Sea Otter

The sea otter is the smallest marine mammal and, in a way, the least equipped for the job — it has no blubber, it must eat a quarter of its body weight every day just to stay warm, and it grooms almost obsessively. And yet it is one of the most influential animals in the North Pacific: where sea otters live, whole underwater forests stand or fall on their appetite.

Overview

Sea otters live along the rocky coasts of the North Pacific, from central California up through Alaska and across to Kamchatka in Russia, in and around kelp forests and shallow reefs. Unlike river otters, they spend almost their entire lives in the water — feeding, sleeping, grooming, mating, and giving birth at sea — and many rarely, if ever, come ashore.

Surviving the cold

Cold water pulls heat out of a body far faster than cold air. A sea otter has no fat layer to slow that loss, so it relies entirely on fur — the densest coat of any animal, with as many as a million hairs per square inch. Long guard hairs shed water while a dense underfur traps a warming blanket of air against the skin.

That system only works if the fur is spotless and fluffed, so a sea otter spends hours a day grooming, rolling, blowing air into its coat, and squeezing out water. It also runs a metabolism roughly three times faster than a land mammal its size, which is why it has to eat so much.

Tools and feeding

Sea otters dive to the seabed for invertebrates — sea urchins, crabs, abalone, clams, mussels, sea snails, and octopus — bringing catches to the surface and eating while floating on their backs, using the belly as a table.

To get into hard-shelled prey they use tools: a rock is balanced on the chest and the shellfish struck against it until it cracks. Individual otters have favourite rocks, kept in a baggy fold of skin under the forearm, and specialise in particular prey and techniques that they learn from their mothers.

The keystone effect

Sea otters keep sea urchins in check, and sea urchins eat kelp. In areas with a healthy otter population, urchins stay small and hidden and the kelp forest thrives, providing food and shelter for hundreds of species and locking away carbon. Remove the otters — as the fur trade did along much of the coast — and urchin numbers explode, stripping the kelp to bare rock and turning a forest into an “urchin barren.” The return of sea otters to parts of their old range has been followed by the return of the kelp.

Reproduction

A single pup is born at sea after a four- to nine-month pregnancy. The mother carries it on her chest, grooms it, and dives for food while it floats, sometimes wrapped in kelp so it does not drift off. Pups are dependent for six months to a year, and raising one is so demanding that a female may lose a third of her body weight and, in poor conditions, abandon the pup to save herself.

Threats and conservation

The sea otter is listed as Endangered. Having recovered from a few thousand animals to around 125,000, it now faces:

• Oil spills. Oil destroys the insulating fur; the 1989 Exxon Valdez spill killed several thousand sea otters.
• Disease. Parasites and pathogens washing off the land in coastal runoff — including Toxoplasma from cat faeces — are a significant cause of death in California.
• Orca predation in southwest Alaska, where killer whales turned to otters as larger prey declined, driving a regional collapse.
• Shark bites off California, where great whites bite and release otters they do not eat, and entanglement and food limitation as populations approach the carrying capacity of a much-reduced range.