Sea otters ("Enhydra lutris") are classified as marine mammals. They live along the Pacific coast of North America. Their historic range included shallow waters of the Bering Strait. It also included Kamchatka, and as far south as Japan. Sea otters have about 26,000 to 165,000 hairs per square centimeters of skin. They have a rich fur for which humans hunted them almost to extinction. They frequently carry a rock in a pouch under their forearm. They use this to smash open shells. This makes them one of the few animals that use tools. They grow to in length and weigh . Although once near extinction, they have begun to spread again, from remnant populations in California and Alaska.