River otter vs sea otter? Watch how the animal floats

The short answer

Watch how the animal holds itself in the water. A sea otter lies on its back at the surface with its paws up, riding the swell. A river otter swims belly down like most animals, head and shoulders pushing a small wake ahead of it.

The National Park Service in Alaska puts it plainly: "Only sea otters float on their backs. They are clumsy on land and rarely leave the ocean. River otters, on the other hand, glide effortlessly between shore and water."

Size is the second check. The Seattle Aquarium puts sea otters at two to three times the size of river otters. Stand the two side by side and the resemblance falls apart.

How big each one actually gets

Alaska's Department of Fish and Game lists adult river otters at 15 to 35 pounds and 40 to 60 inches long. That is a long animal and a light one, and most of the length is spine and tail.

An adult male sea otter weighs 80 to 100 pounds and reaches about five feet, with females about a third smaller. The lightest male on that scale still outweighs the heaviest river otter.

The gap shows in how each body is built. A sea otter carries one made to hold its heat in open ocean through the winter. A river otter carries one made to climb a muddy bank and keep going.

One lives in the sea, the other commutes

The habits separate even further than the bodies do. The Seattle Aquarium describes sea otters as animals that spend most of their lives in the water, where they breed, give birth, hunt and sleep, while river otters live most of their lives on solid ground and use the water to travel and find food.

River otters really do commute. The same Alaska profile records them travelling several miles overland between bodies of water, wearing in well-defined trails that get used year after year.

A river otter in salt water is still a river otter, and that is where most of the confusion starts. Olympic National Park reports them on lakeshores, riverbanks and the outer coast, often foraging in the intertidal zone close to shore. So a brown otter in the surf settles nothing by itself, and you are back to watching how it floats.

The smaller differences

The tails separate them at a distance. A sea otter's tail is short and flattened. A river otter's is long and pointed.

Cold is handled differently too. Sea otters have no blubber. They hold their heat with air trapped in their dense underfur, and they will dive as deep as 250 feet to feed, sometimes hauling a rock up to the surface to break shellfish against.

Breath-holding runs the opposite way from what the size suggests. Olympic National Park clocks a river otter at up to eight minutes underwater, against a sea otter's five.

Sea otters also rest in groups with their paws linked. We looked at what actually holds a raft of sea otters together in an earlier field note.

Pups and cubs

A sea otter mother raises one pup at a time, and gives birth in the water. River otters may have several cubs in a litter.

That same Alaska profile puts the litters at one to six pups, usually two or three, born toothless and blind in a den that is usually a subterranean burrow. One otter starts life at sea. The other starts it underground, in the dark.

A field note from the bank

If the animal is on its back, let it be a sea otter. If it climbs out, shakes, and walks off down a path it clearly already knows, you have a river otter, and most likely a regular. Those trails get used for years.

The river otter who moved in

Our own otter is a river otter, and he reads like one. He is ordinary river-otter size, down to the long pointed tail, and built for a life between a creek and a trail. The red eyes are the one part of him no field guide covers.

The people at the sanctuary took one look at those eyes and that unbothered stare and landed on Murder Otter. He has never shown much interest in the reputation. Most days he is out in the shallows with his attention on something none of them can identify. He turns up early in the story, in the chapter called The Otter Paradox, and he stayed.

To see where he sits among the rest of them, the Atomic Critters Universe page lays out the cast. If you only came for the otter, he has a collection of his own, including a sketch of him holding a red balloon that leans all the way into the name.

Either way, the test stays the same. Find the animal in the water and watch how it lies.