Wild bison eye a visitor at the Rocky Mountain Arsenal National Wildlife Refuge outside Denver August 6, 2009. [Photo/Agencies]

SALMON, Idaho - North America's bison, a living emblem of the Western frontier that roamed the continent by the millions before being hunted to near extinction in the late 1800s, was officially designated on Monday as the national mammal of the United States.

The bison, whose image adorned the back of the US nickel for 25 years and has graced the US Interior Department seal since 1912, was bestowed symbolic status equal to that of the American bald eagle through an act of Congress.

The measure, signed into law by President Barack Obama, proclaims the bison's role as a symbol for America's heritage as a whole. It cites the animal's history as "integrally linked with the economic and spiritual lives of many Indian tribes through trade and sacred ceremonies."

Bison, also widely known as buffalo, rank as North America's largest living land mammals, with males of the shaggy, hump-shouldered species weighing up to 2,000 pounds (900 kg) and standing 6 feet (1.8 metres) tall.

They once ranged by the tens of millions across the continent, most notably the Great Plains. But unregulated hunting and government extermination programs reduced their numbers to just a few hundred by the late 19th century.

Fewer than 50 of the last surviving bison ultimately found refuge in Yellowstone National Park, where they were initially guarded by the US Cavalry.

Their numbers have since rebounded, and today Yellowstone - spanning parts of Wyoming, Montana and Idaho - is home to some 4,900 descendants of that remnant herd, representing the largest band of wild, pure-bred bison.

Despite their newfound celebrity status, bison provoke controversy, particularly surrounding the government's practice of rounding up and sending hundreds of buffalo to slaughter each year when their numbers in Yellowstone exceed a population target of 3,000.

The long-standing policy was designed to keep stray Yellowstone bison from infecting cattle from neighboring ranches with brucellosis disease.

The four-year push to designate bison as the national mammal was led by the Wildlife Conservation Society, a group tied to the Bronx Zoo in New York, which in 1907 sent 15 captive-bred, native bison to the first US wildlife refuge in Oklahoma.

Public lands managed by the Interior Department in 12 states, including Alaska, support 17 bison herds in all, or about 10,000 head of buffalo.

Privately raised bison are estimated to number more than 160,000, mostly in the West and consisting mainly of animals that carry cattle genes and are bred for commercial production.