By the end of the 19th century, North America's many millions of bison were reduced to just a few hundred. They've since recovered to around a half million, most raised as livestock and crossbred with cattle. Now conservationists manage over 60 herds, such as the one at the American Prairie Reserve in Montana, to restore the bison's genetic integrity and ecological function. But new technology is revealing cattle DNA even in herds identified as purebred a few years ago. James Derr at Texas A&M University, who has genotyped over 35,000 bison, says it ranges from miniscule traces to heavy hybridization. "It's hard to prove a bison herd is free of cattle genes. All herds may have some hybrids," he says. The American Prairie Reserve bison await results from recent genetic tests. If they are found to carry cattle DNA, how much is acceptable? Manager Bryce Christensen says, "A lot of conservation herds will be trying to figure (that) out this year."