Are Whales Fish? Why Whales Are Mammals, Not Fish

Why are whales mammals?

Biologists group animals by features inherited from shared ancestors, not by where they live. Whales, from the blue whale to the smallest porpoise, share the key traits of mammals:

• They breathe air. Whales have lungs, not gills, and must come to the surface to breathe through blowholes on top of the head.
• They are warm-blooded. Whales keep a stable body temperature of roughly 35–37 °C (95–99 °F), insulated by a thick layer of fat called blubber.
• They give birth to live young. A blue whale or humpback mother carries her calf for about 10–12 months.
• They nurse their calves with milk. Whale milk is very rich, with about 16–46% fat depending on the species, compared with around 4% in cow’s milk.
• They have hair. Only a little: many whales have a few bristles on the head, especially before birth.
• They have mammal skeletons. Whales have three tiny bones in each middle ear and a single bone on each side of the lower jaw, as all mammals do.

The humpback whale shows several of these traits clearly. The knobs on its head, called tubercles, usually contain at least one fine hair, and a young calf has to surface to breathe every few minutes, staying close beside its mother as it drinks her milk.

How are whales different from fish?

On the outside, whales and fish can look alike, but nearly everything about how their bodies work is different.

The tail is one of the easiest ways to tell them apart. A whale or dolphin swims by moving its tail up and down, just as a running land mammal flexes its back, while a shark or tuna sweeps its tail from side to side. For a closer look at how whales breathe, see How do whales breathe?

There are some overlaps. A few fish, including tuna and some sharks, can keep parts of their bodies warmer than the water, and some sharks give birth to live pups. But no fish breathes air with lungs like a whale, has hair or feeds its young with milk.

Why do whales look like fish?

Water is about 800 times denser than air, so any large, fast swimmer benefits from the same shape: a smooth, torpedo-like body, a powerful tail and fins or flippers for steering. Over millions of years, unrelated animals evolved this shape independently. Biologists call this convergent evolution.

Sharks, tuna, dolphins and the extinct marine reptiles called ichthyosaurs all arrived at a similar streamlined design. The resemblance is only skin-deep. Inside a whale’s flipper are the same bones found in a human arm and hand, and deep in its body are small, leftover hip bones from ancestors that walked on land. The dolphin vs shark comparison shows how a mammal and a fish solve the same problems in different ways.

How did whales evolve from land mammals?

Whales belong to the same large group as even-toed hoofed mammals such as hippos, cattle, pigs and deer. DNA studies show that their closest living relatives are the hippos, as explained in Are hippos related to whales?

Fossils show the move from land to sea step by step. Pakicetus, which lived nearly 50 million years ago in what is now Pakistan, was a wolf-like animal 1–2 m (3–7 ft) long with four legs that lived in and around shallow water. Later forms such as Ambulocetus were more amphibious, and by about 40 million years ago whales such as Basilosaurus lived entirely in the sea, with tiny hind legs that could no longer support them on land.

In 2007, scientists described Indohyus, a small, deer-like hoofed mammal from India with thick bones and ear features found in whales. It showed that the whale family tree grew out of small, water-loving hoofed mammals.

Why were whales once called fish?

For most of history, “fish” simply meant any animal that lived in water. Whales were grouped with fish in many cultures, and several European languages still show it: the Dutch word for whale is walvis, and an older German name is Walfisch, both meaning “whale fish”.

Some thinkers saw the differences early. In the 4th century BCE, the Greek philosopher Aristotle noted that whales and dolphins breathe air, have lungs and nurse their young. Even so, when Carl Linnaeus published the first edition of his classification of living things in 1735, he placed whales among the fishes. In the 10th edition of 1758, he moved them into the mammals, where they have stayed.

The old view hung on outside science. In 1818 a New York jury, in the case of Maurice v. Judd, decided that whale oil counted as fish oil under a state inspection law, in effect treating the whale as a fish. In Herman Melville’s Moby-Dick (1851), the narrator Ishmael insists that a whale is “a spouting fish with a horizontal tail”.

Are whales related to fish at all?

In a distant sense, yes. All four-limbed animals, including whales, birds, frogs and people, descend from fish-like ancestors with lobed fins that began moving onto land more than 360 million years ago. Whales later returned to the water, but they did so as mammals, not by turning back into fish.

That is why scientists say whales are mammals that live like fish, rather than fish that look like mammals. The same logic explains why bats are mammals rather than birds, as shown in Are bats mammals?