Sea otters have the thickest fur of any animal. Their fur contains between 600,000 to 1,000,000 hair follicles per square inch. Unlike most other marine mammals, otters lack a blubber layer. Instead, they depend on their dense, water-resistant fur to provide insulation. To keep warm, sea otters spend a large portion of their days grooming and conditioning their fur. This traps air and heat next to their skin.