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6 Main Types of Animals

Different animal types against the earth backdrop.

Have you ever wondered how many types of animals exist in this world?

A fun fact: Nobody, not even researchers, and scientists, have been able to find out the exact count of animal species. And it is believed that by the time they will find it out, most of them will be extinct!

It stands to reason that animals coexist with humans. We may not be clear on the full statistics of animals but we do observe so many of them around us all the time. But then there are distinct, exotic animals that we only come across on the TV or the Internet, belonging to different parts of the world.

You may have noticed that animals share a variety of similarities and differences; some have similar features, behaviors, food preferences, eating habits, and choice of habitat while others are polar opposites.

The reason why many animals are so similar to each other in appearances and characteristics is that they belong to the same animal category.

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Learning about different kinds of animals, their distinct features, and habits are always enlightening which is why we have prepared this blog for you. Here you will find everything that you must know about 6 basic categories of animals.

1. Mammals

A cat and a dog leaning against each other.

Who are we? Humans, of course! But are we just humans? Well, not really… because we happen to be mammals too and so are our dear pets like cats and dogs. The question that arises is – how can we tell which of the animals are mammals and which aren’t? Well, it’s easy to know it if you are aware of some common characteristics that they share.

Note that mammals include all those warm-blooded animals that have hairy bodies, give natural birth, and feed milk to their young ones from their mammary glands.

What’s more?  Mammals also have endothermic vertebrates, four-chambered hearts, teeth, diaphragm, and a single jaw bone. However, the defining feature of mammals is that they all have backbones considering the fact that they are vertebrate.

Did you know that there are about 5400 species of mammals all over the world? They significantly differ in sizes, case, and point, there are Bumblebee bats that are just 1 inch long while on the other hand, there are blue whales that reach lengths of 108 feet. And don’t forget that blue whales are considered to be the largest animal on planet Earth.

According to various biological studies, mammals were first spotted during the dinosaur era which means that they are over 200 million years old.  In the beginning, mammals were small in their size but after the extinction of dinosaurs, their size drastically changed as they transformed into more prominent beings.

Biologists suggest that mammals can be categorized into three classes – Animalia, Chordata, and Mammalia.

A List of Mammals

  • Horses – (we have more info about horses here.)
  • Kangaroos – (we have more info about kangaroos here.)
  • Dolphins
  • Whales – (we have more info about whales here.)
  • Rats/mice
  • Lions
  • Tigers
  • Sheep
  • Donkeys
  • Monkeys – (we have more info about monkeys here.)

2. Birds

Three chestnut-headed bee-eaters sitting on a branch.

If you think that birds can only fly, think again. With over 10,000 species in the world, there are birds that can run, jump, swim, and even dive. The distinguishable feature of birds that sets them apart from the rest of the animals is the fact that they possess feathers, wings, and beaks.

Feathers, the defining feature of birds, are crucial to their flight abilities. But that’s not it. Feathers also provide warmth and protection against the heat and cold. You may not know this but in many species, feathers are a prominent feature for males to attract their mates.

Research tells that once or twice a year, birds shed away their old feathers and grow out new ones. There are birds that despite having wings and feathers can’t fly. Some of the flightless birds include ostrich, emu, rhea, kiwi, and cassowary.

Every bird has its wings in a different shape which provides them with advantages that differentiate them from other notable birds. For example, falcons have sharp-tipped wings that make them one of the speediest birds ever.

Songbirds usually have even, elliptical wings that allow them to make quick movements in the limited spaces of trees. On the other hand, penguins – a swimming bird – possess flipper-shaped wings that help propel their body through water.

Unlike mammals, birds never swallow their food by chewing. They simply grind their food into smaller pieces and swallow them quickly. The overall shape of a bird’s beak signifies the bird’s diet and their way of food’s consumption.

For example, Carnivorous like hawks and owls typically have a pointed, sharp beak, which is used for ripping and tearing their food apart.  Ducks have a flat yet broad beak that helps them pick their food straight out of the water.

Like other types of animals, birds lay eggs as well. However, some of them lay bright, vibrant eggs while others lay the ones that are plain or covered with tiny spots. While mammals have a thick, strong skeleton, most birds have a lightweight skeleton which keeps them light enough to fly with ease.

