What Are The Three Types Of Mammals?

There are three different types of mammals, and I wanted to find out what these three groups were.

The first group is placental mammals.  These are mammals that have placentas.  The second are marsupials, which have a pouch that is used to nourish their young.  The third are monotremes which lay eggs.

I wanted to find out if all three of these live in North America.  If you want to know then please read on.

Phylogenetic chart of mammals

Placental Mammals

The placental mammals give birth to live young. They are the most diverse group of mammals and consist of over 4,000 known species. There are several different forms of placental mammals. These include whales, bats, elephants, tigers, and humans.

They are referred to as placental mammals because they have a true placenta. This is a significant and dominant characteristic of this group of mammals. 

The fetus stays in the womb until it reaches full development and is ready for birth. During this time, the fetus receives nourishment through the placenta. This organ facilitates the exchange of oxygen, nutrients, and waste products between the fetus blood and the mother’s blood.

Marsupial and placental mammals are quite different, although some people consider marsupials to be similar to placentals. The marsupial may have a placenta, but it is short-lived and does not significantly contribute to the fetus’s development. The placenta of placental mammals is efficient and well-developed to support the growth of the young to full term.

Placental mammals feed their young ones exclusively on milk during the first few days after birth. They have nipples from which their babies suckle to get the necessary nourishment for them to grow and thrive.

True placental mammals come from the stem-group members of the clade Eutheria, which is thought to have existed in the Jurassic period. Modern placental orders might have originated in the Paleogene period.

They gradually evolved and took over different ecological niches left bare when the dinosaurs disappeared. They also increased in size with some mammals such as primates developing specialized hands and feet.


With time, the groups expanded to various continents such as Africa, North America, and South America.


There are three Superorders of placental mammals. These are as a result of a geographically separate evolution a long time ago.

The first is Afrotheria, which is made up from several Orders from Africa. These comprise of the dugongs and manatees, elephants, tenrecs from Madagascar, which are shrew like creatures, elephant shrews, the aardvark, the golden mole and the rock dassies.

The second Superorder is named Xenarthra. These mammals come from South and Central America. Xenarthra is made up of anteaters, armadillos and slots. All of these creatures of this Superorder contain additional joints that connect the verterbrae.

The third placental Superorder is Boreoeutheria. The countries that these are associated with are now North America, Greenland, and Eurasia. However, these were once part of a single land mass.

Boreoeutherians consist of primates, rabbits, rodents, flying lemurs and tree shrews. These are part of the Superoreder Euarchontoglires.

There also exists a separate branch which consists of hedgehogs, bats, moles, dogs, cats, mongoose, hyaenas, bears and the pandas. This second branch also includes horses, rhinos and other odd-toed ungulates. Even-toed ungulates are also in this Superorder.

These includes mammals such as bighorn sheep, camels and giraffes. hese are part of the Superorder Laurasiatherians

Want to know how mammals are classified. I have you covered. Read this article here.

Marsupial mammals

The name marsupial is derived from marsupium which means pouch. They are a group of mammals that are characterized by a premature birth.  The newborn continues to develop while attached to the nipples on the lower belly of the mother. 

The pouch, otherwise referred to as the marsupium is a flap of skin that covers the nipples. There are also species that have the nipples exposed or covered with just the remnants of a pouch.

The young marsupials are born very early, even before they have fully developed. The baby is born only one month after conception and is usually a mere embryo.

The young baby remains attached to the milk-filled teats for a period that corresponds to the remaining gestation period in a placental mammal.


The reason as to why marsupials give birth early is because they lack a placenta. The placenta connects the embryo to the mother’s blood supply.

The placenta works to provide the embryo with the much-needed nutrients as well as oxygen. The placenta also takes away all the waste products. 

Since these animals lack a placenta, they are born at an early stage in their development. Throughout the remaining part of their early lives, the baby marsupial is nourished by milk from the mother. 


Marsupials are divided into two groups consisting of over 500 species of animals. The two basic groups include the American and Australian marsupials.

The American marsupials can be found in most parts of north, central, and South America. North America has only two species of marsupials, with the rest believed to have gone extinct. 

A Koala {bear} an Australian native marsupial in the fork of a Eucalyptus tree. Differential focus.

The most common marsupial is the Virginia opossum. Australia has the highest number of species, with about 120. They include kangaroos, possums, koalas, wallabies, and wombats.

The smallest marsupial in the entire group is the long-tailed planigale. It is tiny, measuring just 2.3 inches and weighing approximately 4.3 grams. This little mammal inhabits most parts of northern Australia such as grasslands and woodlands.

