Why Whales Are Mammals and Not Fish

Whales are more closely related to humans than to fish

Humpback Whale swimming underwater
Kate Westaway/Stone/Getty Images

Whales are a member of the cetacean family, and as such, despite being wholly water-resident, whales are mammals, not fish. There are only 83 species of cetaceans in the world organized into 14 families and two main subcategories: 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. Instead of teeth, Mysticeti have a shelf of bony material called baleen that filters tiny prey like zooplankton out of ocean water. All cetaceans, toothed or baleen, are mammals.

Key Takeaways: Why Whales Are Mammals

  • Whales are cetaceans and fall into two categories: baleen (that eat plankton) and toothed (that eat penguins and fish).
  • Mammals 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. 
  • Whales share a common ancestor with hippopotamuses.

Whale Characteristics

Whales and their cetacean relatives 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 up to 80 ft (24 m) in length. 

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 sub-cylindrical except for their tails, which are flattened at the end. 

What Are 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.

Cetaceans vs. Fish

Prince of Whales whale watching, Vancouver Island, BC
Prince of Whales Whale Watching Tours

To understand what makes a whale a mammal, compare it to an ocean-inhabiting fish of the same general size: a shark. The main differences between cetaceans like whales and fish such as sharks are:

Cetaceans breathe oxygen. Whales have lungs, and they breathe through blowholes in their skulls, choosing when to come to the surface to breathe. Some species like sperm whales can stay underwater for as long as 90 minutes, although most average about 20 minutes between breaths.

In contrast, sharks extract oxygen directly from the water using gills, specially built feathery slit structures located on the sides of their heads. Fish never need to come to the surface to breathe.

Cetaceans 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 they generate heat by swimming and digesting food. That means the same species of whale can thrive in a wide variety of environments from polar to tropical oceans, and many migrate back and forth during the year. Every year, whales travel alone or in groups called pods, moving long distances between their cold-water feeding grounds to their warm-water breeding grounds.

Sharks are cold-blooded and cannot regulate their body temperature, so they must stay in whatever environmental zone they evolved in, generally temperate or tropical waters. There are some cold water sharks, but they have to stay in the cold to survive. 

Cetacean offspring are born live. Whale babies (called calves) take about 9–15 months to gestate, and are born from the mother one at a time. 

Depending on their species, mother sharks lay up to about 100 eggs in egg cases hidden in seaweeds, or they keep eggs within their bodies (in ovipositors) until they hatch.

Cetacean offspring are tended by the mothers. Female whales have mammary glands which produce milk, allowing the mother to feed her calves for a whole year, during which time she teaches them where the breeding and feeding grounds are located and how to protect themselves from predators.

After newborn shark eggs are deposited, or the babies (called pups) hatch from the mother's ovipositor, they are on their own and must break out of the egg case and forage and learn to survive without help.

Cetaceans have vestigial hair. Many of the species 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. 

Cetacean skeletons are built of bone, a strong, relatively inflexible material that is kept healthy by blood flowing through it. Bony skeletons are good protection from predators.

Sharks and other fish skeletons are primarily made of cartilage, a thin, flexible, light, and buoyant material that evolved from bone. Cartilage is resistant to compression forces and gives the shark the speed and agility to hunt effectively: Sharks are better predators because of their cartilaginous skeletons.

Cetaceans swim differently. Whales arch their backs and move their tail flukes up and down to propel themselves through the water.

Sharks propel themselves through the water by moving their tails from side-to-side.

Evolution of Whales as Mammals

Model of Eocene Ancestor of Whales, Indohyus
Model of Eocene Ancestor of Whales, Indohyus. Museo di Storia Naturale di Calci - Pisa. Ghedoghedo

Whales are mammals because they evolved from a four-legged, strictly terrestrial mammal known as a pakicetid beginning in the Eocene, about 50 million years ago. During the Eocene, different forms used various methods of locomotion and feeding. These animals are known as archaeocetes, and fossil archaeocetes' body forms document the transition from land to water. 

Six intermediate whale species in the archaeocetes group include semi-aquatic ambulocetids, which lived in the bays and estuaries of the Tethys Ocean in what is today Pakistan, and the remingtonocetids, which lived in shallow marine deposits in India and Pakistan. The next evolutionary step was the protocetids, the remains of which are found throughout South Asia, Africa, and North America. They were primarily aquatic-based but still retained hind limbs. By the late Eocene, dorudontids and basilosaurids were 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 Africa
Kraig Becker

For well over a century, scientists debated whether hippopotamuses and whales were related: The relationship between cetaceans and land-based ungulates was first proposed in 1883. Before the breakthroughs in molecular science of the late 20th and early 21st centuries, scientists relied on morphology to understand evolution, and the differences between land-living hoofed animals and marine cetaceans made it difficult to believe how these two animals could be closely related.

However, the molecular evidence is overwhelming, and scholars today agree that hippopotamids are a modern sister group to cetaceans. Their common ancestor lived at the beginning of the Eocene, and probably looked something like Indohyus, basically a small, stocky artiodactyl about the size of a raccoon, the fossils of which have been found in what is today Pakistan.

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