Animals: Characteristics and Classification

Animal Definition

In scientific terms, an Animal is defined as a multicellular organism that is usually mobile and derives energy solely from the consumption of other organisms. An animal is any member of the kingdom Animalia aside from humans.

In science, Humans are seen as higher animals and go by a scientific name – Homo sapiens.

Living organisms are grouped into five main categories in biology. These groups are Kingdom Animalia,  Kingdom Plantae, Protista, monera, and fungi. Kingdom Animalia is the largest of all groups.

The term ‘Animalia’ is scientific name for the animal kingdom. The animal kingdom consists of various animals globally. They are the most diverse and populated organisms occupying the earth. They are everywhere around us, inhabiting water bodies, air, and land.

Animal characteristics and animal classification is complex is extremely complex due to the sheer number of animal species.

From evolutionary studies, it is said that

The earliest animals evolved from protists more than 600 million years ago and the first animals to live on land are invertebrates while amphibians were the first vertebrates to live on land.

Animals are eukaryotic and multicellular organisms. They are heterotrophic (they feed on plants or other organisms), unlike plants that manufacture their own food. They can easily move from one place to another. This is why their diversity density index tends to be higher in a habitat.

Some animals are native to a single defined geographical location while some could be found in various areas. This species distribution can be attributed to migration.

For instance, Africa is best known for its large diversity and wildlife richness with animal species like; zebra, elephant, giraffe, and hippopotamus, and the bald eagle is native to the United States.

In most cases, migration happens as a result of climate change, food hunting, seasonal times, or mating reasons. Throughout history some animals have evolved and some have gone extinct.

Characteristics of Animals

Why is it easy to see a lion, a snake, or an elephant and identify it as an animal? Or easily choose a house fly over a spider when asked to identify the animal among these two living organisms?  This is because there are characteristics they exhibit that distinguish them from other organisms.

We can narrow these characteristics into seven key distinguishing features; Cell structure,  Multicellularity, Specialized tissues, Sexual reproduction, Motility, Heterotrophy, and Advanced nervous system.

The cell structure of animals

An organism’s cell structure could either be prokaryotic or eukaryotic. All animals are eukaryotic but not all eukaryotic organisms are animals. Eukaryotic cells have a membrane-bound nucleus and internal organelles whereas prokaryotic cells have no membrane-bound organelles.

These eukaryotic cells can come together in a group to form multicellular organisms, unlike prokaryotic cells. Being eukaryotic distinguishes animals from other organisms like bacteria.

Multicellularity

One major characteristic of an animal is its multicellularity. This means animals are made up of many cells. These cells are specialized for various functions. This is an advantage as they have a longer lifespan and continue to live even when single cells die.

Specialized tissues

This specific characteristic is crucial. Animals are unique because of how their cells are specialized. As they develop, cells group together and diversify into; Nervous tissues,  Epithelial tissues( which line blood vessels and organs), Connective tissues, and muscle tissues.

A collection of tissues makes up organs and these various organs have specific functions in the body. This sort of anatomy makes animals more unique and complex than plants, bacteria, or fungi.

Sexual reproduction

Most animals carry out sexual reproduction. This is a huge advantage over animals that use asexual reproduction. Two individual animal mates, combine their genetic traits and produce offspring that bear the DNA from both parents.

For evolution reasons, an animal formed by the sexual reproduction method has various genetic combinations that enables it to adapt faster to new ecosystems compared to asexual organisms. Their ability to reproduce has also helped in their species richness on earth.

Motility

This is the ability to move.  All animals are capable of movement at some stage in their life cycle. Fish swims, snails slide, horses gallop, wolves run, snakes slither, birds and insects fly, and some animal crawls.

This feature distinguishes an animal from other organisms like plants and fungi. Such characteristics allow animals to easily dominate new ecological niches, find their prey or food, or outrun their predators.

Heterotrophy

Heterotrophy involves eating other plants or animals for energy and nutrients. Some organisms are autotrophs  (e.g plants) meaning they manufacture their own food. This is not the case with animals.

They depend and feed on other organisms. Since they are heterotrophs, they are classified into three groups: Herbivores, Carnivores, and Omnivores.

