BISMARCK (AP) — The work of two wildlife management agencies has resulted in the release of 28 endangered black-footed ferrets on the South Dakota side of the Standing Rock Sioux Reservation, which straddles the Dakotas.

The push behind the effort by the Standing Rock Sioux Tribe Game and Fish Department and the U.S. Fish and Wildlife Service is twofold: maintain control of black-tailed prairie dogs in an area of the reservation used primarily for grazing, and save from extinction the black-footed ferret, which is considered the most endangered mammal in the U.S.

They are yellowish-buff in color and weigh 1 ½ to 2 pounds. The forehead, muzzle and throat are white and the feet are black. They have a black mask around the eyes, which is well-defined in young ferrets.

The ferrets were released Oct. 20 in prairie dog towns between McIntosh and Bullhead, said Michael Gutzmer, ecologist with New Century Environmental, the Nebraska company that provides biological services to the tribe. The 16 males and 12 females had left the National Black-footed Ferret Conservation Center in Colorado that morning, The Bismarck Tribune reported.

Prairie dogs make up 90% of the black-footed ferrets’ diet. The ferrets live in prairie dog burrows, and each eats a prairie dog about every three days, said Seth Gutzmer, biologist on the Standing Rock project and Michael Gutzmer’s son. A loss of habitat, declines in prey, and diseases such as plague have reduced black-footed ferret numbers, according to Fish and Wildlife. About 2% of their original habitat — some 100 million acres — remains. The ferrets were considered extinct in the wild in 1987, but reintroduction efforts throughout North America brought them back.

The roots of the Standing Rock effort date to 2012, when a ferret was found on the reservation south of the Grand River near Mobridge. In a month’s time the company and tribal officials came across three.

“That got a lot of people’s attention,” said Michael Gutzmer, who holds a doctorate in environmental sciences and wetland ecology.

Prairie dog towns are “the ideal place” for ferrets, he said. The recent effort placed ferrets at five areas of three prairie dog towns. They are spaced to allow one ferret an area of about 50 acres. Each is implanted with an identifying chip.

“If one is captured or run over we can document that it came from this release,” Gutzmer said.

There are small pockets of ferrets on the reservation, according to Standing Rock Game and Fish Director Jeff Kelly. His crew and Fish and Wildlife mapped them all and found a suitable area for introduction.

“We’re not trying to save the prairie dogs,” Kelly said. “We’re adding another tool to control them.”

Known ferret populations exist only as a result of reintroduction efforts, and pockets on Standing Rock may have dispersed from introductions on the Cheyenne River Reservation south of Standing Rock, Michael Gutzmer said. The populations are small, fragmented and intensively managed, and only a few produce wild-born adults, according to New Century.

Ferrets mate in March and April and on average birth three or four kits in May or June. It could be three to five years before there is a noticeable increase in the Standing Rock population. Based on incidental sightings and road kills, Michael Gutzmer estimates the newly released ferrets represent about one-third to one-half of the Standing Rock population.

The Fish and Wildlife Service and tribal Game and Fish will conduct follow-up monitoring using cameras over bait, conducting nighttime surveys, and possibly luring the ferrets into camera range using calls that imitate the sound of a distressed prairie dog. Ferrets are reclusive, solitary and usually nocturnal. They don’t truly hibernate in winter but will go into an extended deep sleep and venture out when they need food, Gutzmer said.