Two Otters, Two Worlds
Otters are meat-eating mammals in the weasel family, not rodents. This guide follows two of them. The North American river otter lives in and around fresh water: rivers, streams, lakes, ponds, and marshes. The sea otter lives almost entirely at sea, in shallow waters along the North Pacific coast.
The two differ at a glance. A river otter has a thick, muscular tail about one-third of its body length. A sea otter has a shorter, flatter tail and spends much of its time floating on its back.
Field Methods and Ethics
Tracking wild otters takes quiet eyes and patience. Biologists who study river otters look for clues along the shore: belly slides in mud or snow, tracks of webbed feet, and shared scent sites called latrines.
Always give wild otters plenty of space. Use binoculars to watch from far away. Getting too close can scare otters away from their food or their young. Good naturalists record the date, place, weather, and what the animal does.
Chapter 1
Swimming and Thermal Biology
Cold water pulls heat from a body far faster than air does. Yet otters have no blubber. Instead, they rely on dense fur that traps air and on a fast metabolism that works like a furnace. Long, flexible bodies make them strong swimmers.
Built for Water: Propulsion
An otter has a long, very flexible spine. It can bend almost double to groom its fur. In the water, the same spine ripples in waves from back to belly. Scientists call this up-and-down motion dorsoventral swimming.
When an otter swims slowly, it paddles with its feet. At top speed, it drives forward with powerful up-and-down waves of its body and tail, much as dolphins and seals do.
Paddles and Rudders: Feet and Tails
River otters swim with webbed toes on all four feet and a thick, muscular tail that tapers to a point.
A river otter's tail makes up about one-third of its body length. It is thick and muscular at the base. Underwater it works both as a rudder for sharp turns and as a thruster for bursts of speed.
A sea otter's tail is shorter and flatter. While floating on its back, the otter sculls it from side to side to steer. Its hind feet are broad, webbed flippers, and the outer toe is the longest. These flippers power its swimming.
Locomotion: Water vs. Land
River otters and sea otters use their limbs differently. North American river otters spend much of their time on land. They travel miles overland between waterways and rest in dry shelters on shore.
Sea otters spend almost their entire lives at sea. They do not swim with their front paws. Instead, they use their small, nimble forepaws like hands, prying food from rocks with claws that can partly pull back.
The Warmth Trap: Fur Architecture
Sea otters have the thickest fur of any mammal on Earth, and because otters have no blubber, this dense coat is what keeps them warm in cold water.
A sea otter's fur, or pelage, packs up to one million hairs into a square inch of skin. Long guard hairs form the outer layer. When they are wet, microscopic scales on these hairs lock together like a zipper. Beneath them, soft underfur traps a blanket of tiny air bubbles against the skin.
Grooming for Survival
For a sea otter, grooming is survival, not just keeping clean. Sea otters spend several hours each day cleaning their fur. They rub it with their paws and blow warm air deep into the underfur to refresh the insulating air layer.
All otters, river otters included, groom for hours each day. They comb their fur with paws, claws, and teeth. They also spread natural oils from skin glands that help waterproof the coat.
High-Octane Furnace: Metabolism
Without blubber, otters need a hot internal furnace. An otter's resting metabolic rate is two to three times that of land mammals. To fuel it, an adult sea otter eats about 25 percent of its body weight in seafood every day.
Sea otters also save the heat they make. In their limbs and flippers, arteries carrying warm blood run beside veins bringing cool blood back. Heat passes across to the returning blood, a system called countercurrent heat exchange.
Senses in the Shallows: Whiskers and Eyes
Hunting in dark water takes keen senses. Otters have thick, stiff whiskers called vibrissae around the snout. Each whisker grows from a pocket of blood richly supplied with nerve endings.
The whiskers pick up the tiny water movements made by swimming fish and moving crabs. That lets otters hunt well even in murky or pitch-black water.
Otter eyes adjust too. Water cancels much of the eye's usual focusing power, so strong muscles squeeze the lens rounder underwater. Behind the retina, a mirror-like layer called the tapetum lucidum boosts sight in dim light.
Lungs, Kidneys, and Fur Density
Inside the chest, a sea otter has huge lungs, about two and a half times larger than those of a land mammal of the same size. They store oxygen for dives and give the otter lift, so it floats easily on its back.
