River Otter vs. Sea Otter Size, Fur Density and Food Preferences

You might think that all aquatic otters must be the same. But did you know that there are major river otter vs. sea otter differences?

Sea otters are marine mammals that are found in the North Pacific Ocean, whereas river otters live all around the world in freshwater habitats like rivers, lakes and wetlands.

Unlike sea otters, which spend most of their time in the water, river otters spend much of their time on land, moving between waterways and even burrowing in riverbanks.

Habitats: Freshwater vs Saltwater

River otters and sea otters may belong to the same family, but their homes couldn’t be more different.

The North American river otter (Lontra canadensis) thrives in fresh water. Sea otters (Enhydra lutris), on the other hand, live almost entirely in salt water.

Because sea otters live full-time in saltwater, which can dehydrate animals, these marine otters have special kidneys that help them filter out the salt and retain fresh water in their bodies. They also rely heavily on their dense fur to keep warm since ocean water can be extremely cold.

Their fur traps air and creates an insulating layer, but it needs to stay clean and fluffed to work properly, so these sea otters spend a lot of time grooming.

The river otters' freshwater environment gives them easier access to drinking water and allows more flexibility in how they live and hunt compared to their ocean-dwelling cousins.

Size and Physical Traits

One of the key differences that distinguishes sea otters from river otters is their size.

Adult river otters tend to be smaller and more slender, with males reaching about 30 pounds (13.6 kilograms). They have four webbed feet adapted for both swimming and walking on land.

They have long tails that help them steer through rivers and lakes, and they’re known for their playful behavior, often sliding down muddy banks for fun.

Sea otters, in contrast, can weigh up to 100 pounds (45.3 kilograms), making them much bulkier. Sea otters have the densest fur of any animal on Earth — up to a million hairs per square inch. Their thick fur keeps them warm in cold waters, eliminating the need for a blubber layer like other marine mammals.

These otters rely primarily on their hind limbs for swimming. Unlike the paws of river otters, sea otter hind feet have evolved into broad, flattened flipper-like structures which provide a large surface area for pushing against the water.

What Do They Eat?

Dietary preferences are another big factor in the river otter vs. sea otter comparison.

River otters have a diverse diet that includes fish, crabs and other aquatic creatures. They are excellent swimmers and use their sharp teeth to catch fish in fresh water.

Sea otters rely on marine invertebrates like sea urchins, crabs and other shellfish. They are one of the few animals that use tools, often breaking open shells with rocks to access food.

Their diet plays a crucial role in marine environment. By eating sea urchins, sea otters help maintain healthy kelp forests along the coastline.

Reproduction and Social Behavior

River otters and sea otters also differ in how they raise their young.

Female sea otters give birth to only one pup at a time and spend months floating with their baby on their chest, grooming its thick fur to keep it buoyant. Sea otters live in loose social groups, but they don’t form family groups like some other otter species.

River otters, however, are more social. Male river otters often form small bachelor groups, while females raise their pups in dens along riverbanks. Young river otters reach sexual maturity within two years, and their playful nature helps them develop hunting skills early in life.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.