There are over 30,000 elk—the largest number of any large mammal species in Yellowstone. The northern herd has become much smaller since the mid-1990s. Wolves may cause this. Also, elk use more forested regions to evade the wolves, and make it harder for researchers to count them. The northern herd migrates west into southwestern Montana in the winter. The southern herd migrates southward, and the majority of these elk spend the winter on the National Elk Refuge, southeast of Grand Teton National Park. The southern herd migration is the largest migration of mammals in the U.S. outside of Alaska.