Conservation groups served notice Thursday they intended to sue the U.S. Fish and Wildlife Service for refusing to study whether to give federal protection to prairie dogs known for their ability to warn one another of danger in explicit detail.

One expert, Con Slobodchikoff, a biology professor at Northern Arizona University, said the Gunnison's prairie dog has the most sophisticated communications yet documented among non-humans.

They can whistle different alarm calls for different predators that signal particular manners of escape, his studies have found.

The alarm calls also describe the general size, color and speed of the predator, he said.

The Fish and Wildlife Service says poisoning all but wiped out the Gunnison's prairie dog from 1916 to 1961 but that more recent decades may have sustained a recovery in parts of Arizona, Colorado, New Mexico and Utah.

The agency said in a Federal Register notice that it had no reliable estimates of the current population but acknowledged available habitat for the prairie dog has been shrinking and that the plague can decimate isolated populations, though it's less clear if prairie dogs are occupying new territory when they move from old stomping grounds.

The Fish and Wildlife Service filed a "negative" finding in February, saying it wouldn't conduct a more exhaustive review of possible protections under the Endangered Species Act. The agency, however, said it plans to re-examine the prairie dog's status early next year, after consulting wildlife agencies in the Four Corners states.

Filing notice of a possible lawsuit Thursday were the Santa Fe-based Forest Guardians, a half-dozen other conservation groups, five biologists and dozens of others.

"No one expected a negative finding on our petition," Nicole Rosmarino, conservation director for Forest Guardians, said Thursday.

Just as unexpected, she said, was the agency's pledge to reopen the matter by this February.

"They've never done that when they issue negative findings. So I think it was a clear admission that something's wrong here," Rosmarino said.

Pete Gober, a Fish and Wildlife Service field supervisor in Pierre, S.D., who handled the petition, didn't immediately return a phone call after business hours Thursday from The Associated Press.

Among the biologists suing is Bob Luce, former coordinator of the Interstate Prairie Dog Team, a working group of wildlife officials from Arizona, Colorado, New Mexico and Utah.

In a statement, Luce said the Gunnison's prairie dog was clearly in decline across its range. He called it "an ecological cornerstone of the high desert" that deserved a range of protections.

Forest Guardians and 73 other conservation groups or advocates had petitioned the Fish and Wildlife Service to open an inquiry that could lead to a listing under the Endangered Species Act, which can limit development on public or private land.

Gunnison's prairie dog (Latin name Cynomys gunnisoni) is one of five species found in North America. The Utah prairie dog is listed as threatened, while the Mexican prairie dog is listed as endangered. There are also black-tailed and white-tailed prairie dogs.

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