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Mammals are a class of homeothermic amniotic vertebrate animals that have milk-producing mammary glands with which they feed their young. Most are viviparous.

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The official mammal class is Mammalia. Types of animals that are mammals include warm-blooded vertebrates that have hair or fur and whose babies drink milk. Unlike other animal types like birds and insects, all mammal babies drink milk that comes from their mother’s bodies. This is one of the key ways to know if an animal is a mammal. 

What Are Mammals? – The 10 Characteristics You Need to Know

Mammals have many other unique characteristics compared to other animal types. You can use some of these key mammal markers to identify whether an animal is a mammal:

  1. Warm-blooded vertebrates – Mammals are endothermic, meaning they have a constant body temperature. Mammals are also vertebrates, meaning they have a backbone
  2. Red Blood Cells Without Nucleus – Mammals have adult (mature) red blood cells, known as erythrocytes, that lack a nucleus. This is unique compared to other vertebrates. 
  3. Has hair or fur – Mammals commonly have hair, or fur. Though hair and fur are chemically similar and both made of keratin, they are characteristically different. Hair is normally longer and thinner than fur, while fur is usually shorter and coarser. Both serve as insulators for animals. 
  4. Contains a Malleus, Incus, and stapes – The Malleus is a small bone in the middle ear that is shaped like a hammer. It connects the inner surface of the eardrum with the incus. Given it’s shape, the name is derived from the Latin word for ‘mallet’. The incus is shaped like an anvil and serves a similar function, vibrating to transmit sound. The stapes is a stirrup shaped bone. Together they form the ‘hammer, anvil, and stirrup’, three of the smallest bones in the human body. The stapes is the actual smallest. 
  5. Able to produce milk – Mammals contain mammary glands. These glands give them the ability to produce milk to feed their offspring.
  6. A diaphragm – Mammals have a diaphragm sitting between their heart and lungs 
  7. Hinged Lower Jaw Connected To Skull – Mammals have a hinged lower jaw that connects directly to the skull. By comparison, all over vertebrates connect through another bone known as the quadrate. This mammal articulation allows them to more easily grasp and chew food, as well as chew sideways. 
  8. Left aortic arch – In mammals, the fourth (left) pharyngeal artery is most commonly the arch of the aorta. This differs compared to other animal types. There can be rare cases where mammals have a right aortic arch variant. 
  9. Neocortex – Mammals commonly have a neocortex in their brain. This specialized region allows them to both hear and see.
  10. Live Birth – Mammals are viviparous, meaning they give birth to live babies. There are three different types of mammal birth. Monotremes, marsupial, and placental mammals, discussed further below. 
Mammal Exceptions

There are some exceptions to the mammal criteria. Whales are mammals but they are born without hair or fur as it has typically gone after the fetal stage. Yes, that means the blue whale is in fact a mammal!

Monotremes like the duck-billed platypus and echidnas are a whole category of egg-laying mammals that are the only mammals which do NOT give birth to live young. Instead, animals like the platypus and echidnas actually lay eggs.

The Three Different Types of Mammal Births


A rare and unusual category of mammal birth where the animal lays eggs. There are only five known monotremes mammals on the planet, with the platypus and echidna’s being the most well known.


Marsupial mammals give birth to their young before they are fully developed. The babies then complete their growth outside of the mother but in a ‘pouch’. Well-known marsupial examples include kangaroos, opossums, and wombats. 

Placental Mammals

Members of this birth group have a birth procedure containing a placenta. The presence of a placenta transfers important nutrients between the mother and offspring, helping protect the young and ensure a well-developed birth. Placental births are the most common among all mammals with examples including dogs, horses, cats, and humans. 

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