The North American Bison (Bison bison) was designated the official National Mammal of the United States on May 9, 2016. The Plains Bison (Bison bison bison) once roamed the majority of the open grasslands across the United States to northern Mexico. Their historically massive herds are now significantly smaller and mostly found in geographically isolated populations within parks and preserves. A herd of bison from House Rock Valley Wildlife Area has expanded their range inside of Grand Canyon National Park boundaries, exponentially increasing from one confirmed sighting in 1996 to a current estimated population size of 400 to 600 individuals.