Which marine mammal is the largest member of the weasel family?

The sea otter is the largest member of the weasel family, Mustelidae.

Unlike most marine mammals, sea otters do not have a thick layer of blubber. They rely on exceptionally dense fur, which can contain hundreds of thousands of hairs per square inch, to retain body heat in cold Pacific waters. Regular grooming keeps that insulation effective.

Sea otters often float on their backs while feeding, resting or caring for pups. They are famous for using stones as tools to break open hard-shelled prey such as clams, mussels and sea urchins. This behaviour makes them one of the clearest examples of tool use among nonhuman mammals.

Sea otters once ranged widely around the North Pacific, but the fur trade nearly eliminated many populations. Reintroduction and legal protection have supported recovery, although habitat loss, disease and food-web changes remain concerns.