Which animal has the thickest fur of any mammal?

A sea otter carries up to a million hairs per square inch - more on a patch the size of a postage stamp than most of us have on our whole head. It needs them because, alone among marine mammals, it has almost no blubber. Its insulation is a layer of air trapped in the fur, which is why an otter spends so much of its day grooming: the coat only works if it is clean and the air stays in.

The cost is a metabolism that will not stop. A sea otter eats something like a quarter of its body weight every day simply to keep warm in cold water, and it cannot go long without food. It is also why an oil spill kills them so efficiently - not by poisoning, primarily, but by matting the fur so it no longer holds air, at which point the animal freezes.

That fur nearly finished them. The maritime fur trade hunted sea otters from perhaps 300,000 down to a couple of thousand by the early twentieth century. Their recovery has had visible consequences: sea otters eat sea urchins, urchins eat kelp, and where the otters came back the kelp forests came back with them - one of the clearest demonstrations anywhere of a predator holding an entire ecosystem in shape.