While Yellowstone National Park struggles to keep its bison herd within park boundaries, managers at Grand Canyon National Park in Arizona are facing the opposite problem.

Drought has recently driven a herd of bison into the park from the House Rock Valley, a region of steep, wooded canyonlands in the Kaibab National Forest just north of Grand Canyon. The herd's ancestors were imported into the state in the 1930s for Charles "Buffalo" Jones' infamous "cattalo" experiment. When early attempts to breed bison with cows failed, they were sold to the state. Since 1953, state-permitted hunts have culled the herd to about 100 animals. But over the last two years, poor hunter success has caused the population to rise to about 175. At the same time, drought has left forage scorched on the herd's normal range, sending the animals in search of greener pastures inside the park.

Biologists with the National Park Service say the huge animals (males can weigh up to 2,500 pounds) are wallowing in riparian areas and damaging cultural sites. They considered building a fence, but that would affect other wildlife, such as mule deer. And any fence designed to halt bison would have to be a real bear: The animals are reported to be able to clear obstacles up to six feet high.

But some biologists - and the Arizona Game and Fish Department - believe bison belong here. They were here during the Pleistocene Epoch, more than 10,000 years ago, although there-s no record of them between that era and the early 20th century. So last October, the National Park Service hired Jim Mead, a paleontologist at Northern Arizona University in Flagstaff, to dig through historical and archaeological records.

Published evidence of bison north of the Grand Canyon, Mead says, is spotty at best. But he claims a rock art panel in the new Grand Canyon-Parashant National Monument shows a "definite" bison, along with horses and a rider. Since horses weren't here until the Spanish brought them in the 16th century, that points to historical evidence of bison, he says.

More research remains to be done: According to Mead, no one has really looked for historical evidence in suitable bison habitat, such as the forests frequented by the current herd.

Scientists also want to conduct genetic tests to be certain the bison aren't harboring cattle genes. If they're "tofu-eating half-breeds," Mead says the park will have a strong case for their removal. If they're pure - and native - they'll almost be guaranteed the right to remain.

Allowing the herd to stay will present serious management challenges, says Jeffrey Cross, Grand Canyon's chief of natural resources. Visitors to the park will need to know how to behave if they find themselves face-to-face with a truck-sized bison in the woods.

If there is no evidence that animals are native, and the state is asked to remove the herd, Grand Canyon managers know they're up against dicey politics. After all, says Cross, "the Park Service has a buffalo head on its emblem.

The author is a freelance writer based in Flagstaff, Arizona.