Advanced nervous system

The advanced nervous system in animals gives them the sense of sight, sound, hearing, taste, and touch. These features however distinguish them from other organisms. For instance, some fishes and sharks sense magnetic disturbances in the water using their lateral line.

Snails use their sense of smell and touch to locate another snail in the same area. Dolphins and bats use echolocation.

Classification of Animals

Animal classification can be really complex. Hence, there are various ways in which they can be classified.  Animal Characteristics and Classification go hand in hand.

An animal can be classified based on their morphology and physiology, habitat, feeding habits, or uses. However, because of the complexity associated with animals, they are grouped with other animals sharing similar features to distinguish them from others.

In zoology, animal species are arranged and grouped by their similar characteristics based on the Linnaeus method. Animal characteristics and morphology are used for animal classification.

This method is also known as Linnaean taxonomy which was developed by a Swedish botanist, Carlos Linnaeus. The more characteristics a group of animals has, the more particular their classification group is. The system of animal classification is called taxonomy.  There are seven main taxonomical ranks:

Kingdom

Phylum

Class

Order

Family

Genus

Species

In biology, when classifying animals, other subgroups are branching out from the primary group- Kingdom Animalia.  However, the animal kingdom is broken down into about nine main phyla (phyla is the plural form of phylum).

These phyla are phylum Chordata, phylum Mollusca, phylum Porifera, phylum Platyhelminthes, phylum Nematoda, phylum Annelida, phylum Arthropoda, phylum Echinodermata, and phylum Cnidaria.

These nine phyla include a majority of all animal species even though there are other 26 phyla (though some are seen as subphylum) making it approximately 35 phyla altogether.  Animals are now broken down into two main types: VERTEBRATES and INVERTEBRATES.

Vertebrates

Animals with a backbone are called vertebrate animals. They have a spinal cord surrounded by bone or cartilage. They are differentiated by their vertebral column(the spinal column that protects the spinal cord). The vertebral column is another name for the backbone.

A Vertebrate belongs to the phylum called phylum Chordata. As Chordates, all vertebrates have similar anatomy and morphology.

An animal in this group possesses all of the following characteristics: a post-anal tail, a notochord( a flexible rodlike structure that forms the main support of the body in the lower chordates), a dorsal hollow nerve cord, and pharyngeal slits( a filter-feeding organ).

Vertebrates are further broken down into five classes: Amphibians, Aves (birds), Mammals, Pisces(fishes), and Reptiles.

	Where they live	How they breathe	Body covering	How their young ones are produced	Some examples of their kind
Amphibians	On land and in water	Lungs and Skin	They have soft moist skin with no scales	They lay eggs in water	Salamander, Frog, Toad, Newt
Birds	On land	Lungs	They have feathers	They lay eggs	turkey, duck, goose, parrot, guinea fowl, pigeon
Mammals	On land	Lungs	They have hairy or furry skin	They give birth to their young ones alive	Blue whale, elephant, bats, giraffe, cheetah, leopard, pig
Pisces (fish)	In water	Gills	They have scales	They lay eggs	shark, tilapia, goldfish, hagfish
Reptiles	On land	Lungs	They have tough skin with scales	They lay eggs	lizards, snakes, crocodile, turtles

Invertebrates

Animals without a backbone are referred to as invertebrate animals. They lack the vertebral column and are generally soft-bodied. Most invertebrates belong in the phylum Arthropoda. They are economically important as agricultural pests, parasites and even serve as agents for parasitic infection transmission. They do not possess an internal skeleton (endoskeleton), rather many have structures on the outsides ( exoskeleton).

Invertebrates can live on land, in freshwater, in marine water, and even on other animals. Terrestrial invertebrates like spiders, insects, millipede, worms, centipedes, land hoppers, slaters live on land.

Those that stay in water are the freshwater and marine invertebrates. Such examples are an octopus, corals, prawns, zooplankton, jellyfish, crayfish, lobsters, sponges, and many others.

Most invertebrates change form as they grow. They go through a process known as metamorphosis. They reproduce by two reproductive cells, or gametes coming together to produce a new organism.