A List of Birds

  • Parrots – (check out our article all about parrots here.)
  • Penguin
  • Owl
  • Pigeon
  • Crow
  • Goose
  • Woodpecker
  • Bulbul
  • Kingfisher
  • Plover

3. Fish

Clownfishes in an aquarium

Unlike mammals, fish are cold-blooded creatures, which means that they depend on the outside temperature to maintain their overall body temperature.

Research shows that the body temperature of fish immediately changes as the environment’s temperature alters. This striking characteristic of fish is absolutely opposite to that of mammals that depend on their internal body to regulate their body temperature. Fish primarily comprise of backbone and fins.

With the help of fins, they are able to maintain their balance and steer through the water successfully. Typically, fish can have two types of fins – single fins and paired fins.

The internal organ, known as the swim bladder or air bladder, helps fish to float up and down the water. In some kinds of fish, the swim bladder helps them in sleeping and making sounds. But there are also fish that are able to function without a swim bladder. Members of the elasmobranches such as sharks and ray do not have a swim bladder.

Another common feature of fish is the presence of scales that exist in many kinds such as rough, sharp, etc. It is widely believed that due to the evolution of fish, which began 480 million years ago, their bodies have developed scales that protect them against the environment and their predators. However, fish like eels and Agatha do not have scales.

You may have wondered at some point in time – How do fish breathe underwater? Well, that’s because of the presence of gills that every fish possesses from the moment they are born to the moment they die. Gills are a fundamental organ as they help fish to breathe underwater; it allows them to inhale oxygen from the water and exhale carbon dioxide.

Mind you, not all animals that live underwater can be categorized as fish. For example, whales, despite living in water are classified as mammals. That’s because whales do not have gills. Instead, they have lungs that help them rise to the water surface. On the other hand, tadpoles are touted as amphibians as they lose their gills during their transformation into a frog.

A List of Fish

  • Goldfish
  • Guppy – we have more info about guppies in this article
  • Common carp
  • Oscar
  • Wels catfish – learn more about catfish in our article here.
  • Swordfish
  • Tench
  • Common bream
  • European fish
  • Bluefish

For more detailed information about freshwater and saltwater fish, check out our guide here.

4. Invertebrates

A bee on a green leaf.

Invertebrates are, by far, the largest and most diverse group in the animal kingdom. It has been estimated that of all the animals in the world, 97% of them are invertebrates. A rough calculation further shows that invertebrates could be about 5, 10, or even 30 million.

One of the reasons why invertebrates are in such drastic high numbers is because their reproduction level is high. They are able to replicate hundreds and thousands of their kinds in a matter of days. For instance, corals and sponges can produce both eggs and sperm. Insects like bees and ants are able to lay eggs without fertilization.

Also, invertebrates, insects in specific, are adaptive in nature. Being omnivorous, insects can eat almost anything when they are hungry, feeding on plants or decaying food or materials.

They also possess the ability to survive in extreme dry/hot weather conditions and habitats. And some of them even know how to fly to either dodge their predators or to look for food and shelter.

To shed light on their anatomy, you must know that invertebrates such as worms, slugs, and snails possess a soft body, lacking any segmented legs or a backbone. While they may lack a firm internal skeleton, most of them possess a solid outer skeleton such as mollusks and crustaceans.

This helps in protecting their body from all sorts of external harm. Insects, in particular, have a segmented body and flexible legs that help them move around. Moreover, insects have bilateral symmetry which means that both their left and right side of the body are identical.

Invertebrates include a wide group of species which is categorized as insects, worms, sponges, arthropods, mollusks, octopuses, and many other species.

A List of Invertebrates

  • Bee
  • Scorpion
  • Fly – for info about gnats in particular, check out our article here.
  • Cockroach
  • Snail – learn more about snails in our article here.
  • Centipedes
  • Worms – we have more info about worms in this article.
  • Millipedes
  • Hydras
  • Jellyfish
  • Octopus – check out our guide about the octopus here.

5. Reptiles

A frog sitting underneath a chameleon.

