The sea otter (Enhydra lutris) is different than other marine mammals because it has no blubber (fat). Its fur is the densest of any animal on Earth—an estimated one million hairs per square inch. That’s because, unlike its fellow marine mammals, it has no blubber to keep it warm. It was hunted for its fur until the early 20th century, which resulted in a huge population drop. The sea otter is the heaviest member of the family Mustelidae, a diverse group that includes the 13 otter species and terrestrial animals such as weasels, badgers, and minks. It is one of the few mammal species to use a tool to help it hunt and feed. It wedges a rock between its chest and the “armpit” of a foreleg and pounds shells against it to open them up. Sea otters wrap themselves in kelp so they don't drift on the ocean while they sleep.