Holy cow! Hunt begins for pure-bred American bison

It has been one of America's happiest conservation stories. After teetering on the edge of extinction more than 100 years ago, bison are back, roaming the grasslands of national parks and private ranches of the western US in significant herds. But there's a hitch. How many really are bison?

Not that many, it seems. Animal geneticists have determined that, because of a long history of ranchers cross-breeding cattle with bison in search of a better beef animal, fewer than 3 per cent of the roughly 300,000 bison now in the United States can be considered uncontaminated by the genes of cattle.

It is a discovery that has organisations like the American Prairie Foundation rushing to test as many bison as possible to identify those than can be considered pure and to separate them from the hybrid animals. In a sense the campaign to save the American bison - the real ones - has begun all over again.

Established herds of pure bison fortunately already exist, most notably on national parks, including Yellowstone in Wyoming, and Wind Cave National Park. The large private herd owned by Ted Turner, the one-time broadcasting tycoon and conservationist, on his expansive ranches in New Mexico are also believed to be free of hybrids.

"The US federal herds are the crown jewels of the bison herd," Dr James Derr, an animal geneticist at Texas A&M University and a leader in the effort to isolate pure bison, told The New York Times. "They are healthy, there is no inbreeding, they are pure. That's an amazingly good thing."

Bison with strands of cattle DNA are not easy to spot. They look like ordinary bison with the tell-tale shoulder humps, beard and curling horns. Only by corralling animals, sedating them and taking blood samples can researchers like Dr Derr determine which are hybrids and which are not.

Hybrid bison appealed to ranchers not just because of their good meat yields but also because they were tougher in winter conditions on the open prairies and more resistant to disease and parasites. But scientists worry that in the longer term, the DNA confusion could create problems for the bison species.

"When you mix up two different genomes, you get a lot of different traits, and it's not completely predictable," Dr Derr said. "Hybridisation makes it hard to predict and hard to manage because their immune response can be all over the place."