No. Fish are not mammals. The two are separate branches of the backboned animals, and they split apart a very long time ago. Mammals breathe air, keep themselves warm and nurse their young with milk, while most fish do none of these things. The confusion is old and understandable, since whales and dolphins look and swim like fish. The real differences are inside the body, not in shape or habitat.
What Makes an Animal a Mammal
Fish and mammals both have a backbone and hinged jaws, which is why they share a broad family tree.1Steven M. Carr. Traditional versus Phylogenetic classification of Vertebrata But mammals descend from the four-limbed land animals, and then from the branch of those that laid eggs with protective membranes.2Steven M. Carr. Traditional versus Phylogenetic classification of Vertebrata Mammals belong to a line marked by a single opening on each side of the skull behind the eye.3Steven M. Carr. Traditional versus Phylogenetic classification of Vertebrata Some fish, the fleshy-finned group that includes lungfish and coelacanths, are more closely related to land animals than to other fish.4Steven M. Carr. Traditional versus Phylogenetic classification of Vertebrata
A salmon is more closely related to a cow than to a shark, because bony fish and mammals share a more recent ancestor than bony fish and sharks do. If you tried to define “fish” as a natural group, you would have to include all four-legged animals or drop the word as a scientific category. Biologists therefore treat it as a convenient everyday label for water-living backboned animals with gills and fins, not as a true branch.
The everyday checklist is simpler. Mammals have hair or fur at some point in life, even whales, which carry a few bristles.5Perspectives on Science. Nature and Taxonomy, Systems of Affiliation Mammal mothers feed their young milk from mammary glands, a feature that gave the group its name.6Perspectives on Science. Nature and Taxonomy, Systems of Affiliation Most mammals give birth to live young, while most fish lay eggs.
Jaws and ears tell the same story. A mammal has a single lower jawbone on each side, whereas fish and reptiles have a lower jaw built from several bones joined together. Three tiny bones in the mammal middle ear carry sound to the inner ear, and they evolved from bones that were once part of the jaw in distant ancestors. That is why fossil hunters can often tell an early mammal from a reptile by its jaw and ear bones, even when no fur or milk glands survive.
Gills Versus Lungs
Fish gills, amphibian skin and the lungs of birds and mammals all do the same job of trading oxygen for carbon dioxide.7The Journal of Experimental Biology. Respiratory gas exchange at lungs, gills and tissues: mechanisms and adjustments Each organ works with a different medium, though: water for gills, air for lungs.8The Journal of Experimental Biology. Respiratory gas exchange at lungs, gills and tissues: mechanisms and adjustments A whale must surface to breathe and can drown, while a fish suffocates out of water.
Water holds far less oxygen than air, so gills have to pass a lot of water over thin filaments. A fish takes water in through its mouth, pushes it over the gills and out through slits or a flap at the sides of its head. The blood flows opposite to the water’s direction, which lets the fish pull out more of the scarce oxygen. Out of water, the delicate filaments stick together and collapse, so the surface for taking in oxygen shrinks even though air has more of it.
Mammal lungs work differently. They fill and empty like bags, so air goes in and out the same way. A dolphin’s blowhole is simply a nostril that moved to the top of its head.
Lungfish are a common mix-up. They can gulp air, but they are still fish: they keep gills, scales and fins. Breathing air alone does not make an animal a mammal.
Warm-Blooded Versus Cold-Blooded
Warm-blooded animals produce and hold heat to stay warmer than their surroundings, which is typical of mammals and birds.9PubMed. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus Cold-blooded animals, such as most fish, let their body temperature follow the water. The energy gap is huge. For the same body size, cold-blooded animals use energy at rest about 24 times slower than warm-blooded ones.10PubMed Central. A broad-scale comparison of aerobic activity levels in vertebrates: endotherms versus ectotherms At top effort, they run about 30 times lower.11PubMed Central. A broad-scale comparison of aerobic activity levels in vertebrates: endotherms versus ectotherms
Mammals make most of their heat inside their cells by burning food, and it builds up fastest in organs like the liver and in working muscles. Shivering is muscles twitching rapidly to make extra heat when a mammal gets cold. Fur, fat and blubber then hold that heat in like insulation.
Warm blood buys a steady body temperature, which keeps muscles and nerves working in cold places. Dolphins hunt in icy water, using blubber to hold their heat. The cost is a large, constant food demand.
Being cold-blooded is not a weakness. Needing less energy means spending less time hunting for food, so more energy can go into growing and reproducing.12PubMed Central. Comparative Physiology of Energy Metabolism: Fishing for Endocrine Signals in the Early Vertebrate Pool A goldfish can eat a pinch of food a day while a mouse of similar weight must eat constantly.
The word “cold-blooded” is also misleading. A fish in a warm pond has warm blood; the word only means its temperature follows the surroundings.
When the water changes, a fish can move to warmer or cooler water to adjust. As water cools, its body processes slow down, so it eats less and moves more slowly. Fish in stable places, like the deep sea or polar water, live at nearly constant temperatures even without making their own heat.
