At 850,000 to 1 million hairs per square inch, sea otters (Enhydra lutris) have the thickest fur of mammal. Their fur any consists of two layers, an undercoat and long, waterproof guard hairs. As marine mammals without any fat to keep them warm, this double-layer system traps air next to their skin, preventing it from getting wet and protecting them from chilly ocean waters.
To add air to their undercoat sea otters will lay on their backs in the water and tilt their head down toward their stomach where they then blow air into their fur. In addition to staying warm, this helps increase buoyancy that can help the sea otter swim heavier objects up from the bottom of the ocean. Sea otter pups are born with a special coat that acts like a lifejacket and prevents them from being able to dive. At two months old the pup sheds this special coat.
The special qualities of sea otter fur made it a desired resource in the 18th and 19th centuries and they were hunted nearly to extinction. In response, sea otters were made one of the first marine mammals protected in the state of California in 1913. Sea otters are still a threatened species in the state of California and today 100,000 to 150,000 are protected by law.
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