Why Whales Are Mammals and Not Fish

Whales are more closely related to humans than to fish

Humpback Whale swimming underwater
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Whales are a member of the cetacean family. Despite being wholly water-resident, whales are mammals, not fish. There are only 83 species of cetaceans in the world today, in 14 families, and among those, there are two main subcategories of cetaceans, toothed whales (Odontoceti, including killer whales, narwhals, dolphins, and porpoises) and baleen whales (Mysticeti, humpback whales, and rorquals). Toothed cetaceans have teeth and eat penguins, fish, and seals. Baleen whales have a baleen, a shelf of bony material that is for filtering ocean water to capture the huge amounts of plankton they survive on. 

Key Takeaways: Why Whales Are Mammals

  • Whales are cetaceans and fall into two categories: baleen (who eat plankton) and toothed (who eat penguins and fish).
  • They are mammals, who breathe air using lungs, bear live young and feed them using mammary glands, and regulate their own body temperature. 
  • They evolved from a four-legged terrestrial during the Eocene, 34-50 million years ago. 
  • They share a common ancestor with hippopotamuses.

Whale Characteristics

Cetaceans range enormously in size. The smallest cetacean is the Vaquita, a small porpoise living in the Gulf of California, about 5 ft (1.4 m) long and weighing less than 88 pounds (40 kilograms). It is close to extinction. The largest is the Blue Whale, in fact, the largest animal in the ocean, which can grow to more than 420,000 lbs (190,000 kg) and with lengths up to 80 ft (24 m). 

Cetacean bodies are streamlined and fusiform (tapering at both ends). They have small lateral eyes, no external ears, laterally flattened forelimbs lacking a flexible elbow and an indistinct neck. Whale bodies are subcylindrical except for their tails, which are flattened at the end. 

Why Is the Whale a Mammal?

There are four main characteristics of mammals: they are warm-blooded, they give birth to live young and nurse them for the first year, they breathe oxygen, and they have hair. 

Mammals are endothermic, meaning that they are warm-blooded and are able to regulate their own body temperature internally. Whales have blubber, a layer of fat that helps to keep them warm, and generates body heat as other mammals do. Whales also generate heat by swimming and by digesting food. That means whales can thrive in polar to tropic oceans, and many migrate back and forth during the year. 

Fish, like sharks, say, are cold-blooded and cannot regulate their body temperature, so they must stay in temperate or tropical waters. There are some cold water sharks, but they have to stay in the cold. 

What Distinguishes Whales From Fish?

Prince of Whales whale watching, Vancouver Island, BC
Prince of Whales Whale Watching Tours
  • Mammals breathe oxygen. Whales have lungs, and they breathe through blowholes in their skulls. Unlike humans, whales decide when to breathe; they come to the surface to do that. Fish like sharks absorb oxygen from water using gills, specially built feathery slit structures on the sides of their heads. 
  • Whale offspring are born live. Whale babies take about 9-15 months to gestate, are born one at a time and are cared for by their mothers for a year. Sharks lay eggs, or keep eggs within their bodies until they hatch. After the eggs are deposited, or the babies hatch, they are on their own. 
  • Whale offspring are fed by the mothers. Female whales have mammary glands, which produce milk and feed the whale offspring. Sharks don't do that—they're fish. 
  • 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 or near their mouths. Fish do not have hair at any time during their lives. 
  • Whales swim differently. The 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

Whales evolved in what is today South Asia from a four-legged strictly terrestrial animal known as a pakicetid beginning in the Eocene, about 50 million years ago. During that time, they experimented with locomotion and feeding. These animals are known as archaeocetes, and they document the transition from land to water. 

Six intermediate whale species in the archaeocetes group include semi-aquatic ambulocetids, who lived in the bays and estuaries of the Tethys Ocean in Pakistan, and the remingtonocetids, who lived shallow marine deposits in India and Pakistan. The next evolutionary step was the protocetids, who can be found throughout South Asia, Africa, and North America and was primarily aquatic based, but still retained hind limbs. The late Eocene saw dorudontids and basilosaurids swimming in open marine environments and had lost nearly all vestiges of land life. 

By the end of the Eocene, 34 million years ago, body forms for whales had evolved to their modern shape and size. 

Are Whales Related to Hippos? 

hippos in the Okavango Delta
John (Wikimedia Commons)

Scientists debated for a long time about whether hippopotamuses and whales were related. Basing that decision on morphology alone, the scientists fell into two camps: yes, and no. However, molecular evidence is overwhelming, that hippopotamids are a modern sister group to the cetaceans. Their common ancestor probably looked a small, stocky mousedeer.