Why Whales are Mammals and Not Fish

Humpback Whale underwater portrait
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, just like you and me.

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.They also give birth to live young and nurse their young, breathe oxygen from air, and have hair (yes, even whales do!).

What Distinguishes Whales From Fish?

If you're still not convinced, here are some specific ways that whales differ from fish.

  • Breathing: 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.
  • Birthing: Whales give birth to live young. 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.
  • Nursing: 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.
  • Skin: Whales have smooth skin, while fish have scales.
  • Body heat: Whales are warm-blooded (endothermic), while fish are cold-blooded (ectothermic). Whales have blubber, a layer of fat, that helps to keep them warm, and they also generate heat by swimming and by digesting food. Whales 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.
  • Hair: 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 head. Fish do not have hair at any time during their life.
  • Swimming: Whales arch their back and move their tail flukes up-and-down to propel themselves through the water. Fish move their tails from side-to-side to swim. This causes the fish's whole body to undulate.

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 descendents returned to the water as whales, this makes whales only very distant relatives to fish.

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Kennedy, Jennifer. "Why Whales are Mammals and Not Fish." ThoughtCo, Jun. 13, 2018, thoughtco.com/are-whales-fish-4082399. Kennedy, Jennifer. (2018, June 13). Why Whales are Mammals and Not Fish. Retrieved from https://www.thoughtco.com/are-whales-fish-4082399 Kennedy, Jennifer. "Why Whales are Mammals and Not Fish." ThoughtCo. https://www.thoughtco.com/are-whales-fish-4082399 (accessed June 19, 2018).