Mammals

Otter

Lutrinae · also called otters

Not Evaluated carnivore Rivers & wetlands Coasts & shorelines Ocean

An otter is a weasel that learned to swim. It kept the family’s long, muscular, low-slung body and swapped dry-land hunting for a life in and around water, warmed not by fat but by the densest fur of any animal, which it has to groom for hours a day or freeze. The sea otter went furthest, using stone tools and holding hands with its family in its sleep.

Overview

Otters are the subfamily Lutrinae, 13 species, within the weasel family Mustelidae. They range from the 1 kg Asian small-clawed otter to the 45 kg sea otter and the 1.8 m giant otter, and they live on every continent except Antarctica and Australia, in rivers, lakes, marshes, and coasts. All are carnivores — river otters mainly on fish, the sea otter on urchins and shellfish, the clawed otters feeling for crabs with sensitive fingers.

Fur and a fast engine

Otters have almost no blubber and rely on dense fur — up to a million hairs per square inch in the sea otter — that traps a layer of air, so matted fur means death from cold. To stay warm they burn energy hard, and a sea otter eats about a quarter of its body weight a day.

Tools, rafts, and kelp

The sea otter carries a favourite stone in an armpit pouch and uses it as a chest anvil to crack shells, a skill mothers teach pups. Sea otters rest in rafts, holding paws or wrapping in kelp so they don’t drift apart. By eating urchins they protect kelp forests, making them a textbook keystone species.

Status

Otters as a group are Not Evaluated. The Eurasian and North American river otters have recovered, but the sea otter is still Endangered after the fur trade, and the giant otter, hairy-nosed otter, marine otter, and southern river otter are Endangered from habitat loss, poaching, and the pet trade.

Otter: common questions

Are otters related to seals or to weasels?

To weasels. Otters are members of the family Mustelidae, the same family as weasels, stoats, badgers, martens, wolverines, and mink, and they are essentially weasels that took up an aquatic life, keeping the long flexible body, the short legs, and the musky scent glands of the group. Seals are pinnipeds, a separate branch of the carnivores related to bears, and although otters and seals both swim and hunt fish, they evolved for the water independently. The clearest sign that an otter is a mustelid rather than a pinniped is that an otter still walks and runs easily on land on four proper legs, while a true seal can only hump along on its belly.

Why do sea otters hold hands?

To stay together while they sleep. Sea otters rest at the surface of the open sea, where currents, wind, and waves would otherwise carry a sleeping animal away from its group, its pup, or a good feeding area. By linking paws with a neighbour, or by wrapping themselves in strands of kelp anchored to the seabed, otters anchor themselves in place. Mothers also hold their pups on their chests, and will wrap a pup in kelp to keep it from floating off while she dives to feed. The behaviour is practical, though it is also one of the reasons sea otters are so appealing to people.

Do otters really use tools?

The sea otter does, routinely, which makes it one of a very short list of tool-using mammals alongside chimpanzees, some other primates, and a few others. A sea otter that feeds on hard-shelled prey such as mussels, clams, snails, and sea urchins will select a suitable rock, tuck it into a baggy fold of skin under the foreleg, and, floating on its back, use the rock as an anvil on its chest, smashing the shell against it until it breaks. Some otters keep the same favourite rock for a long time. River otters and clawed otters do not use tools in this way, relying instead on their teeth and dexterous paws.

Are otters dangerous to people or pets?

Rarely, but they are wild predators and not the harmless cartoon characters they are often taken for. Otters generally avoid people and will slip into the water if approached. Bites do happen, most often when an otter is cornered, when a mother is defending pups, or when people try to feed or handle habituated animals, and an otter bite is powerful and prone to infection. Giant otters in the Amazon are large, up to 1.8 metres, live in aggressive family groups, and have injured people who came too close to their dens. Otters can also kill small pets and poultry left near water, and they will fiercely defend a fishing spot.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Lutrinae
Diet: Fish above all for the river otters, along with crayfish, crabs, frogs, birds, and small mammals, while the sea otter specialises in sea urchins, crabs, clams, mussels, and snails prised from the seabed, and the clawless otters of Africa and Asia feel for crabs and molluscs with sensitive dextrous fingers
Fur, not fat: Otters have almost no insulating blubber and instead rely on the densest fur of any animal, with the sea otter carrying up to a million hairs per square inch, trapping a layer of air against the skin, and they spend hours each day grooming to keep it clean and waterproof, since matted fur means death from cold
They run a very hot engine: To stay warm in cold water with only fur for insulation, otters burn energy fast, and a sea otter must eat roughly a quarter of its body weight every day, foraging for many hours, which is why food shortages hit otter populations quickly
Tool use: The sea otter is one of the few mammals that habitually uses a tool, carrying a favourite stone in a loose armpit pouch of skin and using it as an anvil on its chest to crack open hard-shelled prey, a skill mothers teach their pups
Rafting and hand-holding: Sea otters rest at the surface in single-sex groups called rafts, sometimes hundreds strong, and often wrap themselves in kelp fronds or hold paws with a raft-mate so they do not drift apart while they sleep
A keystone species: By keeping sea urchin numbers down, sea otters protect the kelp forests that urchins would otherwise graze to bare rock, so the return of otters to a stretch of coast can bring back the whole kelp ecosystem, one of the clearest demonstrations of a keystone predator
Play that is also practice: Otters slide down mud and snow banks, juggle pebbles, wrestle, and chase their tails, behaviour that looks like pure fun and probably is, while also building the strength, coordination, and social bonds a young otter needs
Range: Every continent except Antarctica and Australia, in rivers, lakes, marshes, estuaries, and along rocky and kelp-forested coasts, with the sea otter confined to the North Pacific, the giant otter to the rivers of the Amazon and Orinoco, and the others spread across the Americas, Europe, Africa, and Asia.
Conservation status: Not Evaluated ≈ Not assessed as a group of 13 species, and status varies widely, with the Eurasian otter recovered across much of Europe after near-extinction from pollution and hunting, and the North American river otter secure, but the sea otter still Endangered and far below its historic numbers after the fur trade, the marine otter and southern river otter of South America Endangered, and the giant otter of the Amazon and the hairy-nosed otter of Southeast Asia Endangered from habitat loss, poaching, and the pet trade in the wild