Grizzlies are most active during dawn and dusk. During midday they make daybeds and nap. Their eyesight is poor, however, their ability to smell is extraordinary (they can scent something as much as 15 miles away). They can run at speeds of 15 to 20 miles per hour over rugged terrain. They can sit up, stand and walk upright for a few paces, and have a good sense of balance. Unlike other bears, grizzlies cannot climb trees. They have five non-retractable, sharp, curved claws that help them catch their food. The teeth are heavy, with flattened molars. They do not cut and tear as well as true carnivores. Food has to be chewed and crushed instead of being cut into pieces. The jaws are powerful. A single bear can devour 60 to 70 fish in one hour. They normally live alone, but will congregate to feed with others when there is an abundance of food.