The Grizzly Bear (Ursus arctos horribilis), also known as the Silvertip Bear, is a subspecies of brown bear (Ursus arctos) that lives in the uplands of western North America. Grizzlies are normally a solitary active animal, but in coastal areas the grizzly congregates alongside streams, lakes, rivers, and ponds during the salmon spawn. Every other year, females (Sows) produce one to four young (most commonly two) which are small and weigh only about 500 grams (one pound). A sow is very protective of her offspring and will attack if she thinks she or her cubs are being threatened. Male grizzly bears can reach and stand 2.44 meters (8 ft) tall on their hind legs; the females are on average 38% smaller. This sexual dimorphism suggests that size is an important factor in the male's ability to successfully compete for and attract breeding opportunities. Their coloring ranges widely across geographic areas, from blond to deep brown or red. The grizzly has a large hump over the shoulders, which is a muscle mass used to power the forelimbs in digging. The hind legs are more powerful, however. The muscles in the lower legs provide enough strength for the bear to stand up and even walk short distances on its hind legs, giving it a better view of its surroundings. The head is large and round with a concave facial profile. In spite of their massive size, these bears can run at speeds of up to 40 kilometers per hour (25 miles per hour). However, they are slower running downhill rather than uphill because of the large hump of muscle over the shoulders. Grizzlies can be distinguished from most other brown bear subspecies by their proportionately longer claws and cranial profile which resembles that of the polar bear. Compared to other North American brown bear subspecies, a grizzly's pelt is silver tipped and is smaller in size. This size difference is due to the lesser availability of food in the grizzlies landlocked habitats. They are similar in size, color and behavior to the Siberian Brown Bear (Ursus arctos collaris).