Sea otters also have reniculate kidneys, built from many small lobes. They remove extra salt so well that a sea otter can drink seawater without drying out.
Chapter 2
River Otter Life and Behavior
This chapter follows the North American river otter. It lives along rivers, streams, lakes, ponds, and marshes. A mother raises her young with no help from males. River otters also leave scent messages for each other, and they wrestle, chase, and slide.
Watershed Explorers: Freshwater Habitats
River otters set up home ranges around rivers, streams, lakes, ponds, and wetlands. They need water that lasts all year, with healthy plants and clean banks along the shore.
Along the coasts of Alaska, British Columbia, and Washington, river otters can hunt fish and crabs in salty bays. But salt mats their underfur, so they must return to fresh water to rinse it.
A river otter family is a mother and her young of the year. Adult males take no part in raising pups. They wander alone or travel in loose groups of males.
Winter on Ice: Life Beneath the Freeze
River otters do not hibernate. In the north, they stay active all winter. Even when ice covers a river, they swim and hunt underneath.
As the water level drops under the ice, air gaps open along the banks. River otters breathe in these gaps while they hunt beneath the frozen surface.
Dens, Burrows, and Holts
River otters do not dig their own burrows from scratch. Instead, they take over old beaver lodges, muskrat burrows, hollow logs, or spaces under tree roots and riverbanks. A river otter's den is called a holt.
Holts usually have underwater entrances. Those hidden doors help keep pups safe from predators on land.
Both river otters and sea otters use delayed implantation. After mating, the tiny embryo stops growing and floats free in the womb for several months. The pause lets pups be born in spring or summer, when conditions are best.
Raising River Otter Pups
Female North American river otters usually give birth to two or three pups in early spring. The pups are born blind and helpless inside the den. Their eyes open after about a month.
Young pups are wary of deep water, so their mother must teach them to swim. At about two months old, she coaxes, nudges, or even carries them into the water by the scruff of the neck.
She holds them up until they learn the swimming stroke. She also teaches them to find food and guards them from predators as they grow.
Scent Communication: The Latrine
North American river otters share news by smell. They mark shared sites called latrines on points of land, fallen logs, and rocky outcrops along the water.
At latrines, otters leave droppings called spraint, urine, and strong musk from scent glands near the base of the tail. Together these signals tell other otters about identity, sex, territory, and readiness to breed.
Sea otters lack these scent glands. At sea, currents wash scent marks away quickly. Sea otters rely on calls, visual displays, and touch instead.
The Purpose of Play
River otters wrestle, play tag in the water, and chase each other. This play is not just fun. It builds agility and core strength and trains the fast reflexes they need to catch prey.
River otters also play alone with objects. They balance small stones on their paws, juggle pebbles, and drop shells to fetch them from the riverbed. This play sharpens the skill of their sensitive paws.
Chapter 3
Coastal Kelp and Sea Otter Society
Now we follow the sea otter. It spends almost its entire life at sea in the North Pacific, usually within a mile of the coast. Sea otters sleep floating on their backs, anchor themselves in kelp, dive for food, and rest together in groups called rafts.
Diving Physiology: Into the Kelp
Sea otters and river otters both hold their breath to dive for food, and muscular valves seal their ears and nostrils shut the moment they go under.
The valves clamp shut automatically, keeping water out of the airways and ear canals. Sea otters usually dive between thirty and one hundred feet to collect prey from the ocean floor.
A sea otter's dive typically lasts one to two minutes. A river otter's hunting dives are about as short. But when escaping danger, a North American river otter can stay underwater for up to eight minutes, with its heart rate slowed.
Life in the Raft: Anchoring in Kelp
Resting sea otters wrap strands of kelp around their bodies so ocean currents do not carry them away, and they gather in floating groups called rafts.
Giant kelp is anchored to the sea floor by a sturdy holdfast, so a wrapped otter is moored like a boat. A raft may hold two or three otters, dozens, or even hundreds.
Sea otters sometimes link paws while resting. Holding on keeps pairs, or mothers and pups, from drifting apart where kelp is sparse.
Social Divisions: Rafts by Sex
Sea otter resting rafts are usually divided by sex. Mothers with pups gather in sheltered coastal kelp forests. Adult and young males rest in separate rafts, usually at the outer edges of the population's range.
