
In the Oklahoma Flint Hills, an autumn moon set on The Nature Conservancy's Tallgrass Prairie Preserve as scientists and cowboys gathered at dawn recently 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. They drive part of the herd into a tight group, and then stampede them into a holding trap.
The shaggy beasts' 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.

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 and 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.