Sea otters have the densest fur measured in any mammal, with figures commonly cited around 100,000 hairs per square centimetre in the thickest regions. The coat works by trapping a layer of air against the skin, so the animal is insulated by air rather than by tissue. This is why sea otters groom constantly: fouled or oiled fur loses its air layer and the animal chills quickly, which made the species so vulnerable to the 1989 Exxon Valdez spill.
Insulation alone is not enough. A sea otter's resting metabolic rate is roughly two to three times that expected for its mass, and it eats on the order of a quarter of its body weight each day. Food intake is therefore a continuous requirement, not a comfort.
The ecological consequence is large. Sea otters eat sea urchins, urchins graze kelp holdfasts, and where otters were hunted out during the fur trade, urchin barrens replaced kelp forest across long stretches of coast. The strength of that relationship varies by region, and how much present-day kelp variation is due to otters rather than warming, disease and other grazers is contested.