3 Main Types of Mammals, Characteristics, an Easy Guide

For the types of mammals, the number can be confusing for some. To be clear, there are three main types of mammals, but they have five main characteristics that set them apart from other animals.

So in this article, we’ll get into some detail around the types, and their main characteristics. Starting with a quick summary answer to start us off.

There are three types of mammals: Monotremes (egg-laying mammals), Marsupials (pouched mammals), and Placentals (placental mammals). Among them, the placental mammals are the largest type of mammals to exist. Mammals’ types are divided based on how they give birth and how their babies develop.

In summary, mammals within the animal kingdom, are a group of animals that have a backbone (vertebrates), hair, or fur, are warm-blooded (endothermic), have four-chambered hearts, with a specific jaw structure, and also feed their young ones with milk.

Read on, or watch the video below from our Ranger Planet YouTube Channel.

Mammals on average have about 6,000 different species in their group or class. Mammals are members who belong to the group of animals called Mammalia.

All members of this group share certain physical characteristics as outlined above, and are descended from the same ancestors.

Types of mammals – How mammals are divided

Mammals are divided mainly based on how their babies develop and how they give birth.

The three types of mammals are:

  • Monotremes (egg-laying mammals)
  • Marsupials (pouched mammals)
  • Placentals (placental mammals).

Among these three types of mammals, the placental mammals are by far the largest type of mammals that exist in the world.

Here’s a breakdown of these three types of mammals and all you need to know about each type. 

Monotreme mammals

Mammals that lay eggs fall into the type of monotremes and are therefore members of the mammalian order Monotremes.

Monotremes are different from other types of mammals.

They are the only types of mammals that don’t give birth to developed young and instead lay eggs. 

These mammals have a body temperature that is lower than most warm-blooded animals. This feature makes them more common with reptiles.

The name for these types of mammals comes from one of their features, which is having a single opening to both lay eggs and to pass their waste through. 

Anteaters, platypus, and four species of echidna are the only monotreme mammals to be found. They are located across Australia, Tasmania, and New Guinea. 

Although they don’t give birth to young, the characteristics that make them mammals include nursing their young with milk, having hair, and the possession of three bones in the middle ear.

It’s believed that monotreme mammals were genetically left behind by the branch of mammals that went on to become either marsupials or placental mammals.

Example: Echidna

In the case of Echidnas, similar to porcupines, they have spines scattered all over their body, more like a spiky ball.

The female anteater, on the other hand, lays a leathery shell directly into the pouch of her belly.

The egg will start to hatch at around 10 or 11 days. The newborn baby will be very tiny, measuring only the size of a dime.

The newborns will feed through the mother’s milk, which will seep through the pores on the mother’s skin.

At around 6-8 weeks after birth, the newborn will develop small spines. These spines will irritate the mother, after which she will let the baby out of her pouch. 

types of mammals - monotremes - echidna
types of mammals – monotremes – echidna

Placental mammals

This type of mammal is a member of the infraclass Placental and to the subclass Eutheria.

Of the three types of mammals, this class makes up a more significant portion of the animal kingdom. Placental mammals give birth to babies when young. 

These mammals are classified mainly by the presence of an organ called the placenta

This is an organ that develops during gestation, which in turn facilitates the mother to exchange nutrients and wastes between the blood of her own body and the fetus. 

Any mammal that doesn’t fall into the monotremes and marsupial types are placental mammals.

Some classifications and authorities consider the marsupial mammals as placental mammals as well.

But in fact, the placental type of mammals has a much more developed and more efficient type of placenta that allows for a longer development period within the protection of the womb.

This well-developed placenta is a key factor that has contributed to the evolutionary success of this type of mammal as well.

Some examples of placental mammals include tigers, blue whales, vampire bats, and of course, humans.

Example: Blue whale

The title for the largest mammals also belongs to an animal from this type of mammal, the blue whale.

Blue whales can grow 30 meters in length and weigh around 400,000 pounds. 

During the time the offspring stay in the womb of the mother, they grow into a well-formed structure for a long time, until the mother is ready to give birth.

The time they stay in the womb or when the mother will give birth to the babies differs between the size and shape of mammals.

And as mentioned, during the period the offspring stay in the womb, they receive nourishment for growth and to develop fully through the mother’s diet, and more specifically via the umbilical cord.

The umbilical cord specifically is located inside the placenta. This structure of birth is what gives this type of mammal its unique identity. 

types of mammals - placental - blue whale
types of mammals – placental – blue whale

Marsupial mammals

Marsupial mammals are types of mammals that give birth to babies that have not completely developed.

They are members of the infraclass Marsupial. The babies they give birth to are very tiny and are called “joeys”. 

Once born the newborns crawl and get into a pouch on the outside of the mother’s abdomen. The development stage of these newborns is completed in the pouch.

During this time, the babies will feed on the mother’s milk until they are fully grown/developed.

Most marsupial mammals are found in Australia with some in South & North America and New Guinea.

Koalas, kangaroos, wallabies, Tasmanian devil, and opossums are some well-known marsupial mammals.

Example: Opossum

If you take the opossums specifically, this type of marsupial mammal will give birth to as many as twenty or so babies at once.

But because the mother has only thirteen nipples in her pouch, the first thirteen babies who manage to climb up to the mother’s pouch and get to the mother’s nipples are the only ones who survive.

As the rest are still in an under-developed stage and will not have access to milk to nourish and grow, they will unfortunately perish.

types of mammals - marsupials - kangaroo
types of mammals – marsupials – kangaroo

Type of Mammals – more wildlife help

I hope this has provided you with a good overview answer and an easy guide to the three main types of mammals. Be sure to check out other interesting articles on ranger planet.


A Certified Ecologist and an Entomologist, Michael has been interested in all aspects of Nature for many years. It's only now he's decided, along with his partner Fran, to begin documenting what he knows.

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