With 600,000 to 1,000,000 hair follicles per square inch, sea otters have the thickest fur of any living animal. Why? Unlike most marine mammals, sea otters do not have a blubber layer, so they rely on their thick, water-resistant fur to keep them warm. In fact, they spend a large portion of each day grooming and conditioning their fur. The process traps air and heat next to their skin, which helps keep them warm.