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According to the National Park Service, 60 million bison roamed the prairie when Columbus arrived in the new world. By 1900, only about 500 were left. The bison population had been slaughtered for meat, pelts, and sport. 60 were shipped to Yellowstone Park for safekeeping.

The rest were taken by ranchers, who mated them with domestic cattle. Why? Bison are strong, lean, and able to tolerate extreme weather, so ranchers wanted to cross breed for those characteristics. Now, researchers have discovered, of the half million bison today, most have some cattle genes, and the researchers say that places them at risk. From member station KUAF, Jacqueline Froelich brings us the story.

JACQUELINE FROELICH: Dawn on the Oklahoma Flint Hills, where an autumn moon sets on the Nature Conservancy's Tallgrass Prairie Preserve. Scientists and cowboys are here for the annual bison roundup. Instead of horses, wranglers climb aboard trucks. They rumble toward a herd of 2,600 bison standing quietly in a nearby pasture, drive part of the herd into a bunch, and then stampede them into a holding trap.

(Soundbite of revving truck)

FROELICH: The bison's heaving breath swirls through the corral into a thick, white fog. Over the next 10 hours, ranch hands use plastic paddles to spank the bison through a maze of alleys and corrals toward their annual physical.

(Soundbite of wranglers rounding up bison)

FROELICH: Each bison slams into the examination chute. A brace closes around its neck, and heavy grates squeeze it still. A bull is checked for injuries, given a shot, and weighed. He stands six foot at the shoulder and weighs almost 2,000 pounds.

(Soundbite of a bison huffing)

FROELICH: The huge bison huffs with anxiety. Up close, he seems prehistoric. His shaggy brown head hangs low beneath a humped spine, curved horns showing bits of blood. Preserve director Bob Hamilton says he may look like a bison, act like a bison, even smell like one, but he's a cattle hybrid. And if he carries maternal cattle DNA, that could impair his metabolism and his offspring.

Mr. BOB HAMILTON (Director, Tallgrass Prairie Preserve): That could especially have an impact on the species long-term in terms of their fitness, their ability to withstand cold temperatures or deal with heat, or energy conversion in terms of forage. So that could sure have a fitness impact.

FROELICH: Fertility, disease resistance, and behavior could eventually be affected, too. So in a dozen other conservation herds like this one, bison carrying maternal cattle DNA are being removed.

(Soundbite of bison)

FROELICH: Sequestered in their own roundup pen, bison calves mew for their moms. Soon, the calves will be pushed into the squeeze chute, where researchers will yank their tail hairs for DNA testing. If positive, they will be shipped off to a commercial livestock ranch. Texas A&M University animal geneticist James Derr is studying this herd and others like it.

Dr. JAMES DERR (College of Veterinary Medicine & Biomedical Sciences, Texas A&M University): So we're trying to understand what the genetic architecture of North American bison is and then use those animals to start new herds and to form the foundation of conservation efforts for that species.

FROELICH: Out of the half million bison in the U.S., just two tiny herds - one on Yellowstone National Park, the other on Wind Cave National Park, have never been crossed with cattle. So researchers are using those animals to rebuild the wild species.

FROELICH: Back at the corral, former preserve director Harvey Payne watches the roundup. He used to wander this place on horseback as a boy.

Mr. HARVEY PAYNE (Former Director, Tallgrass Prairie Preserve): You couldn't help but set up on top of one of these hills and look over the landscape and just try to imagine what this country must have looked like several hundred years ago with large herds of bison grazing peacefully over it.

FROELICH: 50 years later, Harvey Payne restored bison to this ancient cross-timber and post-oak savanna. And that's no small miracle, says M.A. Sanjayan, lead scientist for the Nature Conservancy.

Dr. M.A. SANJAYAN (Lead Scientist, The Nature Conservancy): You're seeing, you know, there's a bald eagle flying over this preserve today that I saw. And it's amazing to see that, and you're seeing buffalo come back in the same way. You know, I think it's an incredibly positive conservation story, which we need more of.

FROELICH: And the prognosis for the 2,600 members of this Oklahoma conservation herd now that most of their cow genes have been removed? Their descendants stand a better chance of survival. For NPR News, I'm Jacqueline Froelich.

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