Phylum Arthropoda

Invertebrates in this group are known for their exoskeleton, segmented body, and paired jointed appendages. They are the largest group of invertebrates.

Many of them live in aquatic environments and some in terrestrial habitats. Some are even able to fly.

Common arthropods are aphids, insects, mites, spiders, etc. Other sub-phylum examples are crustaceans too like shrimps, crabs, lobsters.

Phylum Mollusca

Phylum are the second-largest phylum of invertebrates, following Arthropoda. Mollusks are soft-bodied invertebrates. The majority of them, are enclosed in a calcium carbonate shell.

They exhibit a wide range of morphology in each class but share a common characteristic. These characteristics include a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell.

Many of them live in freshwater and terrestrial habitats. Common examples are snails, octopus, squid, and slugs.

Phylum Annelida

Phylum Annelida is made up of invertebrate worms. Annelids are found in most wet environments.  The well-known species are earthworms and leeches

Phylum Echinodermata

This group consists mainly of marine invertebrates. The phylum name was derived from the Greek word “echinos” meaning spiny and “derma” meaning skin. Their skeleton sometimes resembles spikes.

They exhibit a great diversity of body forms and sizes. They all have five-part radial symmetry, a water vascular system, and an internal skeleton.

The general appearance of an echinoderm may be like that of a star with arms or it may be round to cylindrical. Well-known examples of echinoderms are starfish, sea lilies, feather stars, sea urchins, brittle stars, serpent stars, sea cucumber.

There are other ways animal classification is carried out. As discussed earlier, they can be grouped based on their feeding habit. They can also be classified based on their habitat ( where they live) and other common types are domestic, farm, and wild animals.

Endotherms

Animal classification can also be done based on body temperature. Endothermic animals are organisms that maintain their bodies at a metabolically favorable temperature by using the heat released by their internal body functions.

The primary examples are mammals and birds.

Ectotherms

Ectothermic animals are organisms that regulate their body temperature solely based on the environment. Ectotherms usually live in environments with constant temperatures like the ocean or tropics.

Types of animals based on feeding habits

Herbivores

These animals’ primary source of food are plants. They feed only on plants and their digestive systems can only digest plant matter.

They generally have flat, broad teeth. Such examples are goats, cattle, sheep, rabbits, squirrel, chipmunks, etc. These living things produce a majority of our animal by-products.

Some animal species unique to this category are ruminant animals. An animal in this class is an herbivore with 4 stomachs that is also a mammal.

Carnivores

A carnivore is an animal that feeds on other animals. The word carnivore is derived from Latin meaning “meat-eater”.  They rely solely on animal flesh to get their nutrients.

They have sharp pointed teeth to tear the flesh of their prey. Such examples are sharks, snakes, lions, tigers, bears, hyena, polar bears, skunks, cheetah, and many others.

We have two types of carnivore animals: Obligate Carnivores and Facultative Carnivores. The former rely only on animal flesh whereas the latter eat animal flesh and non -animal food.

Omnivores

These animals feed on both plants and animals. Omnivore means “to eat anything” in Latin. Such examples are humans, birds, ostriches, seagulls, woodpeckers, bears, hedgehogs, pigs, great apes. monkeys etc.

Parasitic Animals

These sorts of animals depend totally on another organism to survive. They live on or inside another organism known as the host.

This sort of relationship is usually referred to as parasitism. They are usually small and feed on the host’s partly digested food, depriving them of nutrients. Such examples are tapeworms, fleas, barnacles, hookworm, lice, etc.

Types of Animals based on Habitat

Terrestrial Animals

Animals that live predominantly or entirely on land are called terrestrial animals. This primarily applies to those that stay on land only. There is a variety of them. Elephant, fox, lion, cheetah, ostrich, lizard, dogs, cat, tiger, camel, and many more are all terrestrial inhabitants. They usually possess limbs for movement and forearms.

Saxicolous animals

Saxicolous animals are rock-dwelling terrestrial animals. Examples include mountain goats and ibex.

