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Fish vs. Mammals: What's the Difference?

By Aimie Carlson & Harlon Moss || Published on May 27, 2024
Fish are aquatic vertebrates with gills and scales, primarily breathing underwater, while mammals are warm-blooded vertebrates, usually with hair or fur, and most give birth to live young. Mammals nurse their offspring with milk, unlike fish.

Key Differences

Fish inhabit water environments, relying on gills to extract oxygen from water. Their bodies are adapted to aquatic life, with fins for movement and scales for protection. Mammals, in contrast, breathe air through lungs and are adapted to live in a variety of environments, from aquatic to terrestrial. While some mammals, like whales and dolphins, live in water, they must surface to breathe air.
Reproduction among fish can vary, with many species laying eggs (oviparous), but some are live-bearers (viviparous). Mammals predominantly give birth to live young (viviparous) and have a unique reproductive system characterized by the presence of mammary glands, which produce milk to nourish their offspring. This nursing behavior is a defining characteristic of mammals.
Fish are cold-blooded, meaning their body temperature varies with the environment. Mammals are warm-blooded, maintaining a constant body temperature regardless of the environment. This distinction allows mammals to inhabit a wider range of climates and activities across different temperatures.
Fish have a diverse range of sizes, shapes, and habitats, with over 32,000 species identified. Mammals, while less numerous with around 6,400 species, exhibit significant diversity in size, habitat, and behavior, from the tiny bumblebee bat to the massive blue whale.
The sensory systems of fish and mammals are adapted to their environments. Fish have lateral lines that detect vibrations and movement in water, aiding in navigation and predator avoidance. Mammals rely more on their developed senses of sight, hearing, and smell, which vary widely among species but are generally more advanced than those of fish.

Comparison Chart


Aquatic, breathe underwater through gills.
Various, breathe air through lungs.

Body Temperature

Cold-blooded (ectothermic).
Warm-blooded (endothermic).


Mostly egg-laying, some live-bearers.
Mostly live-bearers, nurse offspring with milk.

Skin Covering

Hair or fur, some have blubber.



Species Diversity

Over 32,000 species.
About 6,400 species.

Sensory Systems

Lateral lines for detecting vibrations.
Advanced sight, hearing, and smell.


Various, including legs, wings, and flippers.

Temperature Regulation

Constant internal regulation.

Nourishment of Young

Eggs or external care.
Mammary glands produce milk.

Fish and Mammals Definitions


Cold-blooded and lay eggs.
Salmon migrate upstream to lay eggs.


Give birth to live young and nurse them.
Kangaroos carry and nurse their young in a pouch.


Aquatic vertebrates with gills and fins.
Sharks are fish known for their predatory skills.


Warm-blooded vertebrates with hair or fur.
Tigers are mammals with distinctive striped fur.


Exhibit a wide diversity of species.
Clownfish are recognized for their vibrant colors and symbiosis with sea anemones.


Breathe air through lungs.
Elephants have large lungs to support their massive size.


Breathe underwater through gills.
Guppies extract oxygen from water using their gills.


Have developed sensory and nervous systems.
Bats use echolocation for navigation and hunting.


Have scales covering their bodies.
Goldfish have shiny scales that reflect light.


Adapted to diverse environments.
Polar bears are adapted to live in Arctic climates.


Any of numerous cold-blooded aquatic vertebrates characteristically having fins, gills, and a streamlined body and including the bony fishes, such as catfishes and tunas, and the cartilaginous fishes, such as sharks and rays.


Any of various warm-blooded vertebrate animals of the class Mammalia, including humans, characterized by a covering of hair on the skin and, in the female, milk-producing mammary glands for nourishing the young.


Plural of mammal


Can fish live outside water?

Fish cannot survive outside water for extended periods, as they require water to breathe through their gills.

How do fish regulate their body temperature?

Fish are cold-blooded and rely on the temperature of their surrounding environment to regulate their body temperature.

What is a unique feature of mammals?

A unique feature of mammals is their mammary glands, which produce milk to nurse their young.

Why are mammals more adaptable to different environments than fish?

Mammals are more adaptable due to their ability to maintain a constant body temperature and their advanced sensory systems.

Are dolphins fish or mammals?

Dolphins are mammals; they breathe air, give birth to live young, and nurse their offspring with milk.

How do fish and mammals differ in their sensory capabilities?

Fish have a lateral line system for detecting water vibrations, while mammals have more developed senses like sight and hearing.

What are the main differences in the limb structures of fish and mammals?

Fish have fins designed for propulsion and maneuvering in water, while mammals have limbs (legs, arms, wings, or flippers) adapted for movement in their specific environments, whether terrestrial, aerial, or aquatic.

What makes a mammal different from a fish in terms of reproduction?

Mammals give birth to live young and nurse them with milk, while most fish lay eggs.

How do mammals contribute to their ecosystems?

Mammals contribute by pollinating plants, dispersing seeds, controlling insect populations, and serving as key predators and prey in their ecosystems.

Can all mammals live in water?

Not all mammals can live in water, but some, like whales and dolphins, are fully adapted to aquatic life.

What is the significance of scales in fish?

Scales protect fish from injuries, parasites, and infection, and play a role in their movement through water.

What adaptations do aquatic mammals have for underwater life?

Aquatic mammals, like whales and seals, have adaptations such as streamlined bodies, blubber for insulation, and modified limbs like flippers for swimming.

What evolutionary paths did fish and mammals take?

Fish are ancient vertebrates that gave rise to the first tetrapods, leading to mammals' evolution. Mammals evolved from synapsid ancestors, diversifying significantly after the dinosaurs' extinction.

Why do some mammals return to the water, like whales and seals?

Some mammals returned to water to exploit aquatic niches, adapting to feeding, breeding, and living in the ocean, which led to evolutionary advantages such as access to abundant food sources.

How do fish reproduce?

Fish reproduction varies; most lay eggs (oviparous), but some give birth to live young (viviparous), and others (ovoviviparous) retain eggs inside their body until hatching.

How do fish and mammals contribute to the balance of their ecosystems?

Fish play roles in aquatic food webs, affecting algae populations and serving as prey. Mammals help in seed dispersal, pollination, and maintaining the balance through predator-prey relationships.

Can mammals and fish coexist in the same habitats?

Yes, mammals and fish often coexist in aquatic ecosystems, with mammals like otters, seals, and whales sharing habitats with fish, each occupying different ecological niches.

Why do mammals need to breathe air but fish do not?

Mammals have lungs that extract oxygen from air, a necessity for their high-energy, warm-blooded metabolism, whereas fish extract oxygen from water through gills, suited to their aquatic environment and cold-blooded nature.

How do the brains of fish and mammals differ?

Mammal brains are generally more complex, especially in areas related to intelligence, memory, and social behaviors, reflecting their more complex behaviors and social structures compared to fish.

What role do teeth play in the diet of fish and mammals?

Teeth vary widely among fish and mammals, adapted to their diets; fish may have sharp teeth for catching prey or flat ones for grinding, while mammals have teeth adapted for diets ranging from carnivorous to herbivorous and omnivorous feeding strategies.
About Author
Written by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
Co-written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.

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