Around 140 recognized species of mammals live in water, ranging from fully aquatic species like whales and dolphins to semi-aquatic ones like seals and sea otters. These animals span three biological orders and have evolved remarkably different strategies for surviving in oceans, rivers, and coastal waters.
Whales, Dolphins, and Porpoises
Cetaceans are the largest group of water-dwelling mammals, with 94 recognized species. They are fully aquatic, spending their entire lives in water, and they split into two major subgroups: baleen whales and toothed whales.
Baleen whales are the filter feeders. Instead of teeth, they have hundreds of comb-like plates made of keratin hanging from their upper jaws. They gulp or skim enormous volumes of water and push it back out through these plates, trapping tiny shrimp-like creatures and small fish. This group includes blue whales, humpback whales, and right whales. Right whales have massive heads that make up a third of their body length just to accommodate their long baleen plates. Baleen whales have two blowholes and do not echolocate.
Toothed whales, by contrast, have a single blowhole and use echolocation to find prey. This group includes sperm whales, orcas, dolphins, and porpoises, with more than 40 species of smaller dolphins and porpoises found worldwide. Their teeth are conical or spade-shaped, designed for grasping fish and squid rather than chewing.
River Dolphins
Not all cetaceans live in the ocean. Seven recognized species live primarily or entirely in freshwater rivers, and most are in serious trouble. Five of these species live in Asia, and two are found in South America’s Amazon and Orinoco river basins.
The Amazon river dolphin (also called the boto) is one of the most recognizable, with a pinkish hue and a long, flexible snout suited for navigating murky, flooded forests. In the same waterways, the tucuxi, a smaller dolphin, has declined by roughly 7.4% per year from 1994 to 2017. In Asia, the South Asian river dolphin still survives in the Ganges and Indus river systems, while the Irrawaddy dolphin population in the Mekong River has dropped to fewer than 100 individuals, mostly due to entanglement in fishing nets. The baiji, a freshwater dolphin found only in China’s Yangtze River, was declared extinct in 2006.
Seals, Sea Lions, and Walruses
Pinnipeds account for 37 species. Unlike cetaceans, they haven’t fully committed to life in water. They hunt in the ocean but haul out onto land or ice to rest, breed, and give birth. The group divides into three families: true seals, eared seals (sea lions and fur seals), and walruses.
The easiest way to tell seals and sea lions apart is their ears. Sea lions have small visible ear flaps, while true seals have only tiny holes. Sea lions also have comparatively large front flippers and can tuck their back flippers under their bodies to walk on all fours. True seals are more awkward on land, shuffling along on their bellies, but they’re built differently for swimming too. Sea lions power through the water with sweeping front-flipper strokes, tucking their flippers tight against their bodies to form a torpedo shape between strokes. Seals, walruses, and most other aquatic mammals rely on the back end of their bodies for thrust instead.
Walruses stand alone in their own family. Both males and females grow long tusks, which they use for hauling themselves onto ice, establishing dominance, and defense.
Manatees and Dugongs
Sirenians are gentle, slow-moving herbivores sometimes called sea cows. Five species are recognized, though one (Steller’s sea cow) was hunted to extinction in the 1700s. The surviving species are three types of manatee and the dugong.
Dugongs live exclusively in warm, shallow coastal marine waters. In Queensland, Australia, they’re found from Moreton Bay up through the Great Barrier Reef and into the Torres Strait, wherever healthy seagrass meadows grow. They spend most of their day grazing the seafloor with a downward-curving snout perfectly shaped for the job. Manatees are more flexible in their habitat, moving between freshwater rivers, estuaries, coastal lagoons, and ocean waters across the Americas and West Africa. Their rounded faces and flexible upper lips let them grab a wider variety of aquatic plants.
You can tell them apart at a glance by their tails. Dugongs have a forked, dolphin-like tail fluke, while manatees have a broad, rounded paddle.
Sea Otters and Polar Bears
Two members of the order Carnivora qualify as marine mammals without belonging to the seal family. Sea otters live almost entirely in coastal Pacific waters, floating on their backs, diving for shellfish, and famously using rocks as tools to crack open their food. The marine otter (or chungungo) of South America also depends on the ocean, though it’s far less well known.
Polar bears are the most land-oriented of all marine mammals. Their skeletons are built for walking rather than swimming, and their large, clawed feet are better adapted for gripping sea ice than propelling through water. Still, they depend on the marine environment for survival, hunting seals on Arctic sea ice and swimming between ice floes. They’re strong swimmers when they need to be, but among marine mammals, they’re the least adapted for life in water.
How Mammals Survive in Water
Water pulls heat from a body 25 times faster than air at the same temperature. That’s a lethal problem for warm-blooded animals, and aquatic mammals have evolved two main solutions: blubber and fur.
Blubber is a thick layer of specialized fat beneath the skin found in cetaceans and pinnipeds. It does more than insulate. It streamlines the body, stores metabolic energy, and adds buoyancy. The insulative quality varies between species depending on both the thickness and the fat content of the blubber. Harbor porpoises, for instance, have blubber about twice as thick as that of tropical spotted dolphins (roughly 1.5 cm versus 0.77 cm), giving them about four times the insulating value. Blubber with higher water content conducts heat more readily, so fattier blubber insulates better. Newborn dolphins aren’t born with especially high-quality blubber. Instead, their insulation simply improves as they grow and accumulate more of it.
Sea otters took a different path entirely. They have no blubber. Instead, they rely on the densest fur of any mammal, trapping a layer of air against the skin that acts as a wetsuit. This makes them uniquely vulnerable to oil spills, which mat down the fur and destroy its insulating properties.
The Diving Reflex
All mammals, including humans, share a built-in response to submersion called the diving reflex. When the face hits cold water and breathing stops, the heart rate drops and blood vessels in the extremities constrict, redirecting blood flow toward the heart and brain. This conserves oxygen for the organs that need it most and extends the time before oxygen deprivation causes damage.
In humans, this reflex reduces heart rate by 15 to 40%, though a small number of people experience drops below 20 beats per minute. In deep-diving marine mammals like sperm whales and elephant seals, the reflex is far more extreme, allowing dives lasting well over an hour. The longer the dive, the more pronounced the heart rate reduction becomes, as dropping oxygen levels trigger additional slowing. Aquatic mammals also store far more oxygen in their muscles than land mammals do, giving them a larger reserve to draw on during extended time underwater.