Troglofauna animals

Related to the Saxicolous class, Troglofauna animals are animals that live in caves and subterranean environments. Animal classification examples include gastropods, millipedes, and spiders.

Arenicolous animals

These types of organisms live in the sand or are animals that live in the desert. Their body, however, is adapted to enable them to survive and live successfully on land. They possess a keen sense of sight, smell, and hearing.

Most of them have a skin color that enables them to blend with their environment especially when hunting or avoiding predators. They also have thick skin that helps them to avoid water loss.

Aquatic Animals

Animals that live only in water are referred to as aquatic animals. Many aquatic organisms have special respiratory organs called gills that enable them to breathe well underwater. The gills take in dissolved oxygen in the water and excretes carbon dioxide.

They lack limbs but their body is streamlined to aid movement (swimming). Some like fishes are covered by scales for protection. Some Common examples are fishes, octopi, corals, sea anemones, and many others.

These are also some of the few animals that live in the arctic.

Aerial Animals

Animals that spend most of their time in the air are called Aerial animals. Such are usually birds. Their body is adapted to help them navigate easily on air.

Their forelimbs are modified into wings for flight and their hindlimbs are moved forward to balance their body weight. They also have flight muscles for additional strength when flying.

Arboreal Animals

An Arboreal animal lives on trees. They are found mostly in the tree canopy and the layer of treetops close to the clouds. They have adapted the ability to reach branches of trees as well as leap and glide their way around trees.

A fun fact about these animals is that they have stretchy membranes between their legs. This helps to increase the surface area of some arboreal animals without weight gain to enable them to glide from tree to tree.

Monkeys, tree pangolins, and opossums have prehensile tails that are specially adapted for gathering food, climbing, swinging, and holding things. Monkeys and flying squirrels are good examples of arboreal animals.

Amphibians

Amphibians live in water as well as land. They are cold-blooded. Amphibians don’t have scaly skin but rather their skin is slimy and permeable. Their body shape is streamlined and has wedded toes for swimming.

Their skin is thin and moist which allows for water and gaseous exchange. They have long hind legs and strong feet to quickly jump great distances and change direction. Common examples of amphibians are frogs, toads, salamanders, and caecilians.

Other Classifications of Animals

Domestic animals

Domesticated animals are those animals that have been tamed by humans. They are being kept either as a pet, for a food source, or work. Therefore, they are usually different from their wild ancestors through selective breeding. Animal domestication comes down to three groupings:

Animal pets

These animal pets are tamed and kept by humans for companionship. They are usually kept at home or the zoo for recreational purposes.  Examples of such pets are dogs, cats, parrots.

Farm Animals

These animals are kept and farmed for food and money. They grow and mature quickly making them viable for farm business. Most of them eat plants making it inexpensive to feed them. Some undergo multiple fertility periods in a single year. They can be reared for their meat, eggs, or their skins.

Some animal skin is being used as raw material for clothing and shoes. Most of our clothes and shoes are made from wool and leather which are gotten from animal skin. Many of these farm animals breed easily in captivity. Examples are poultry birds, sheep, pigs, cows.

Beast of burden

These animals are used for carrying heavy loads or pulling heavy equipment. Humans use them primarily for work e.g donkey, horse, ox, mule, camel.

Wild animals

Wild animals are undomesticated animals. They are usually dangerous and attack any other animal that comes near their territory. They are predators and very difficult to tame. You will find these animals particularly in tropical rainforests.

A lot of effort is being made around the globe to protect and save wild animals and to keep them in the wild, where they belong. They vary across numerous species. Typical examples are the buffaloes, lions, tiger, shark, crocodile, snakes, bear, wolf, fox, eagle, gorilla, vulture, elephant, monkey, kangaroo, hippopotamuses, antelope, and many others.

Endangered Animals

Endangered animals are at risk of extinction, either in a specific political jurisdiction or globally. These organisms are listed as endangered because they are threatened due to changes in their habitat and are likely to be extinct animals in the near future.

An example of an endangered animal is the Arctic Fox. Many endangered animals live in the arctic where the environment is rapidly changing.

This poses a great biological threat to these species as it changes the animal hibernation cycle and there is little available food when they begin their search in spring.