Why Whales are Mammals and Not Fish

Whales are more closely related to humans than to fish

Humpback Whale underwater portrait
A humpback whale. Kate Westaway/Stone/Getty Images

Whales live in the ocean, can stay underwater for long periods of time, and have strong tails to propel themselves. So do fish. So, are whales fish?

Despite living in a watery habitat, whales are not fish. Whales are mammals, sharing many characteristics with humans.

Characteristics of Mammals

There are four main characteristics that set mammals apart from fish and other animals. Mammals are endothermic (also called warm-blooded), which means they need to provide their own body heat through their metabolism. Mammals give birth to live young (as opposed to laying eggs) and nurse their young. They breathe oxygen from air and have hair — yes, even whales.

What Distinguishes Whales From Fish?

Whales breathe through their blowholes, which are basically nostrils on the top of their heads, and have to be at the surface of the water to take in air. They use lungs to take in oxygen and dispel carbon dioxide. Fish take in oxygen from the water through their gills and they can't survive long out of the water in the air.

Per the mammal checklist above, whales give birth to live young, while fish lay eggs. While some fish, such as guppies, incubate their eggs in a pouch and appear to give birth, they do not do so in the same way as mammals.

Mammals are named after their mammary glands, which produce milk to nurse their young. Like other mammals, whales nurse their calves. Fish do not have mammary glands or nurse their young. And where fish have scales, whales have smooth skin.

Whales have blubber, a layer of fat that helps to keep them warm, generating their own body heat as other mammals do. Whales also generate heat by swimming and by digesting food. They maintain their body temperature in a small range, while fish are subject to the temperature of the water around them. Whales have to eat much more than fish do in order to keep warm.

Whales aren't furry like many mammals, but they have hair follicles at some point in their development. Many of them lose their hair before they are born, while others still have some hair on the top of their heads. Fish do not have hair at any time during their lives.

Whales arch their backs and move their tail flukes up and down to propel themselves through the water. Fish move their tails from side-to-side to swim.

Evolution of Whales and Fish

Although they both live in water, whales and fish evolved differently. The ancestors of whales lived on land, as we can tell from their bone structure. The bones in their fins show individual digits that their ancestors may have used to walk and grasp. The movement of their backbone is more like you see with a land animal running rather than the swimming motion of fish.

The ancestors of fish are ancient fish, who also lived in the water rather than on land. While some ancient fish developed into land animals whose descendants returned to the water as whales, this makes whales only very distant relatives to fish.