River Otters vs Sea Otters

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River Otters versus Sea Otters – 10 Main Differences

Lately we have spent a lot of time traveling in wetland areas of the southern United States, including North Carolina, Florida, and the coast of Alabama. We’ve been hoping to spot river otters in their habitat, but without any luck. We’ve researched quite a bit to find out the differences between river otters vs sea otters.

So, let’s take a look at the fact comparing rivers otters vs sea otters.

River Otters vs Sea Otters

The scientific name for the North American river otter is Lontra Canadensis. Lontra is Portuguese for otter, and Canadensis means “of Canada.”  North American river otters are a part of the genus Lontra, which also includes the marine otter, neotropical otter, and southern river otter. These are a part of the subfamily Lutrinae, which consists of all otters.

Sea otters, or Enhydra lutris, Greek for “in the water” and Latin for “otter,” are a part of the genus Enhydra, a part of the subfamily Lutrinae that contains all 13 extant otter species.

rier otter vs sea otter. This is a photo of a sea otter off the coast of California.
Sea otter off the coast of California.

So, sea otters and river otters are both in the subfamily Lutrinae, a part of the Mustelidae family. This family also includes ferrets, badgers, weasels, and wolverines.

There are three subspecies of sea otters, Northern, Southern, and Asian sea otters. These subspecies are based primarily on their location and physiological differences. The largest of the three subspecies is the Asian sea otter and should not be confused with the Asian small-clawed otter.

river otters vs sea otters river otter playing in the water
River Otter

River Otters vs Sea Otters – Physiology Differences & Similarities

Physiology: River otters Live from 12-15 years, becoming reproductively mature by 2 to 4 years of age.  

River otters are noticeably smaller than sea otters, typically weighing 10 to 30 pounds and usually measuring around 2-2.5 feet. They are noticeably slimmer and smaller than sea otters, with longer tails, and the females are especially smaller.

Sea otters typically live between 19-24 years but only become reproductively mature around 5-8 years of age.

Male sea otters can weigh anywhere from 50-100 pounds and measure up to 4’11 inches, whereas females tend to weigh from 15 to 75 pounds and reach a length of 4 feet 9 inches.

Unlike most marine mammals, sea otters do not have blubber to rely upon for warmth but make up for this by having an extremely thick coat; in fact, it is the densest fur of any known animal, with roughly 150,000- 1 million hair follicles per square inch!

River Otters vs Sea Otters – Diet Similarities and Differences

Diet:  River otters are primarily crepuscular or nocturnal and typically become diurnal during the winter. (Crepuscular means the animal is active at twilight. Diurnal means “of the day,” so they are active during the day and either rest during the nighttime hours or mostly rest during the nighttime hours.)

Unlike sea otters, river otters do not rely on the use of rocks nearly as much; they are more driven by hunting after prey. They can swim over 7 miles per hour, hold their breath for 4 minutes, dive up to 60 feet, and use their powerful yet small jaws to capture their favored prey of small fish, lizards, frogs, crayfish, and small waterfowl.

Sea otters are primarily diurnal. They often forage for food around rocky coastlines and the sea floor, where they prey on snails, mussels, and other shellfish. Their favorite prey tends to be spiny sea urchins.

Notice the long whiskers on sea otters? One of the uses for their whiskers is to help locate food in the ocean, as they dig around formations and the sea floor.

Sea otters are incredibly reliant upon their front paws. They use these to turn over rocks in search of prey and catch fish. Another unique trait of sea otters is their arm pouches and obsession with rocks. Sea otters have special pouches under their front arms to store food and small rocks that they save for cracking open hard shells. Below is a sea otter eating a green crab off of the coast of California.

River otters vs sea otters this is a photo of a sea otter eating a green crab off the coast of California.
Sea Otter eating a green crab off the coast of California.

River Otters vs Sea Otters – Similar and Different Behaviors

Behavior: River otters are polygynous like sea otters, meaning that one male will mate with several females. Gestation typically lasts two months. However, river otters also experience delayed implantation, so they may not immediately become pregnant. The mating season is typically between Spring-early Winter.

Unlike sea otters, river otters will find a den in which to have and raise their pups. The pups will be two months old when they can finally be introduced to water, and typically by this time, they are ready to leave the den, but they may stay with their family for up to a year.

River otters are more social than any other species of otter. The social grouping of river otters typically consists of a mother and 1-3 pups, along with some unrelated adults and juveniles that help the leading mother.

A group of river otters on land is called a romp. However, other names have been used for a group of river otters on land, including bevy, family, and lodge. When a group of river otters is in the water, they are called a raft.

Male river otters can also create bachelor groups of 10-20 males. No matter how the group is made up, they all will live in the same area, sleep in the same den, use the same place for the bathroom, share food, and groom one another. Relations within groups are typically friendly, but otters of one group tend to avoid those of other romps (technical term for a group of otters).

Unlike the clunky, ill-adapted for-walking bodies of sea otters, river otters are exceptionally lithe and nimble on land. This makes their infamous love of playfulness, chasing, and mock-fighting possible. Play is an important part of developing survival skills for otters.

Sea otters like to congregate and float in large groups known as rafts. When not foraging, they often cling to each other and sea kelp to keep from drifting out to deeper waters when resting. Many rafts are often of the same sex, as females prefer to avoid males until they enter estrus, which may occur at any time.

A raft of sea otters. Credit the USFWS

There is no set breeding season for sea otters. However, it primarily occurs in autumn, with gestation lasting six months.

Sea otters are one of few mammals capable of experiencing delayed implantation, which means that the embryo created will not implant immediately and will only fertilize once favorable environmental conditions are met. Once pregnant, sea otters primarily only have one pup per mating period, but multiple pups rarely occur. Sea otters are polygynous by nature, meaning that a single male will mate with several females.

Although male sea otters have territories and will defend them, their defenses are usually nothing more than splashing and vocal expressions; fighting amongst sea otters is highly uncommon. Sea otters can give birth at sea.

River Otters vs Sea Otters – Habitat Similarities and Differences

Habitat: The North American river otter is found primarily along coastal regions. However, their populations have decreased steadily within the Midwestern United States.

They prefer to stay by bodies of fresh and marine water. River otters do not base their habitat on temperature or terrain but primarily on food and water access. They do not dig their dens but instead steal those other mammals make.

The Northern sea otter is found from Alaska down to northern Oregon. The Southern sea otter is found primarily along the coast of California. The Asian sea otter lives from the Northern Kuril islands of Japan to the Eastern shores of Russia.

seaotter 1
Sea Otter off the coast of California.

River Otters vs Sea Otters – Are Otters Endangered?

Conservation: Sea otters are considered Endangered, which is alarming as they are also considered a keystone marine species.

 Sea otters are voracious carnivores and often hunt herbivorous invertebrates, which can severely harm kelp forests if their populations are not kept under control.  Sea otters were nearly hunted to extinction due to the fur trade, which has now been outlawed. Sea otters are also impacted by fecal runoff from domestic cats and other terrestrial wildlife as they carry various diseases and parasites that severely impact sea otter populations.

Mass oil spills, commercial fishing, illegal poaching, plastic pollution, and ocean acidification are significant causes for concerns in terms of otter populations. All of these issues stem from problems humans create and can change. To help save the endangered sea otter, consider donating and advocating for sea otter rescues and research. Help replant forests, local wildflower populations, and your garden without inorganic fertilizers to help reduce run-off and prevent algal blooms. Always pick up your pets and properly dispose of their waste to prevent the spread of parasites and diseases. Help clean up litter in your neighborhood or along the coast. And most importantly, learn all you can about ocean issues and share your knowledge.

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