What kinds of animals can be categorized as reptiles? The ones that are cold-blooded, with their bodies covered in scales. Unlike birds and mammals, reptiles are unable to regulate their internal body temperature.

Since they are deprived of feathers and sweat glands, they can’t stay warm in winters and cool off in summers respectively. Therefore, they seek shelter or soak up the sun as needed.

It should be noted that the outside temperature affects the metabolism of reptiles which is why they become inactive during winters. Their reproduction is also dependant on the environment’s temperature. For instance, on the basis of the soil temperature, it can be easily determined whether the hatchlings would be male or female.

If we touch base on facts, it comes into highlight that reptiles are the oldest animals in the world. Their first appearance in the fossil was first recorded 315 million years ago. History points out that these creatures ruled supreme during the Mesozoic era that lasted for almost 270 million years.

While reptiles ruled the animal kingdom during the Mesozoic era, most of them became extinct, with only four living orders alive, which are the Squamata, Chelonia, Crocodilia, and Rhyncocephalia. Among these living orders, the Squamata is the largest and most diverse order of living reptiles, holding a rich fossil record.

In Rhyncocoephalia, the most distinguishable reptile is Tuatara that bears a notable resemblance with lizards. One might think tuatara to be a distant relative of the lizard family, but in fact, they are a prominent member of a lineage – Rhynchocephalia which means “peaks on the back”.

Tuatara, belonging to an ancient lineage, is only found in predator-free islands of New Zealand. In Crocodilia, currently, there are 23 known species from regions such as North and South America, Africa, Asia, and Australia.

Chelonia, another prominent type of reptile, is referred to as the “crown group” and includes turtles and tortoises. These kinds of animals primarily live in both water and on land.

A List of Reptiles

  • Lizard
  • Snakes
  • Turtle
  • Skink
  • Crocodile
  • Tortoise
  • Alligators
  • Chameleons
  • Caimans
  • Vipers

6. Amphibians

A frog nestled in between the leaves,

Amphibians form a major class of vertebrates alongside birds, reptiles, and mammals. As compared to mammals, amphibians are more diverse species having over 4700 of their kinds. It may be surprising to you but amphibians, some 400 million years ago, were fish.

These fish succumbed to the weather conditions and therefore, started to develop limbs and lungs. With the help of new-formed limbs, they can crawl wherever they wanted to and with the lungs, they could breathe underwater. This way, a new group of animal came about which scientists named as amphibians that means “double life”.

With adverse changes in the temperature, the variety of amphibians has considerably decreased. It has been observed that about 360 million years ago when the weather conditions were favorable for amphibians, the world witnessed the largest variety of amphibians. Most of the species that existed at that time have gone extinct and their existence can’t be traced anymore.

Like other types of animals, amphibians do not possess any distinct structure like birds that have beaks and feathers. However, all amphibians have notable features that they share.  Firstly, amphibians possess soft, moist, and scale-less skin that help them absorb water and oxygen.

However, without moisture, the skin of these cold-blooded animals dries out, and they end up dying. To stay hydrated, amphibians live near areas that are filled with water such as ponds, swamps, and marshlands. During hot, dry weather, most amphibians go into hibernation since the weather conditions are unfavorable.

Although amphibians have super thin skin, yet it contains glands that protect them against hostile situations. For instance, their thin skin consists of poison glands that provide them protection from their predators.

After the female amphibians are fertilized, they lay eggs in water. These eggs hatch into larvae, or tadpoles that are able to breathe through their external gills. With time, the larvae start to develop and feed on vegetation.

Then the period of metamorphosis occurs where physical transformation in tadpoles starts to take place as their external gills change into lungs. From mere plant eaters, these tadpoles become meat eaters. Within 3 to 4 years, amphibians enter into their adulthood.

A List of Amphibians

  • Frog – check out our article about toads (which are a type of frog) here.
  • Salamander
  • Caecilian
  • Diplocaulus
  • Amphiuma
  • Anthracosauria
  • Pacific giant salamander
  • Torrent salamander
  • Labyrinthodontia
  • Newt

All kinds of animals, whether they are reptiles, amphibians, birds, mammals, invertebrates, or fish, play a significant role in the ecosystem. Indeed, a world without animals would be a baron one.