Northern Rocky Mountain wolves, a subspecies of the gray wolf ("Canis lupus"), were native to Yellowstone when the park was established in 1872. Predator control was practiced in the late 19th and early 20th centuries. Between 1914 and 1926, at least 136 wolves were killed in the park; by the 1940s, wolf packs were rarely reported. By the 1970s, scientists found no evidence of a wolf population in Yellowstone; wolves persisted in the lower 48 states only in northern Minnesota and on Isle Royale in Michigan. Canadian grey wolves were introduced into Yellowstone in 1995. This move has returned wolves to land that was once ruled by the canine. They currently exist in several packs, the largest of which are the Slough Creek, Yellowstone Delta, and Leopold packs. The wolves had to "re-learn" an instinct that their ancestors once had: hunting bison. Being used to the elk in Canada, the wolves were dumbfounded by the large, burly bison found all over Yellowstone. However, their main prey remains elk. They have made many appearances on National Geographic Channel documentaries. Wolves in Yellowstone can live as long as 10 years in the wild, though 6 to 8 is normal.