Mammals: The Ultimate Guide – Mammal Facts, Information & Pictures

Mammals – the ultimate guide: fun facts, pictures and in-depth information, with links to further reading.

Mammals – The Ultimate Guide

On this page you'll discover what makes a mammal a mammal, how mammals evolved, and the different types of mammal. You'll also learn some amazing facts along the way!

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Air Sea Land Mammals
Although most mammals live on land, they are also found in the sea... and some can fly!

Mammals: An Introduction

Here’s a quick question:

Think of an animal – any animal – and picture it in your head.

What animal did you think of?

Was it a wild animal such as a tiger, an elephant or a polar bear? Perhaps you thought of a pet animal such as a dog or a cat, or a farm animal such as a cow or a pig?

Whatever animal you did think of, it was most likely to have been a mammal. (Okay, you may have chosen a crocodile, or an eagle, but we’re pretty sure that most of you would have thought of a mammal!).

Perhaps it’s because we are mammals ourselves that when we think of ‘animals’, we usually end up thinking of ‘mammals’.


tundra wolf
Mammals are found in many different environments. Some mammals, such as the Gray Wolf, are found in a variety of habitats. Others are more specialized.

Mammals form a part of the animal kingdom known as Mammalia.

Mammalia is a ‘class’: a group of animals with shared characteristics and ancestors. A class is just one of the many groups that biologists separate living things into; a process known as ‘classification’.

  • You’ll find out more about how mammals are classified in Part 3 of our Mammals series.

Mammal Facts

Blue Whale Largest Mammal
The Blue Whale is the largest mammal that has ever lived.

Before we get down to the nitty gritty of what makes a mammal a mammal, here are some quick questions about mammals to get you thinking:

How many mammal species are there in the world today?

There are between 5,400 and 5,500 mammal species alive today.  (Wilson & Reeder’s Mammal Species of the World makes the figure 5,416; a 2008 IUCN study makes it 5,488.)

Rodentia (rodents) is the biggest group of mammals in terms of the number of species it contains. It is followed by Chiroptera (bats) and Soricomorpha (shrews and shrew-like animals).

What is the biggest mammal alive today?

The biggest living mammal (and indeed, the biggest animal that has ever lived) is the blue whale. It grows to lengths of up to 30 meters (98 ft.) and weighs up to 180 metric tonnes (397,000 lb.)!

What is the biggest land mammal?

The biggest land mammal alive today is the African bush elephant. It weighs up to 10.4 metric tonnes (22,930 lb.), and is 3.2 meters (10.5 ft.) tall at the shoulders.

What is the smallest mammal?

The smallest mammal alive today is the Etruscan shrew, which tips the scales at a mere 2 grams (0.07 oz.)

When did mammals appear?

The first mammals appeared around 225 million years ago, during the Triassic Period. (We’ll find out more about this further down the page!)

Mammals: Classified

Flying Mammal Bat
Bats are flying mammals.

Although a tiny bat looks very different to a huge whale, both share certain characteristics that make them mammals rather than any other type of animal. Let's find out what makes a mammal a mammal ...

Mammals have backbones

Mammals are vertebrates, which means that they have backbones. Go on – feel your own backbone; it’s the bumpy bit that runs up the center of your back!

All mammals have backbones, from the tiniest shrew to the largest elephant.

Mammals are warm-blooded

A mammal’s body is able to raise or lower its temperature, and isn't dependent on its surroundings to run at the correct temperature. For example, we shiver when we’re cold – this warms us up. When we’re hot, we sweat. This cools us down.

Animals that can do this are ‘warm-blooded’, or ‘Endothermic’, to give it its scientific name.

The opposite of this is being ‘cold-blooded’, or ‘ectothermic’. If an ectothermic animal such as a lizard needs to get warm, it has to bask in the sun!

(Most) Mammals give birth to live young

Nearly all mammals give birth to live young. However, there is one pesky group of mammals that don’t obey this rule! They are the monotremes, and instead of giving birth to babies, they lay eggs! You’ll be finding out more about these incredible mammals further down the page!

Mammals have hair

All mammals have hair, although some (e.g. whales and  dolphins) only have hair follicles, the organs that produce hair. No other type of animal has hair, so if you see a hairy animal, then it’s a mammal!

Female mammals feed their babies with milk

This is perhaps the most important characteristic of mammals. All female mammals feed their young with milk.

Milk is a special liquid that is produced in regions in the mother’s skin called mammary glands. Milk provides new-born (and not so new born) mammals with all the nutrients they require.

Just imagine the advantages that having a pre-made source of food gives a young mammal! Baby birds (and their parents) must be extremely jealous!

