Which living marine mammal has the densest fur?

The sea otter has the densest fur of any living mammal.

Its coat can contain roughly 100,000 to 150,000 hairs per square centimetre, creating a dense insulating layer. Unlike seals and whales, sea otters do not rely on a thick layer of blubber to stay warm. Instead, they trap air in their fur and maintain insulation through frequent grooming.

Sea otters spend much of their lives in cold coastal waters of the North Pacific. Grooming removes debris and helps preserve the coat’s water-repellent structure. Loss of fur condition can quickly increase heat loss, making clean, well-maintained fur essential.

Sea otters are also important predators in kelp-forest ecosystems. By eating sea urchins, they can reduce grazing pressure on kelp. Their historical populations were severely reduced by the maritime fur trade, but conservation and reintroduction programs have helped them return to parts of their former range.