Fans of bison are lobbying Congress to declare America's largest land mammal the "national mammal" -- putting it right up there with the bald eagle as an American icon. But bison have their critics, as well as competition: killer whales are mammals, too, and then there's that other widespread mammal: humans.

"The North American Bison is an enduring symbol of America, its people and a way of life," Sen. Michael Enzi, R-Wyo., said in a statement Friday announcing the "National Bison Legacy Act."

"Throughout history, the bison has been the center of the economic and spiritual lives of American Indians and is an important historical symbol of the United States," added co-sponsor Sen. Tim Johnson, D-S.D.

They introduced the bill at the request of an alliance among the Wildlife Conservation Society, the Intertribal Buffalo Council and the National Bison Association.

"The bison is the nation’s most culturally recognizable animal and as such deserves recognition through designation and celebration," the alliance noted. "Bison currently appear on two state flags, on the seal of the Department of the Interior, and on U.S. currency. In addition, bison have been adopted as the state mammal of Wyoming, Oklahoma and Kansas."

Tens of millions of bison, or buffalo, once roamed North America, but the species dwindled to about 1,000 by the early 1900s. The numbers are back to an estimated 20,000 in the wild today. In addition, about 500,000 commercial bison are on 4,000 ranches.

In recent years, efforts have been made to put wild bison on more parts of the West -- including the transfer in March of about 60 Yellowstone bison to Montana's Fort Peck Reservation. 

But the symbolic act would not provide added protections for wild bison, which have their enemies.

In Montana, The Associated Press noted, livestock producers and property advocates have filed lawsuits to stop free-roaming bison, arguing they tear down fences, spread disease and compete with domestic cattle for grass.

And Boulder, Colo., this week rebuffed billionaire Ted Turner's donation of a bison herd for public viewing. It cited cost concerns and public opposition.

John Calvelli, spokesman for the Wildlife Conservation Society, told the AP that the effort is meant to transcend such issues.

"This isn't about getting into the middle of these issues of bison and property rights," he said. "No matter what political stripe you come from, we can all agree on the important role that bison have played."

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Dennis Lingohr / American Prairie Reserve

Some of the 71 bison calves trucked from Canada are released onto grassland in Montana.

Dozens of bison calves trucked in from Canada are now roaming a stretch of Montana countryside that conservationists hope to restore to its original shape.

The bison, even though they were born across the border, are from the genetic stock of herd that used to roam what's known as the American Prairie Reserve. Hundreds of their ancestors were from that same area when they were sold to Canada in 1906 for its Elk Island National Park.

"We knew from the beginning that returning bison to the land would be an important step in restoring the reserve’s full biodiversity," Sean Gerrity, president of the American Prairie Reserve, said in a statement. "When the Canadian government purchased the herd, it helped the species survive near extinction. Now we are bringing them back to help restore a complete grassland ecosystem."

The 71 calves released Thursday join 140 bison already on the 123,000-acre reserve.

The nonprofit says it aims to "create the largest wildlife reserve in the continental U.S., culminating in three million acres of private and public land and connecting one of the last large sections of untilled temperate grasslands on the continent."

Some 500,000 bison roam rural U.S. lands today, a fraction of the tens of millions that once populated the Great Plains.

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