Bison attack!

Most Americans love bison - they call them buffalo - from afar. But there are plenty who trek to Yellowstone National Park each year. The visitors may or may not see a bear or a moose, but bison are sure to be enjoyed - provided people don't do the dumb tourist thing and walk up close to a bison for a better picture, only to get the potentially fatal sharp-horned heave-ho.

Now a scientist suggests bison may also have given the final heave-ho to most of America's now-extinct big mammals. Beavers the size of bears, mammoths, horses, camels and saber-toothed cats all were gone by 11,000 years ago. The main cause was climate change that reduced the food and water supply (human hunters may have helped, too) but, suggests Eric Scott, curator of paleontology at the San Bernardino County Museum in Redlands, Calif., it was the ever-growing bison herds that may have forced a "tipping point" for the doomed species.

Bison would have had advantages over other large herbivores, such as their multiple stomachs that probably allowed them to obtain maximum nutrition from their food. Their population growth since migrating from Asia may have malnourished nursing mothers of other big species just enough to cause their numbers to collapse. And, with far fewer large herbivores to feed on, dire wolves, lions and other big carnivores would have starved as well.

It wasn't until plains Indians obtained horses - and Buffalo Bill and his fellows went to work - that bison too finally faced the threat of extinction.