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.