Is a Fish a Mammal?

When you think about a fish what comes to your mind? 

A sea creature that is caught with a net or a bait on a hook, and tastes delicious when cooked?

Most people don’t give it much of a thought other than when it lies on their plate. 

To take it even further when you think about the characteristics of a fish, what are the things you can still remember from elementary class, and what will your answer be when asked, “Is a fish a mammal?”

Before you answer that question, let’s define a mammal.

Is a Fish a Mammal
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What is a mammal?

The word Mammal sounds similar to a gland that’s responsible for producing milk: The mammary gland.

The easiest way to define mammals is by classifying them as animals that nourish their young ones with milk produced in their mammary glands.

Mammals are warm-blooded vertebrate animals. 

Vertebrates are animals with backbones. These backbones form a core of the skeletal framework of mammals.

However, that’s not all that defines a mammal. 

Characteristics of mammals

  • The females have special mammary glands that produce milk to nourish their babies. These special glands are also called breasts.

This is the most defining characteristic of a mammal.

  • They have hair or fur on their skin. The hair or fur serve many purposes, one of which is temperature regulation. Ever wondered why your hair stands when you are cold and relaxes when you’re sweating?

Your hairs help to regulate temperature.

Humans have hair so we’re mammals. Dogs have fur.

  • Mammals give birth to their young ones alive. Their offspring are born with their body intact, not as a liquid in shells on Earth surfaces that undergo biological processes. 

Mammals don’t lay eggs. 

Now, let’s talk about fish

What is a fish?

According to Oxford Advanced Learner’s English dictionary, a fish is “a creature that lives in water, breathes through gills, and uses fins and a tail for swimming”. 

The points to note are that a fish lives in water, breathes through gills and uses fins and a tail to swim.

A fish is a limbless cold-blooded vertebrate animal with gills and fins living wholly in water. A fish is classified as Pisces.

One peculiar feature of a fish compared to other vertebrate animals is that it does not have limbs with digits such as toes and fingers. The fish can’t pick things with fingers or scratch itself because it has no limbs. It only possesses fins for swimming.

A fish’s skin could be scaly and sometimes slimy which makes it especially adapted to living in the water. It makes it easy to move in water at an increased speed.

You can’t expect a fish (pisces) to survive out of water for more than an hour, just as you can’t expect a human (mammalia) to survive in water for that same amount of time without breathing equipment.

Common differences between fish and mammals are tabulated below

1 They lay eggs They give birth to live babies 
2 They are cold-blooded They are warm-blooded
3 The breath through gills They break through the lungs
4 They have scales They have hair or fur
5 No mammary glands They have mammary glands

Why Do People Think All Fishes Are Mammals? 

While it is an established fact that fish are not mammals, a lot of people think they are mammals. Perhaps, you have also thought fish are mammals. 

You might be wondering why this is so, why does this common mistake occur?

One of the reasons is that many fish are devoted to caring for their young ones in a way that is usually associated with mammals. 

Now that we’ve separated the characteristics of a fish from that of a mammal, let’s answer the pertinent question.

Is a fish a mammal?

A fish is not a mammal. 

However, some sea creatures are mammals. 

These creatures give birth to their young ones alive and produce milk to nurse their young ones. 


These creatures are the reason there’s so much confusion and common misconceptions that all fishes in the ocean, river, lake and pond are mammals.

They are

  • Dolphins 
  • Whales
  • Seals
  • Sea lions
  • Sea otters
  • Porpoises
  • Fur seals
  • Dugongs
  • Manatees
  • Walruses

Why are Whales classified as mammals and not fish? 

Whales are mammals because of the following reasons: 

  • They are warm-blooded.
  • They feed their babies with milk.
  • They breathe in air through their lungs just like humans.
  • They give birth to live young ones.

How are fishes and mammals alike? 

Despite being vastly different, some similarities exist between fish and mammals.

  • They are both vertebrates.
  • They both have a skeletal system.
  • They both require oxygen to survive. Fish have gills that make it possible to get oxygen from water. Mammals have lungs.
  • They both carry out the characteristics of all living things, breath, eat, reproduce, react to stimuli, and die.

Some fun facts about Fish 

  • Did you know that the plural of fish is fish, it is the most common plural form of fish, especially when all the fish being referred to are of the same species. It is only in a few cases that features is used as a plural of fish. 
  • It might interest you to know that fishes have feelings and can express both negative and positive emotions.
  • Fish are social animals. They form beneficial relationships with the same species as well as different species. There are some fish called “cleaners”. They eat parasites off larger fish, which serve as food for them while reducing the parasitic load on the larger fish. 

Fish also hunt in groups and give visual signals about the location of prey to Others to facilitate hunting.

  • Fish have an excellent memory. A cleaner fish can remember as much as 100 different larger fish it cleans while larger fish can also recognize their specific Cleaners. 

Fish can also remember humans who give them food rewards.

  • Fish are smart! Fish will actively seek out rewarding and pleasurable experiences and in the same vein, avoid painful or harmful experiences. They can also learn how to use tools in their day to day life. For instance, many fish use rocks to crack open oysters, clams and other bivalves to get food.

More fish query you may like

Is an Octopus a fish (A Mysterious Creature)?

Is Tilapia A Real Fish?

What Is a Group of Jellyfish Called?

What Kind Of Fish Is Dory?


Fish are an incredible class of animals with amazing characteristics. One only needs to dive into the ocean to see the different colourful species that exist.

However,  a fish is not a mammal. 

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