Hearts, Skin and Brains
Fish with jaws have a two-chambered heart, with one receiving chamber and one pumping chamber.13PubMed Central. The vertebrate heart: an evolutionary perspective Mammals, like birds and crocodiles, have a four-chambered heart.14PubMed Central. The vertebrate heart: an evolutionary perspective The mammal heart beats much faster, yet its basic electrical rhythm looks surprisingly similar to a fish’s.15PubMed. Evolution and development of the building plan of the vertebrate heart
A fish heart sends blood to the gills, then on to the body, and back again in one loop. Passing through the gills cuts the pressure, so blood reaches the body at a lower force than a mammal’s does. That is enough for a cold-blooded animal that burns little energy, so the simpler heart is no flaw.
A mammal heart runs two loops, one to the lungs and one to the body. Each trip gets a fresh push, and the wall between the two sides stops oxygen-rich blood mixing with oxygen-poor blood. The body receives blood at high pressure and full oxygen, which supports the warm-blooded way of life.
Skin differs too. Many fish have an outer skin made entirely of living cells, without the tough dead top layer mammals have.16EBSCOhost. Skin (comparative anatomy) Fish skin is thinner and is often covered by slime and scales instead.
Cold-blooded animals, including fish, have brains about ten times smaller than warm-blooded animals of similar size.17CrossRef / Preprint. The Fish Challenge to Vertebrate Cognitive Evolution Yet fish still show many of the same thinking skills.18CrossRef / Preprint. The Fish Challenge to Vertebrate Cognitive Evolution They can learn routes and recognize other fish.19PubMed Central. Comparative Transcriptomics Between Zebrafish and Mammals: A Roadmap for Discovery of Conserved and Unique Signaling Pathways in Physiology and Disease.
Why Whales and Dolphins Look Like Fish
Herrings, dolphins and extinct marine reptiles called ichthyosaurs all evolved the same streamlined body.20Wikipedia. Convergent evolution A body tapered at both ends lets an animal move fast through water, which resists movement strongly.21Wikipedia. Convergent evolution Scientists call this convergent evolution, unrelated animals evolving the same shape.22Wikipedia. Convergent evolution
The tail gives the mammals away. Whale and dolphin tail flukes lie flat and sweep up and down, because the back third of their body bends vertically to drive them.23West Chester University (Plenum Press). Biomechanical Perspective on the Origin of Cetacean Flukes Most fish tails stand upright and sweep side to side. Land mammals run by flexing the spine up and down, and whales inherited that motion from four-legged ancestors. Fish bend side to side, a far older swimming style.
Other clues are on the body. A dolphin has a rounded forehead and a beak, while most fish have mouths set into a pointed head with no bump. Its flippers hide arm bones, with a shoulder, elbow and finger bones inside. The dorsal fin is made of tough tissue without bones, whereas a fish fin is supported by rays or spines.
Fish That Bend the Rules
The opah is the first fish known to warm its whole body, including its heart.24PubMed. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus It produces heat by constantly flapping its wing-like side fins.25PubMed. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus Special blood vessels in its gills keep that heat from escaping into cold water.26PubMed. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus In the gills, warm blood leaving the body passes close to cold blood coming in, handing over its heat. So it stays alert while hunting in deep, cold water.27PubMed. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus Still, it is a fish: it has gills and scales, lays eggs and makes no milk, and it lacks hair and lungs.
Bluefin tuna and great white sharks also warm parts of their bodies.28PubMed Central. Genomic basis of evolutionary adaptation in a warm-blooded fish But they make heat mainly with swimming muscles, unlike mammals.29PubMed Central. Genomic basis of evolutionary adaptation in a warm-blooded fish A web of tiny blood vessels captures warmth before it leaves the body. Their warmth is concentrated in areas such as muscles, brain or eyes, so they are only partly warm-blooded. That allows faster swimming and hunting in cold water.
Why People Once Called Whales Fish
Aristotle noticed that whales and dolphins work more like four-legged animals and set them apart from fish.30Perspectives on Science. Nature and Taxonomy, Systems of Affiliation He watched dolphins breathe air and bear live young, with no microscope or genetics, only careful observation. Yet some 16th-century naturalists grouped dolphins with fish simply because they lived in water.31Perspectives on Science. Nature and Taxonomy, Systems of Affiliation His idea had been lost or ignored, as people returned to the simpler habit of sorting by habitat.
In 1693, John Ray argued whales share almost nothing with fish beyond habitat and shape.32Perspectives on Science. Nature and Taxonomy, Systems of Affiliation He looked inside whales and found a heart, lungs and warm blood like those of land animals. Linnaeus placed whales with mammals in 1758, coining the group’s name.33Shells and Pebbles. The Murky Waters of Classification: Ordering Fish in Eighteenth-century Europe34Perspectives on Science. Nature and Taxonomy, Systems of Affiliation His system put whales beside cows and humans in a group named for the mammary glands, and it gave scientists a shared set of categories. Outside science, whales were still often called fish.35Shells and Pebbles. The Murky Waters of Classification: Ordering Fish in Eighteenth-century Europe
That habit is still alive. Fishing communities from Hong Kong to Brazil count whales and dolphins as “fish.”36PubMed Central. Whales, dolphins or fishes? The ethnotaxonomy of cetaceans in São Sebastião, Brazil Their category means any animal that lives in water, even turtles and crocodiles.37PubMed Central. Whales, dolphins or fishes? The ethnotaxonomy of cetaceans in São Sebastião, Brazil