What Are Animals: Definition, Characteristics, and Classifications

We often think of animals as living beings that move, breathe, and have limbs or bodies. While this general idea fits many species, it does not fully capture what science defines as "animals." In this article, we’ll explore the scientific definition of animals, their biological traits, and how they are classified based on their structure, diet, habitat, reproduction, and daily activity patterns.

Definition of Animals and the Animal Kingdom

Biologically speaking, animals are:

• Multicellular organisms,

• Heterotrophic, meaning they rely on other organisms for food,

• Lacking cell walls (unlike plants and fungi),

• Equipped with a nervous system that allows response to stimuli,

• Undergo a blastula stage during embryonic development,

• Made of specialized cells that form tissues and organs.

Any organism that meets all these criteria is classified under Kingdom Animalia. Animals are one of the most recently evolved groups in the tree of life, first appearing during the Cambrian Explosion, around 525 million years ago. They initially evolved in the oceans and later diversified into nearly every ecological niche on Earth.

Humans are also part of the animal kingdom and depend entirely on other kingdoms such as plants, fungi, and bacteria. This interdependence highlights the interconnected nature of life on Earth.

Main Characteristics of Animals

Animals share several biological and behavioral traits, including:

• Eukaryotic cells: Their cells have a nucleus containing DNA.

• Multicellularity: Composed of many cells organized into tissues and organs.

• Heterotrophy: They consume organic matter to obtain energy.

• Aerobic respiration: Use oxygen to break down food molecules.

• Adaptability: Evolved for a wide range of environments—land, water, or air.

• Behavioral responses: They actively respond to stimuli (e.g., hunting, fleeing).

• Advanced sensory systems: Most have eyes, ears, or other organs for perception.

Given their diversity, animals are categorized in various ways to better understand their biology.

Animal Classification: Types and Categories

1. By Skeletal Structure

Vertebrates (Vertebrados)

These animals have a backbone or spinal column. They belong to Phylum Chordata, which includes:

• Vertebrates proper: Mammals, birds, reptiles, amphibians, and fish.

• Urochordates (tunicates): Only show chordate traits in their larval stage.

• Cephalochordates (like lancelets): Retain their notochord throughout life.

Invertebrates (Invertebrados)

These animals lack a spinal column and make up the vast majority of animal species. Groups include:

• Sponges (Porifera)

• Cnidarians (jellyfish, corals)

• Echinoderms (sea stars, sea urchins)

• Nematodes (roundworms)

• Annelids (earthworms, leeches)

• Mollusks (snails, octopuses, clams)

• Platyhelminthes (flatworms)

• Arthropods (insects, spiders, crustaceans)

Many invertebrates have an exoskeleton and grow by molting.

2. By Diet

Herbivores

Feed only on plants. Subtypes include:

• Frugivores (fruit-eaters)

• Polinivores (pollen-eaters)

• Xylophagous (wood-eaters)

• Folivores (leaf-eaters)

• Ruminants (cows, deer): have multi-chambered stomachs

Some insects, like bees and butterflies, are adapted for specific plant-based diets.

Carnivores

Eat other animals. May be active predators or scavengers. Examples:

• Raptors (eagles, hawks)

• Crocodiles

• Big cats, wolves

• Snakes and lizards

Carnivores typically have sharp teeth, claws, and keen senses for hunting.

Omnivores

Consume both plant and animal matter. Examples include:

• Mice

• Pigs

• Chimpanzees and bonobos

• Humans

This versatility provides adaptability to different environments.

3. By Habitat

Terrestrial Animals

Live on land. Examples:

• Horses, zebras, deer

• Spiders and ants

Adaptations include limbs for walking and lungs for breathing air.

Aquatic Animals

Live in water. Examples:

• Fish

• Jellyfish, sponges

• Dolphins, whales

They breathe through gills or have streamlined bodies and blowholes for breathing air.

Aerial or Aeroterrestrial Animals

Primarily live on land but can fly. Examples:

• Birds

• Bats

• Flying mammals (like sugar gliders)

• Flying insects (butterflies, dragonflies)

4. By Reproductive Strategy

Oviparous

Lay eggs that develop outside the mother’s body. Examples:

• Birds, reptiles, amphibians

• Insects

• Monotreme mammals (platypus, echidnas)

Viviparous

Give birth to live young. The embryo develops inside the mother. Examples:

• Most mammals

• Marsupials (kangaroos, koalas)

Ovoviviparous

Eggs hatch inside the mother, and the young are born alive. Found in:

• Some sharks

• Certain snakes

• Invertebrates like some insects

5. By Activity Cycle

Diurnal Animals

Active during the day, sleep at night. Examples:

• Humans

• Most birds

• Cattle, dogs

• Butterflies, bees

These animals rely heavily on vision for survival and navigation.

Nocturnal Animals

Active at night, rest during the day. Examples:

• Bats

• Owls, nightjars

• Foxes, rodents

• Moths, fireflies

Nocturnal species often rely more on hearing or smell and are adapted to low-light environments.

Final Thoughts

Understanding what defines an animal goes far beyond common perception. Through this comprehensive breakdown—from cellular structure to ecological role—we see how vast and varied the animal kingdom truly is. Whether on land, in the sea, or in the sky, animals play essential roles in Earth’s ecosystems.

If you’re interested in learning more, explore additional articles under our Wild Animals category.

animal tags： Animals Animal classification