They are both semi-aquatic animals – medium-sized, brown, furry, adorably cute and excellent swimmers. Both live in family groups. Both are intelligent. Both nearly went extinct because of humans. Beavers and otters share enough similarities to cause confusion, but the species also possess some very distinctive characteristics.
Beaver is a Rodent
The beaver exists as two species, one in North America and one in Europe. It is the second-largest form of rodent–animals known for their gnawing, chisel-like teeth. As a rodent, it's related to rats, mice, chinchillas and porcupines.
The feature everyone spots on a beaver is its tail. The tail is furless, flat and wide, like a ping-pong paddle. If a beaver sees danger while out on the water, it slaps the water with its tail to alert other beavers.
Otter is a Weasel
The otter, with 18 species, is distributed across the world. It's in the weasel family along with minks, skunks and badgers. The relationship is seen in an otter's sharp teeth and claws. Otters can be found near rivers and in the sea, depending on the species. They are famous for their playful nature. A group of otters is often called a romp.
Being semi-aquatic means both animals spend a large amount of their time in the water, but some time on dry land.
Beavers stick to ponds and wetlands. They want deep, still water in which to store tree branches for food and avoid predators. If there is no pond, they create one by building a waterproof dam across a stream. The only other animal that changes its environment more is the human animal.
Otters, on the other hand, may dig a den out of a riverbank, but they don't otherwise change their environment. Some otters live next to rivers. Sea otters live in the sea just off the coast. They don't build dens. Instead, when night falls, they wrap themselves up in kelp so they don't float out to sea.
Otters are obligatory carnivores. They must eat meat to survive. Usually, this is fish. Otters also eat other small animals they can catch, such as insects and birds. Sea otters are famous for bringing shellfish up to the surface and cracking them open with a rock on their chest.
Beavers are herbivores. They are only interested in plants, especially trees. Not only will a beaver bring down a tree to build a dam or lodge, it will also store leafy branches and whole trees in its pond to eat over the winter.
Because neither species has a layer of fat, they depend on a thick, glossy pelt to keep them warm. Hats made of beaver fur were all the rage in the 1800s, and sea otter fur was demanded for coats. Hunting killed off almost the entire population. Now laws protect both the beaver and the otter, and their respective populations have made a comeback. Both have been spotted in places where they hadn't been seen for decades.