Mammals have lungs, and breathe air

All mammals have lungs – organs that extract oxygen from the air and expel carbon dioxide.

Even aquatic mammals such as whales and dolphins need to come up to the surface to breathe.

Other Characteristics of Mammals

There are several other characteristics that scientists use to identify a mammal. If you want to know more about what makes a mammal a mammal, jump on over to Part 2 of this series, What Is a Mammal?

Different Types Of Mammal

The quokka is a 'macropod'; a type of marsupial, or 'pouched mammal'.

As we’ve found, mammals are a diverse group of animals. Mammalia is made up of many smaller groups of mammals that are even more closely related to each other than they are to other mammals.

In this section, we’re going to find out about the main mammal groups.

There are 3 main types of mammal:

  • Monotremes (egg-laying mammals)
  • Marsupials (pouched mammals)
  • Placental mammals (mammals whose young develop in the womb)

Monotremes (Egg-Laying Mammals)

The Platypus is one of just five monotreme species.

The first mammals began to appear many millions of years ago. The early mammals were all egg-laying mammals. This group split into two main branches. One branch remained egg-laying mammals. The other group, known as Theria, evolved the ability to give birth to live young.

The monotremes are the descendants of the group that remained egg-layers. There are only five monotremes alive today: the platypus, and four species of echidna.

Marsupials (Pouched Mammals)

Red Kangaroos
Red Kangaroos are the largest marsupials alive today.

The Therians themselves branched into two groups: the marsupials and the placental mammals.

Marsupials give birth to young that are relatively undeveloped. New-born marsupials crawl into a special pouch in the mother’s body in which they continue to grow.

A marsupial’s mammary glands are located within the pouch, so the young marsupial – known as a ‘joey’ – has 24/7 access to a nutritious meal!

Placental Mammals

Polar Bear
The Polar Bear is a placental mammal.

You’re a placental mammal, as are cats, dogs, bats, whales and many other familiar mammals!

Like marsupials, Placental mammals develop in a region of the mother’s body called the uterus, or womb. However, placental mammals are born at a later stage of development than marsupials.

An organ called a ‘placenta’ provides the fetus (the developing animal) of a placental mammal with nutrients and oxygen. The placenta also transports waste products away from the bloodstream of the fetus.

The fetus is connected to the placenta by a tube-like structure called an umbilical cord.

Your belly button is all that remains of the umbilical cord that attached you to your mother!

Other Mammal Groups

North American Beaver
This North American Beaver is a rodent - just one of the many types of mammal.

Although there are only two types of monotreme, the marsupial and placental groups of mammals contain many different groups.

Marsupial groups include macropods (kangaroos, wallabies, quokkas, etc.), possums, opossums and bandicoots.

Placental mammal groups include Primates (apes, monkeys and lemurs), Pinnipeds (seals, sea lions and walruses), Cetacea (whales, dolphins and porpoises), Rodents (Rats, mice, beavers, etc.) Carnivora (dogs, cats and bears).

  • You can find out about the many types of mammal in Part 2 of this series: Types Of Mammals.

How Did Mammals Evolve?

Mammals began to appear around 225 million years ago, during the late Triassic Period. The first mammals were small, shrew-like animals. They ate insects and scurried around in the undergrowth at night.

The ancestors of the first mammals were reptilian animals that had evolved from amphibians. They were called amniotes. The Amniotes branched into two main groups.

One branch of amniotes, called synapsids, would eventually become mammals. The other branch, called sauropsids, were the ancestors of modern-day reptiles … and of the dinosaurs!

At one point the synapsids (the ancestors of the mammals) were the dominant land animals. However, it all changed in the Mesozoic Era with the arrival of the dinosaurs!

Many scientists believe that it was due to the dominance of the dinosaurs in the Mesozoic Era that many mammalian characteristics were formed.

The early mammals had to be warm-blooded, so they could hunt at night. They had to have excellent senses to move around and hunt in the dark, and brains large enough to be able to make sense of the extra information. Their jaws and teeth may have been shaped to enable them to catch fast-moving and small insect prey.

Perhaps we have the dinosaurs to thank for making us what we are!

  • You can discover more about the amazing world of the early mammals and their ancestors here: Mammal Evolution.

Mammals – The Ultimate Guide: Conclusion

We hope that you have enjoyed this introduction to mammals! On this page you’ve learned that mammals form part of the animal kingdom called Mammalia. You’ve found out what makes a mammal a mammal, and you’ve also found out about the three main types of mammal.

You’ve also met your early mammalian relations!

In the next parts of this series, you’ll be able to discover more about these subjects.

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