Sea otters have the densest fur of any mammal, with up to one million hairs per square inch.
Sea otters spend several hours a day grooming their fur to keep it clean and fluffed full of air. Without this constant attention, the fur becomes dirty and matted and loses its insulating properties. Sea otters also stay warm by generating a lot of body heat. They have rapid metabolisms and require lots of food calories to fuel them. In fact, most sea otters need to consume about a quarter of their body weight in food every day.