On the other hand, the largest animal in this group is the red kangaroo. These mammals have a rusty red color and can weigh up to 200 pounds. On average, they measure 3.25 to 5.25 feet in length.

Reproduction and Pregnancy

When a pregnant marsupial is expecting a young one, she starts preparing for birth by preening the pouch. She does so to make way for the incoming embryo. The cleaning involves typically licking off pee as well as droppings from the prior baby.

The reproductive systems of this group of mammals are quite complex. They have a double reproductive tract. The female marsupials possess two uteruses. Each of the uteri has its lateral vagina, and they both join up to form a birth canal that is centrally located.

In some species, males have a two-headed penis that is located behind the scrotum.

Did you know that there is only type of mammal that can fly? Find out what it is in an article I have written here.


A monotreme is used to refer to a group of unique mammals that don’t give birth to young ones, but they instead lay eggs. The monotremes have only five species of animals.

These include the duck-billed platypus and four species of spiny anteaters or echidna. Both animals are native to the islands of Australia and New Guinea, with none in North America.

An echidna walking over a grassy field.

Aside from the fact that they lay eggs, monotremes are more or less similar to other mammals. They produce milk to feed their newborn. Unlike the other mammals that have nipples, monotremes have mammary gland openings in their skin.

The openings secrete fluid which accumulates in the mother’s abdomen. The young feed on the milk from here by lapping it up. 

Do Monotremes Lay Eggs?

Monotremes are the only mammals that lay eggs, and in this way are entirely different from marsupials and placental mammals. The body temperature of a monotreme is also very different from most warm-blooded animals.

They have a lower body temperature that is more in common with that of a reptile.  

The name of a monotreme is due to a huge difference between them and the other types of mammals. Whereas placental mammals and marsupials have two openings, one for waste and one for eggs to pass through, monotremes only have one. Through this opening, both eggs and waste pass.

Want to know what the ten most deadly mammals of North America are? Read this article I have written to find out.


Apart from the fact that monotremes lay eggs, and a couple of technical features in the skeleton, monotremes are not very different from other mammals in their habitats.

The echidna and the platypus are both specialized in their respective ways. They both have short limbs, which extend sideways rather than downwards.

Duck-billed Platypus

The platypus uses these for swimming down to the bottoms of lakes to collect food such as invertebrates. The echidna use their limbs to burrow and to dig out its and termites.

The duck-billed platypus is an odd-looking type of mammal. They have a broad bill  similar to a duck, a tail that resembles a beavers and webbed feet. They spend time both on land and in the water. The platypus is an exceptional swimmer and can dive underwater for up to 30 seconds when looking for food.  

The platypus lays between one and three, small eggs. They develop in the uterus for 28 days and then are incubated by the mother for ten days. The mother then curls around them to keep warm. 

Although they do have mammary glands, they do lack teats, something that most other mammals do have. They release the milk to the young through pores in the skin.

Short-beaked Echidna, Tachyglossus aculeatus, or Spiny Anteater, a unique animal, one of only two monotremes, the egg-laying mammals.


The echidna is a monotreme with a body that is covered with spines except for the belly and face. The spines are mainly for protection from predators.

There are two types of echidnas; the short-beaked and the long-beaked. The short-beaked are smaller than their counterparts and possess short beaks just as their name suggests. The long-beaked have longer and down-curving beaks. They also have shorter spines.

The echidnas have long snouts with long front claws that they use to plow through the forest litter, break ant and termite mounds and even to turn over hollow logs.

When it finds insects, it uses it long and sticky tongue to suck them up into its mouth. The echidna mostly feeds on ants, and this has earned it the name spiny anteater.

A female echidna lays an egg directly into a temporary pouch that develops on her abdomen during breeding. The pouch works to incubate the egg for about ten days until the baby hatches. The echidna only lays one egg.  

After hatching, the baby echidna is small and helpless and stays in the pouch to further grow. The baby remains there for about eight weeks sucking the yellowish milk seeping out of the mother’s pores.

Once the baby echidna has fully developed, they are deposited out of the pouch and into a nursery burrow. From here, the baby will spend a further six to eight weeks in the pouch.

After this, the spines begin to grow, irritating the mother and forcing her to eject them from the pouch. The mother frequently returns to nourish them until the young are five months old. 

Want to know more about the different orders of mammals. Read this article I have written to find out.

Bryan Harding

Bryan has spent his whole life around animals. While loving all animals, Bryan is especially fond of mammals and has studied and worked with them around the world. Not only does Bryan share his knowledge and experience with our readers, but he also serves as owner, editor, and publisher of North American Mammals.

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