Head to head
North American Beaver vs Giant Otter
Seen as a brown head and a wake crossing a pond at dusk, a beaver and an otter can look the same. Up close they are unmistakable, and they are not related. The beaver is a rodent, the second-largest in the world, and a strict vegetarian that reshapes whole landscapes by building dams. The otters are a group within the weasel family, sleek carnivores that hunt fish and build nothing. The examples here are the North American beaver and the giant otter of the Amazon, the largest of the otters, but the contrasts hold for river otters everywhere.
| North American Beaver | Giant Otter | |
|---|---|---|
| Group | A rodent (family Castoridae) | A mustelid, the weasel family |
| Diet | Herbivore: bark, cambium, leaves, aquatic plants | Carnivore: mainly fish, plus crabs and other prey |
| Tail | Broad, flat, scaly, paddle-shaped | Long, round, tapering, muscular |
| Builds dams? | Yes; dams, lodges, and canals | No; digs a simple bank den |
| Teeth | Huge, ever-growing, iron-orange incisors | Sharp carnivore teeth for gripping fish |
| Social life | Monogamous family colony in one lodge | Noisy extended family groups, up to about 20 in the giant otter |
| On land | Waddling and slow | Bounding and agile |
| History | Nearly trapped out for fur; widely recovered and reintroduced | Nearly trapped out for pelts; giant otter still Endangered |
Beaver vs Otter: common questions
How do I tell a beaver from an otter in the water?
Watch the tail and the way it moves. A beaver swims with just its broad head and the top of its back showing and, if alarmed, rears up and brings its flat tail down with a loud crack before diving. An otter is more snake-like at the surface, its long body undulating, often with the tail visible as a thick rope trailing behind, and it dives smoothly and silently, frequently rolling and resurfacing nearby. A beaver seen out of the water looks heavy and hunched; an otter bounds. And an otter is very often in a group, whereas an adult beaver is usually alone or with one or two others from its family.
Are beavers and otters related?
No. The beaver is a rodent, part of the vast group that includes mice, squirrels, and porcupines, and its closest relatives are the mountain beaver of the US Pacific coast and, more distantly, the pocket gophers. The otter is a carnivoran in the family Mustelidae, alongside weasels, badgers, wolverines, and minks. The two lineages diverged far back in the mammal tree. They look alike only because both have adapted to a semi-aquatic life with dense waterproof fur, webbed feet, and valves that close the ears and nose underwater, a set of features that evolution keeps arriving at independently.
Do otters build dams like beavers?
No, and this is one of the clearest ways to tell which animal has been active on a waterway. Beavers are the builders: dams of branches and mud that pond water back, dome-shaped lodges with underwater entrances, and sometimes long canals dug to float wood. Otters modify almost nothing. They use existing holes and undercut banks, or dig a short burrow with a couple of entrances, one of them usually below the waterline, and they leave latrine sites and grassy "rolling" patches. If a stream has a stick dam and a pond, that is a beaver; if it has fish scales, crayfish shells, and slides on the bank, that is an otter.