(Reuters) - The U.S. government proposed protecting old-growth forests in Idaho and Washington state on Tuesday to save the nation's dwindling population of mountain caribou, popularly known as wild reindeer.

Under the plan, roughly 375,000 acres of mostly U.S. Forest Service land in the Selkirk Mountains in northern Idaho and northeastern Washington would be designated as critical habitat for the reclusive caribou.

The estimated 46 mountain caribou in the Selkirks, which bridge the border between the United States and Canada, are all that remain in the country, said Susan Burch, branch chief in Idaho for the U.S. Fish and Wildlife Service.

It is unknown how many of the woodland mammals once inhabited the high country in Idaho, Washington and British Columbia, but their numbers had dwindled to just 30 when they were added to the federal endangered species list in 1984.

Unlike other types of caribou, some of which live in Alaska, the Selkirk band inhabit elevations above 4,000 feet and rely on old-growth forests for food and protection from predators, government scientists said.

The greatest threat to survival of the animal is fragmentation of its territory by logging, wildfires, road-building and recreation trails, according to the service.

Both male and female caribou have antlers and range in weight from 175 to 400 pounds. They seasonally migrate up and down mountains, climbing over packed snow in the winter to feed on lichens on century-old cedars and spruces.

Their reclusive habits and the remote country they inhabit are among reasons there are gaps in historical data, Burch said.

"By the time anyone realized something had died, there was nothing left to look at," she said.

The plan to earmark habitat, which would likely limit such activities as timber harvests, mining and development of recreational areas, is open for public comment until the end of January.

After years of litigation, environmentalists and the government struck a deal in 2009 that required the service to propose habitat protections for Selkirk caribou this month. The service is meant to make a final decision on the measure in a year.

It was not immediately clear if groups promoting access on public lands would object to the proposal. Officials with the BlueRibbon Coalition, a national nonprofit in Idaho that advocates for off-highway vehicle and other forms of motorized recreation, declined to comment when contacted on Tuesday.