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There is some discussion over how many groups of marine mammals there are. Some organizations say four; others say five. They all include the same mammals, so I wanted to research what these are.

NOAA recognizes four different groups: cetaceans (whales, dolphins, and porpoises), pinnipeds (sealssea lions, and walruses), sirenians (manatees and dugongs), and marine fissipeds (polar bears and sea otters). The Marine Mammal Center separates polar bears and sea otters into five groups.

Here I present the four (or five) groups of marine mammals.

All marine mammals are protected in the United States by the Marine Mammal Protection Act of 1972. Climate change, habitat loss, and illegal hunting are some of the threats they face, and many marine mammals are considered endangered.


The cetaceans family comprises whales, dolphins, and porpoises. They all belong to the order cetacean, a word derived from the Latin language and translated to a large sea mammal.

There are approximately 89 species of cetaceans have been discovered so far. More species are still being identified and classified.

Blue whale

The cetaceans occupy all oceans in the world and many large rivers. The smallest marine mammal in this family is the hector’s dolphin, while the blue whale is the largest.

During the prehistoric period, it is believed ancestors of the whales lived on land. The land ancestors are thought to have resembled small dogs, cows, or camels.

Later, they started living entirely in the oceans and adapted to the marine environment. There, they gradually lost their hind legs, and the front legs ultimately turned into paddle-like flippers. This is how cetaceans are believed to have evolved.

There are different types of cetaceans, and they are mainly classified according to their feeding methods. They are divided into two main groups; the odontocetes, the toothed whales, and the mysticetes, baleen whales.

Want to know more about how whales feed? I have written an article which you can find here, on how baleen whales feed.

Just like the name suggests, the toothed whales typically have teeth. They are more prominent in numbers than their counterparts. Currently, there are more than 72 species of toothed whales. These include sperm whales, killer whales, dolphins, porpoises, belugas, and narwhals. 

These species have cones or spade-shaped teeth to catch prey before swallowing. Many of these prefer to feed on fish. The toothed whales are exceptionally social marine mammals and like to live in groups, also known as pods.

The baleen whales from the second type of cetacean. Currently, only 14 species of baleen whales have been discovered. These include the blue whale, right whale, fin whale, and humpback whale. 

These mammals lack teeth and possess comb-like structures on top of their upper jaws. They feed by filtering the food from the water. A baleen whale gulps a mouthful of water filled with lots of fish and plankton.

It then forces the water through the baleen plates while the food remains inside the mouth, where it is swallowed afterward.

Cetaceans have blowholes that they use to breathe. When they want to live, they rise to the water’s surface, inhaling and exhaling through their blowhole. They also have a thick layer of fat to keep them warm, preventing excessive heat loss to the water.

Want to know more about the largest animal on the planet? Check out my 101 facts article on the blue whale. You can find it here.


The word Pinnipeds is used to refer to a group of fin-footed marine mammals. This group comprises seals, walruses, and sea lions. 

Pinnipeds are divided into three families; the Phocidae, the earless or actual seal; the Otariidae or eared seals; and the Odobenidae, the walruses. All three families are comprised of 33 species of animals. 

These kinds of marine mammals are adapted to spending time both in the water and on land. Pinnipeds spend time in the water but occasionally come onshore to suckle newborns, molt, or mate.

Pinnipeds live in various habitats, including ice, tropics, coastal, and the open ocean. They may also be migratory or sedentary. They have flippers on both the front and rear of their bodies. These animals can drag themselves onto land and survive for long periods.

The earless or authentic seals can be identified by looking at their ears. They have ear holes but lack external ear flaps. Also, they have smaller front flippers and glide on land using their bellies.

The eared seals comprise sea lions and fur seals. You can tell these animals from the earless seals by their flippers and ears. They have large front flippers and can walk on them while on land. They also have external ear flaps.

Walruses have air sacs located on their necks. The air sacs inflate so that they can float on water. Walruses are among the largest Pinnipeds, with males reaching weights of about 3,000 pounds. They live in the arctic regions in the north Pacific and north Atlantic oceans.


The Sirenians from the order Sirenia include manatees and dugongs. These are herbivorous marine mammals and feed exclusively on aquatic vegetation. Sirenians are collectively given the name sea cows. They have large, thick bodies and tend to move at a slow pace.

These mammals prefer shallow waters and like to be submerged at least two meters deep. This is due to plenty of vegetation and kelp to feed on in the shallow waters. 

Manatees can be found in the warm latitude areas on the coast of the Atlantic. Dugongs occupy the coastlines of the Pacific Ocean.

Dugongs are found entirely in the ocean. Manatees can be found in both marine and inland waterways. They also differ in the shape of their tails. Dugongs have deeply notched seats that closely resemble a whale’s tail. Manatees have rounded bottoms that look like a paddle.

Sirenians tend to be solitary creatures and only come together whenever they want to mate. These marine mammals can breed all year round, but seasonal peaks exist. Manatees have mountains during the wet season when there is plenty of food. Dugongs have increased birth rates in the warmer seasons. Sirenians have an average lifespan of about 50 to 70 years.

Want to know more about manatees? Check out my 101 facts about them here.



The Mustelids are a large group of mammals, including weasels, badgers, martens, and otters. Only one species is classed as a marine mammal from this entire group. Sea otters can be found on the Pacific Ocean’s coastline from Alaskaifornia.

Sea otters are famous and adorable marine mammals. They are mainly recognized by their furry bodies and their ability to lie on the back and float on water. Sea otters are carnivorous and must eat at least 25% of their body weight daily. Their diet comprises crabs, mussels, sea urchins, and clams.

Unlike other marine mammals, sea otters lack blubber. This is why they have thick fur to keep them warm. The dense fur insulates their bodies from the water’s cold and prevents heat loss. 

Sea otters must constantly groom their fur to keep it in good condition. By doing this, the hair can trap air and heat next to the skin. The hair also keeps them buoyant in water.

Sea otters are social creatures and stay in groups of either females with their pups or males alone.

Sea otters are considered a keystone species because they help keep the number of sea urchins down. If sea urchins are left to increase in numbers, they would, in turn, eat almost all the kelp. This would affect other sea animals relying on kelp for their food and habitat.

Ever wondered why otters hold hands? Find out here.

Polar Bears

Polar bears may look like land mammals but are technically marine mammals. Polar bears spend a significant part of their lives on ice, not land. 

Polar bears hunt, mate, and do most of their activities on the ice. Their ideal habitat is the sea ice in the arctic ocean in Canada.

Polar Bear

They have large webbed feet that they use to swim and walk on the ice. Polar bears are carnivorous and feed mainly on seals. They will scavenge on carcasses or hunt smaller animals and birds when there is limited food

Polar bears are excellent swimmers and can swim up to speeds of 6mph. They have great endurance and can swim for hours to get to a piece of ice. 

They have large paws that paddle through the water while the hind legs are held flat. The paws are widespread and can measure up to 12 inches in width.

Polar bears have whitish-looking fur that keeps them warm in the arctic’s freezing temperatures. These animals can occasionally be spotted washing their hair in ice to keep it clean.

For 101 facts on polar bears, click here.