According to the National Park Service, there are about 5,450 bison as of 2021, which includes two primary breeding herds (Northern and Central). You can see them year-round in both the Hayden and Lamar valleys. In the winter, they may be seen near thermal features around the park, including Upper Geyser Basin, Biscuit Basin, and Mammoth, as well as along the Firehole River, Madison River, Blacktail Deer Plateau, Tower, and the Gardiner Basin. The average male weighs 2,000 pounds and the female weighs 1,000 pounds. Bison might live 12 - 15 years, with some living as long as 20 years and they feed primarily on grasses and sedges. Bison can be aggressive and fast, with speeds up to 30 miles per hour.