Herd of buffalo grazing by river

For bison, summer is always a great time to put on a few extra pounds before winter comes—but even when looking to beef up, bison still order the salad. Grazers of grass and other wild brush, these great American ungulates are also ruminants: Their specialized digestive systems process and metabolize food extremely slowly, squeezing as many calories as possible out of a few mouthfuls of grass. Every little bite helps for this lumbering, hungry herd—there's a long Yellowstone winter ahead of them.