WASHINGTON — Animal keepers at the National Zoo’s conservation center in Virginia sent 26 black-footed ferrets to “boot camp” Wednesday to prepare the critters for life in the wild as part of an ongoing effort that has fueled the recovery of a species once declared extinct.

Black-footed ferrets, the only ferret species native to North America, disappeared in the late 1970s. Then in 1981, a ranch dog in Wyoming killed a small animal, which led biologists to discover a colony of wild black-footed ferrets. By 1985, though, there were just 24 left.

Over time, scientists decided to collect those last ferrets to try to save them. Only 18 survived. Many scientists worried it was too late to save the species, said David Wildt, now the head of the Center for Species Survival at the Smithsonian Conservation Biology Institute, a branch of the National Zoo.

“Can you go down to as few as 18 animals and be able to bring those animals back?” Wildt recalled wondering at the time. “There aren’t a lot of examples of successful reintroduction programs.”

The ferret’s struggle may surprise those who keep ferrets from Europe as pets. American ferrets used to be common across the Great Plains. Tens of thousands once lived across 12 states.

Prairie dogs are their main food source, but disease and extermination of prairie dogs, considered a nuisance on land for cows, starved the ferrets.

Thirty years later, the ferret population is on the rise.

Zoos in Louisville, Ky., Toronto, Phoenix, Colorado Springs, Colo., and the Smithsonian’s National Zoo joined with the U.S. Fish and Wildlife Service to breed the endangered animals.

The Smithsonian developed the first artificial insemination technique for ferrets, which has produced 139 kits, and scientists are building a ferret sperm bank to maintain the population’s genetic diversity. So far, five kits have been produced using frozen sperm.

“That has never been done before with respect to endangered species,” Wildt said. “Here you have a model, not just in terms of producing animals for reintroduction, but the science.”

Still, reintroducing ferrets to the wild sparks controversy in places like Kansas, where some residents are angry that protecting the ferret means also protecting prairie dogs that some ranchers want to poison. More than 7,000 ferrets bred in zoos have been released into prairie dog colonies since 1991. There are 19 sites stretching from Canada to Mexico where the ferrets have been reintroduced to the wild.

Scientists estimate about 1,000 black-footed ferrets live in the wild today. There are four self-sustaining populations in South Dakota, Arizona and Wyoming.

Ferrets born in captivity, though, must first make it through boot camp. Not all of them survive that taste of nature.

While accustomed to digging in spacious enclosures in Virginia, the Smithsonian-bred ferrets have much to learn. They will be sent to the National Black-Footed Ferret Conservation Center in Colorado for “preconditioning” for the wild.

The “reintroduction candidates” spend at least 30 days in training. It exposes them to underground burrows and prairie dog tunnels — and the chance to hunt and kill live prey amid sounds and smells of the prairie.

“This is actually going to be the first taste of prairie dog for these animals,” said U.S. Fish and Wildlife Service biologist Paul Marinari.

“They might look nice and cute and cuddly, but they’re by no means domesticated or timid when it comes to going after their primary prey.”

Printed on Thursday, September 29, 2011 as: Zoos help restore nearly extinct ferret in US West