Are Sharks Mammals?

Sharks have ruled the ocean waters for over 400 million years. There are more than 500 species of these fierce predators living today. Some people think sharks are mammals, but they are not. Sharks are members of the fish family. Are you wondering why? Continue reading to learn more about sharks and find out why they don’t fit the mammal criteria.

What Is a Mammal?

The class Mammalia includes over 5,000 species. All mammals are warm-blooded vertebrates whose offspring drink milk from their mother’s mammary glands. Mammals are viviparous, which means they give birth to live young. The exceptions to this rule are monotremes like the platypus and the echidna, which lay eggs.

Mammals also have hair or fur at some point in their lives. This includes whales, which are born with sensory hairs on their heads and jaws. Unlike other vertebrates, the lower jaw of a mammal is directly connected to the skull. Additionally, mammals have three special bones, the malleus, incus, and stapes, that transmit sound waves from the outer to the inner ear. Mammals have a few other unique features, but we can count sharks out on these alone.

Sharks Are Not Mammals

So, as you can see, sharks don’t fit the criteria for mammals in the most important respects. They don’t produce milk, they don’t have hair, and most of them are cold-blooded. The only major characteristic they share with mammals is that the majority of shark species give birth to live young, although others lay eggs.

Mako sharks, great whites, hammerheads, tiger sharks, and blue sharks are just a few of the shark species that give birth to live pups. Before birth, baby sharks rely on a yolk sac for nutrition. However, some species develop a connection to the mother’s placenta that is similar to the umbilical cord in mammals. The pups even have a scar like a belly button in between their pectoral fins.

Why Do People Think Sharks Are Mammals?

Because sharks have live babies, some people think this automatically makes them mammals. Sharks are also sometimes confused with sea mammals. The confusion is understandable because sharks and marine mammals share some characteristics. Dolphins, porpoises, killer whales, manatees, and dugongs spend their entire lives underwater, just like sharks. However, these marine mammals must surface to breathe occasionally.

A shark is a fish, so a shark doesn’t need lungs to breathe. Rather, it breathes by taking oxygen in through its gills. Instead of a skeleton, the shark has tough, flexible cartilage that is less dense than bone. In fact, this characteristic makes sharks special, classifying them as a type of fish called an elasmobranch. Other fish that belong in this category include rays, skates, and sawfish.

Most sharks are also like most fish in that they are ectothermic. This means their body temperature is dependent on the temperature of the water. Marine mammals are endothermic, so their body temperature is consistent and generated internally. So although sharks share some characteristics with mammals, they are much closer to most fish species.