The grizzly bear (Ursus arctos ssp.) is any North American subspecies of brown bear, including the mainland grizzly (Ursus arctos horribilis), Kodiak bear (U. a. middendorffi), peninsular grizzly (U. a. gyas), and the recently extinct California grizzly (U. a. californicus†) and Mexican grizzly bear (U. a. nelsoni†). Specialists sometimes call the grizzly the North American brown bear because the grizzly and the brown bear are one species on two continents. In some places, the grizzly is nicknamed the silvertip bear for the silvery, grizzled sheen in its fur. It should not be confused with the black grizzly or Ussuri brown bear (U. a. lasiotus) which is another giant brown bear subspecies inhabiting Russia, Northern China, and Korea. Since the mainland grizzly is so widespread, it is representative and archetypal for the whole subspecific group. Even so, classification is being revised along genetic lines. Except for females with cubs, grizzlies are normally solitary, active animals, but in coastal areas, grizzlies gather around streams, lakes, rivers, and ponds during the salmon spawn. Every other year, females (sows) produce one to four young (usually two) which are small and weigh only about at birth. A sow is protective of her offspring and will attack if she thinks she or her cubs are threatened.