Many large animals, or megafauna, including horses, camels, mammoths, mastodonts, ground sloths, sabre-tooth cats ("Smilodon"), the giant short-faced bear ("Arctodus simus"), and the cheetah, became extinct in North America at the end of the Pleistocene epoch (ice ages), at the same time the first evidence of humans appeared, in what is called the Holocene extinction event. Previously, the megafaunal extinctions were believed to have been caused by the changing climate, but many scientists now believe, while the climate change contributed to these extinctions, the primary cause was hunting by newly arrived humans or, in the case of some large predators, extinction resulting from prey becoming scarce. The American bison ("Bison bison"), brown bear or grizzly bear ("Ursus arctos"), moose or Eurasian elk ("Alces alces"), and elk or wapiti ("Cervus canadensis") entered North America around the same time as the first humans, and expanded rapidly, filling ecological niches left empty by the newly extinct North American megafauna.