Mammals are warm-ʙʟᴏᴏᴅed animals that give birth to live young (except for a few exceptions that lay eggs) and feed those young with milk. Most mammals are hairy – but not all! Humans and ɴᴀᴋᴇᴅ mole rats are both mammals but they aren’t exactly fluffy.
Mammals do not have gills. That means that any aquatic or marine mammals need to be able to come to the surface often enough to breathe. In contrast, fish and amphibians with gills can stay underwater indefinitely.
Whales, dolphins, and porpoises (a.k.a Cetaceans) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer. The transition from land to water is documented by a series of intermediate f͙o͙s͙s͙i͙l͙s, many of which are known from India and Pakistan.
Cetaceans have lungs and come to the surface to breathe air, like other mammals and unlike fish. And even though modern cetaceans have bodies fully adapted for life in water, traces of their land ancestry are still present in cetacean e̼m̼b̼r̼y̼o̼s: modern cetaceans lack hind limbs, but their e̼m̼b̼r̼y̼o̼s still have the beginnings of hind limbs.
Cetaceans are unrelated to other marine mammals, the sirenians (manatees and dugongs) and the pinnipeds (seals, sea lions, walruses). Sirenians are most closely related to elephants, and pinnipeds are related to land carnivores (e.g., dogs and bears). In some regards, all cetaceans, sirenians, and pinnipeds are similar; they are all adapted to life in water. For instance, they all have streamlined bodies, short limbs, and fin-shaped hands and feet. In other regards, these three groups are dissimilar. For instance, cetaceans and sirenians lack (nearly all) body hair, whereas pinnipeds have dense fur. On the other hand, whereas the main propulsive organ of cetaceans and sirenians is the tail, sea lions swim with their forelimbs, and seals with their hind limbs.
Are Sharks Mammals?
On the other hand, sharks are a subgroup of fish. Some sharks do give birth to live young, but they do not nurse and they’re not warm-ʙʟᴏᴏᴅed. In fact, sharks don’t even have true ʙᴏɴᴇs. Their sᴋᴇʟᴇᴛᴏɴ is made up entirely of cartilage. Sharks don’t have ribcages, so they’re easily ᴄʀ ᴜsʜed under their own weight on land.
Unlike most other fish, sharks don’t have a swim bladder. They “float” within the water column thanks to their cartilaginous sᴋᴇʟᴇᴛᴏɴ, which is far less dense than ʙᴏɴᴇ. Sand sharks can also store air in their stomachs to help them keep afloat. Most sharks also rely on ｄｙｎａｍｉｃ lift to avoid sɪɴᴋing in water. This means they need to keep moving and rely on their pectoral fins like wings to stay still, and they can’t swim backward.
What about Seals, Sea Lions, and Sea Otters?
Seals and sea lions are also mammals, but they’re not very closely related to Cetaceans. In fact, seals, sea lions, and sea otters are all part of the order Carnivora. Carnivora also includes dogs, cats, bears, and skunks. The closest living relatives of sea lions and seals are probably bears. The closest living relatives of sea otters are other otter species.