Floating Marine Nurseries
Most southern sea otters give birth right in the ocean. Sea otters almost always have a single pup. The mother carries her newborn on her chest as she floats on her back.
A newborn pup has fluffy natal fur that traps so much air that it floats like a cork and cannot dive. When its mother dives for food, she wraps the pup in kelp so it does not drift away.
Sea otter mothers nurse their pups for six to eight months. During that time, a mother grooms her pup's fur and shows it how to crack shells.
Chapter 4
Foraging Ecology and Tool Use
Sea otters are among the few mammals known to use stone tools. They feed mostly on hard-shelled animals from the sea floor and crack them open against a rock on the chest. River otters work differently: they seize slow-swimming fish and crayfish in their jaws.
Stone Age Mammals: Anvils and Pockets
Sea otters float on their backs and crack hard shells against flat stones on their chests, tucking a stone into a skin pouch under one arm between dives.
Under each front leg, a sea otter has a pouch of loose, stretchy skin. On the sea floor, the otter stashes clams, crabs, or urchins there. That leaves its front paws free to keep gathering food.
North American river otters have no armpit pouches. They seize prey in their jaws and often eat small prey right away in the water.
Percussive Power: Strike Speeds
To open a hard shell, a sea otter holds the prey in both front paws and pounds it against the rock on its chest. Researchers have timed sea otters striking up to three times a second.
Some sea otters keep a favorite rock with the right weight and shape. They carry it across many dives and tuck it under an arm between uses instead of finding a new one.
Some sea otters also use shoreline boulders as permanent anvils. After years of smashing mussels against the same rock, they leave worn marks and piles of broken shells that archaeologists study.
River Otter Hunting: Catching Prey
North American river otters are flexible hunters. They mostly catch slower fish such as minnows, suckers, sculpins, and carp. When they are plentiful, crayfish, frogs, salamanders, and big water beetle larvae are on the menu too.
An adult river otter has 36 teeth. Pointed canines grip slippery fish, and sharp premolars slice like shears. These teeth also crunch through crayfish shells. Biologists can even count yearly rings in the teeth to learn an otter's age.
Teeth for the Job: Shearing vs. Crushing
Skulls and teeth show how each otter feeds. An adult sea otter has 32 teeth. Instead of the slicing carnassial teeth of most land carnivores, it has broad, flat molars with thick enamel.
These molars crush the armor of sea urchins, crabs, clams, and snails. Sea otters are also the only carnivores with just four lower front teeth instead of six. Together the four form a scoop for scraping meat from cracked shells.
Diet Specialization and Traditions
Sea otters feed mostly on hard-shelled invertebrates from the sea floor. Yet individual sea otters can be picky. In the same kelp forest, one otter may specialize in crabs and clams while another targets urchins or abalone. These specialties cut down competition among neighbors.
Chapter 5
Keystone Predation and Cascades
Some predators shape whole habitats. Biologists call them keystone species. Sea otters are a classic example: by eating plant-eating sea urchins, they set off a chain reaction called a trophic cascade. River otters matter in other ways, as signs of clean water and as movers of nutrients.
Guardians of the Kelp: Urchins
Sea otters are classic keystone predators in kelp forests. They eat many purple and red sea urchins, which graze on giant kelp. Without sea otters, urchins multiply unchecked and chew through the holdfasts that anchor kelp to the sea floor.
The result is an urchin barren: bare rock where a forest stood, with far fewer fish and invertebrates. Where sea otters keep urchins in check, giant kelp can grow up to two feet a day, taking in carbon dioxide. By protecting kelp, sea otters indirectly help coastal waters store carbon.
Freshwater Bioindicators: Clean Waterways
River otters are bioindicators whose presence signals clean fresh water, and along the California coast, marine protected areas give sea otters safe havens with plenty of prey.
River otters sit near the top of freshwater food webs. They need clean water rich in oxygen, with plenty of fish and frogs, and they vanish quickly from polluted rivers.
Pollutants such as mercury, PCBs, and pesticides settle in mud and small animals, then build up in fish. River otters eat so many fish and crayfish that these chemicals collect in their livers and kidneys.
