Otter

The fur problem

Sea otters have the densest fur of any mammal — up to 1,000,000 hairs per square inch (compared to about 100,000 hairs on an entire human head). The fur is critical because, unlike most marine mammals, sea otters don’t have a thick blubber layer. They keep warm by trapping a layer of air between two coats of fur — a fine underlayer for insulation and a longer guard layer to keep water out.

This fur was their near-undoing. Russian, English, and American hunters drove sea otters nearly to extinction during the 18th and 19th centuries, hunting them for the dense fur valued in Asian markets. From an estimated 150,000–300,000 in the 1700s, populations collapsed to perhaps 1,000–2,000 by 1911, when international protection finally arrived.

Today, populations have recovered to about 125,000 globally — still a fraction of pre-hunting numbers, with the largest concentrations in Alaska. The species is still listed as Endangered.

Tool use

Sea otters are one of the few non-primate mammals that systematically use tools. They favor a particular stone — usually about fist-sized — that they keep tucked into a loose fold of skin under one armpit. To open a clam, mussel, sea urchin, or abalone, an otter floats on its back, balances the prey on its chest, and uses the stone like an anvil to crack the shell.

Different individuals prefer different tools, and most carry the same stone for a long time. Mothers teach pups the technique. Recent research shows that older otters with consistent stone-use have measurably less tooth wear than those that crack shells with their teeth.

Keystone species

Sea otters are a textbook keystone species. They eat sea urchins; sea urchins eat kelp. Without otters, urchin populations explode and graze kelp forests down to barren rocky bottoms. With otters, kelp forests thrive — and kelp forests support a diverse community of fish, invertebrates, and seabirds.

When sea otters were near-extinct, North Pacific kelp forests collapsed in many areas. Their recovery has restored kelp forest ecosystems across much of their former range — a rare unambiguous conservation success story.

A floating mammal

Sea otters spend almost their entire lives in the water, including giving birth. They float on their backs to rest, eat, and sleep, often holding hands with other otters or wrapping themselves in kelp to keep from drifting apart in their sleep — a habit that produces some of the most-shared wildlife images in social media.

A pup is born already covered in dense fur and floats like a cork. Mothers spend much of the first weeks holding the pup on their chest, grooming and nursing it.

Other otter species

Several other otter species exist:

• River otter (Lontra canadensis) — North American freshwater rivers.
• Eurasian otter (Lutra lutra) — Eurasian rivers.
• Giant otter (Pteronura brasiliensis) — Amazon Basin; up to 1.8 m long.
• Asian small-clawed otter (Aonyx cinereus) — smallest otter, increasingly popular and worryingly traded as exotic pets.
• African clawless otter — sub-Saharan Africa.