Northern Yellowstone supports one of the largest and most diverse populations of free-roaming wildlife on earth and is often called "America's Serengeti." About half of the park's approximately 30,000 elk spend the winter on the northern range after summering elsewhere in Yellowstone. Lower elevations and drier conditions there, along with wind and sun that keep ridge tops and south-facing slopes relatively free of snow in winter, give animals easier access to forage. Most large grazing animals in Yellowstone, especially elk, migrate seasonally, moving to higher elevations in spring as warming weather melts the snow and fresh grasses and forbs emerge, then returning to lower elevations in the fall after summer in the high country.