wild: reintroduced animals coming back in Wyoming.

The first wild population of endangered black-footed ferrets that started from captive-bred animals, once feared to have died out, has survived and is growing, researchers say.

The latest survey, from 2006, reports nearly 200 ferrets in Wyoming's Shirley Basin, says Martin Grenier of the University of Wyoming in Laramie. They descend from animals released there between 1991 and 1994. The new census fits in with an exponential population growth that Grenier and his colleagues have seen in surveys since 2000, they say in the Aug. 10 Science.

The ferrets' future depends on the well-being of their main food item, the prairie dog. The small ferret population also remains vulnerable to disease. Even so, Grenier and his colleagues say that the resurgence of the Shirley Basin ferrets largely relieves fears that the population's founders were too inbred to reproduce well in the wild.

"The black-footed ferret reintroduction to Shirley Basin is quite a famous case study," says Doug Armstrong of Massey University in Palmerston North, New Zealand, a biologist who studies reintroductions.

North America's only native ferret, the black-footed species has thrived or failed in accord with the fortunes of three prairie dog species on the western plains. Prairie dogs make up 90 percent of the ferrets' diet.

Prairie dog populations declined as people tried to eradicate them and diseases such as sylvatic plague swept the West. The ferrets grew rare too. In the early 1970s, a captive-breeding attempt for ferrets failed when a vaccine against canine distemper that had worked safely in domestic ferrets gave wild ones the disease.

Biologists thought that wild ferrets then went extinct, but in 1981 a rancher's dog rediscovered the animals near Meeteetse, Wyo. State wildlife managers trapped as many as they could--18 animals--for another try at breeding the black-footed ferret in captivity.

"We were thinking it was similar to the domestic ferret, but it wasn't similar at all," remembers reproductive physiologist JoGayle Howard of the Smithsonian Institution's National Zoological Park in Washington, D.C. She and others eventually persuaded seven ferrets to breed, and from that initial group more than 4,800 kits have been raised.

The first reintroduced animals, 228 in all, went to the Shirley Basin, where prairie dogs inhabit some 150 acres. Sylvatic plague hit the prairie dogs--and biologists discovered that they had been wrong in believing that ferrets were immune.

Since then, captive-bred ferrets have been released in 12 other places. At least two populations seem to be thriving, although they haven't been surveyed, says Grenier.

Surveying the animals and what affects them after the release is "a key part of any reintroduction," says Philip J. Seddon of the University of Otago in Dunedin, New Zealand. He decries the "bad old days of species reintroductions of 'Let's just chuck them out there and come back later to see if any survived.'"

Reintroduced animals need a place where they're protected from whatever menaced them in the first place, notes Seddon. "An example is the Arabian oryx in Oman, held up as a success with a healthy, self-sustaining wild population before collapsing following resumption of poaching."

Howard says, "It comes down to habitat."