The Busy Floaters
Floating on its back in the cold waters off the Pacific coast, a sea otter can look as relaxed as a person in a swimming pool. But the otter is usually busy. It may be grooming its thick fur, cracking open a meal or simply holding onto a strand of kelp so it does not drift away while it naps.
Unlike whales and seals, sea otters have no thick layer of blubber to keep them warm. Instead, they rely on fur, the densest of any animal, with as many as a million hairs in a single square inch. Air trapped in the fur keeps the otter's skin dry and warm. Because of this, otters spend hours each day cleaning and fluffing their coats. A dirty, matted coat would let cold water reach the skin.
Staying warm takes a lot of energy, so sea otters eat a great deal, often about a quarter of their body weight each day. They dive to the sea floor for crabs, clams and sea urchins. Some otters carry a flat rock tucked into a loose pouch of skin under one front leg. Floating on its back, the otter places the rock on its chest and bangs a shellfish against it until the shell cracks. Sea otters are among the few mammals other than apes and monkeys known to use tools.
Their appetite matters for the whole coastline. Sea urchins feed on kelp, the giant seaweed that forms underwater forests. Where otters are plentiful, they keep urchin numbers in check, and kelp forests thrive, giving shelter to many fish and other creatures. Scientists call an animal like this a keystone species: remove it, and much of the living system around it changes.