Estuary Protectors: Sloughs and Salt Marshes
In California, sea otters also protect salt marshes. In Elkhorn Slough, striped shore crabs burrow into muddy creek banks and eat the roots of pickleweed, a plant that holds the marsh together.
Forest Fertilizers: The Latrine Link
River otters link rivers to the land. They eat fish, crabs, and crayfish in the water. Then they leave nutrient-rich spraint on shore at their latrines. This carries nitrogen and phosphorus from the water into the soil along the bank.
Those nutrients help fertilize the trees, grasses, and shrubs that grow beside the river.
Chapter 6
Conservation and Coexistence
Otter history holds both loss and hope. Fur hunting drove sea otters toward extinction until treaties and laws stopped it. River otters vanished from many states and were brought back by restoration programs in more than 20 states. Yet most otter species worldwide are still in decline.
Shadows of the Fur Trade
In the 1700s and 1800s, fur traders hunted sea otters across the North Pacific for their pelts. An estimated 150,000 to 300,000 sea otters once lived there. By 1911, fewer than 2,000 survived, in about 13 scattered colonies.
In 1911, the United States, Great Britain (for Canada), Japan, and Russia signed the International Fur Seal Treaty, which banned commercial hunting of sea otters at sea. In 1972, the U.S. Marine Mammal Protection Act added strong federal protection. The surviving colonies slowly grew.
Bringing River Otters Back
In the 1900s, pollution and unregulated trapping wiped out North American river otters in 11 U.S. states and reduced them in dozens more. Starting in 1976, state wildlife agencies moved over 4,000 river otters into more than 20 states. There, the otters formed breeding populations that now sustain themselves.
Clean water matters for the return, too. River otters need unpolluted rivers full of fish, so healthy river otters are a good sign for a whole watershed.
Modern Hazards: Oil, Disease, and Sharks
Sea otters still face serious threats. In March 1989, the tanker Exxon Valdez spilled 11 million gallons of crude oil into Prince William Sound, Alaska. An estimated 2,800 sea otters died soon after. Oil soaked their underfur and destroyed its trapped air, so they lost body heat fast.
Some threats wash down from land. Rain carries the parasite Toxoplasma from cat feces through storm drains and rivers to the sea. Snails and shellfish concentrate it, and southern sea otters that eat them can die of brain inflammation.
Toxins from blue-green algae blooms in polluted lakes also flow into bays such as Monterey Bay. Sea otters that eat tainted mussels and clams can suffer fatal liver failure.
Along the California coast, great white sharks take exploratory bites of floating sea otters. The sharks do not eat them, but the wounds can be fatal. These deaths help limit where California sea otters can spread.
Otters of the World: Thirteen Species
Thirteen otter species are traditionally recognized, living in Africa, Asia, Europe, North America, and South America. Sea otters also have three recognized subspecies: southern, northern, and Russian.
The giant otter of South America grows up to six feet long and hunts in vocal family packs. The Asian small-clawed otter, the smallest species, weighs just 4 to 11 pounds. Its short, blunt claws leave its fingers free to probe mangrove mud for crabs.
Most otter species worldwide are in decline. The IUCN Otter Specialist Group reports that 12 of the 13 species are declining. Habitat loss, dams, farm runoff, and poaching all threaten them.
Coexistence and Field Observation
Where otters and people share water, both need care. You can help by joining river cleanups and reporting otter sightings to local wildlife groups.
Glossary
- anvil
- a hard rock surface an otter uses to break open tough shells
- bioindicator
- a living animal that shows how clean and healthy its home is
- carnassial
- slicing cheek teeth that cut meat like scissors
- countercurrent
- a blood flow path that moves heat from warm arteries to cool veins
- delayed implantation
- a pause in early pregnancy that lets pups be born in a good season
- dorsoventral
- an up-and-down body wave from back to belly
- holt
- a river otter's den, often with an underwater entrance
- keystone species
- a species that shapes its whole habitat, like sea otters in kelp forests
- pelage
- the fur coat that covers the body of a mammal
- raft
- a group of sea otters resting together on the water surface
- reniculate kidney
- a lobed kidney built to filter lots of salt from seawater
- spraint
- river otter droppings used to leave scent marks on land
- trophic cascade
- a chain reaction through a food web, set off when a predator changes how many prey there are
- vibrissae
- stiff sensory whiskers that feel tiny water waves