Which animal is the heaviest member of the weasel family?

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

Adult sea otters commonly weigh about 14–45 kilograms, with especially large males exceeding that range. They are nevertheless among the smallest marine mammals because their compact bodies are adapted for life in coastal waters. Unlike seals and whales, sea otters lack a blubber layer and depend on exceptionally dense fur for insulation.

Sea otters are also unusual mustelids because they can spend their entire lives in the ocean. They use stones as tools to crack open clams, mussels and other hard-shelled prey. By eating sea urchins, they help protect kelp forests, making them a classic keystone species.

A common mix-up is the giant otter, which is longer and can look larger, but is much slimmer and lighter. The sea otter’s exceptional body mass—not overall length—makes it the family’s heaviest living member.