Are the many buffalo roaming the Great Plains really buffalo? It turns out that not many are pure bison bison, but are instead hybrids.

“The majority of public herds have some level of hybridization with cattle,” said Kyran Kunkel, a World Wildlife Federation biologist who is doing the sampling. “You can’t see any difference visually. But we don’t know what the long-term ecological or biological impacts would be.”

American bison, which teetered on the edge of extinction more than a century ago, are one of the first and perhaps greatest conservation successes, but there is an asterisk next to their species: while bison were being nursed back to viable populations, ranchers who owned them crossed them with cattle.

By the late 19th century, tens of millions of American bison had been reduced to fewer than 1,000, with two dozen or so in Yellowstone National Park, and another 250 in Wood Buffalo National Park in Canada. The balance of the animals were owned by cattle ranchers who wanted to preserve them.

“They purposely crossed bison with domestic cattle to make a better beef animal,” which they called cattelo, said James Derr, a geneticist at the Texas A&M College of Veterinary Medicine. “Bison did better in harsh conditions and are more resistant to parasites and native viral diseases.” (Bison do not contract Texas fever, for example, which afflicts cattle.)

Over time, cattle genes have spread into many of the remaining herds of American bison. Since the late 1990s, Dr. Derr and his graduate students have traveled to public and private bison herds around the country, taking blood samples. They have concluded that the vast majority of the 300,000 or so bison in the United States are hybrids, though they look like pure bison. Fewer than 10,000 bison are genetically uncontaminated.

The research has led to the stark realization that the battle for the long-term preservation of wild bison is not over.

Though cattle genes in affected bison herds make up less than 1 percent of the bison genome, their presence could create serious consequences like weaker disease resistance. “Hybridization makes it hard to predict and hard to manage because their immune response can be all over the place,” Dr. Derr said.

As a result, scientists are working to preserve the pure strain and restore it to the wild in viable quantities. Indeed, only about 3% of currently extant buffalo are not hybrids (that translates to roughly 10,000), and so we are still a long way from preserving this great national treasure, once hunted nearly to extinction.

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