Animals

Animal Categories

Scientists place animals in the biological kingdom Animalia. Animals are living organisms that can move, eat food, grow, and reproduce. Unlike plants, animals cannot make their own food using sunlight. Instead, animals must get energy by eating plants, other animals, or both. This need to find food influences how animals behave, where they live, and how they survive in different environments.

Members of the biological kingdom Animalia are classified into many other groups based on their physical characteristics, body structure, and evolutionary history. These groups help us better understand how animals are related to one another and how they live in different ecosystems.

Vertebrates and Invertebrates

Animals are commonly divided into two broad categories: vertebrates, which have a backbone, and invertebrates, which do not. Vertebrates include familiar groups such as mammals, birds, reptiles, amphibians, and fish, all of which share an internal skeleton and more complex body structures. Invertebrates make up the vast majority of animal life on Earth and include many different groups, the most diverse of which are insects.

Habitats

Animals are one of the most diverse and fascinating forms of life on Earth. They live in almost every habitat, from deep oceans and tropical rainforests to deserts, mountains, and the frozen Arctic. Some animals are large and powerful, like elephants and whales. Others are small and easy to overlook, like insects and tiny fish. Together they form a vast network of life that has existed on Earth for hundreds of millions of years.

History

Animal life has existed on Earth for a very long time. The earliest animals appeared in the oceans more than 500 million years ago, long before humans existed. Over time, animals evolved into many different forms. Some adapted to life on land, developing legs and lungs. Others evolved wings and took to the air. Many remained in the oceans, where some species grew larger than any animals that have ever lived on land.

Animals live in Ecosystems

An ecosystem is a place where living organisms like animals interact with each other and with their physical environment.

Because animals have been part of Earth’s ecosystems for so long, they play important roles in keeping nature balanced.

In a healthy ecosystem, different species depend on one another.

Predators help control animal populations. Herbivores eat plants and shape vegetation. Pollinators help many plants reproduce by moving pollen from flower to flower. Even small creatures such as insects help recycle nutrients and keep ecosystems functioning.

This incredible variety of life is known as biodiversity. Biodiversity helps ecosystems stay strong and resilient. When many different species live together in a habitat, the ecosystem is better able to recover from changes such as storms, droughts, or disease.

Animals are also important to humans. Healthy ecosystems help provide clean air, clean water, and fertile soil. Many crops that people depend on rely on animals for pollination or seed dispersal. Forests, wetlands, oceans, and grasslands all depend on complex relationships between animals and plant.

Animal Conservation

For these reasons, caring about animals is not only about protecting wildlife. It is also about protecting the natural systems that support life on Earth—including our own. When animal populations decline or habitats are damaged, ecosystems can become unstable. Learning about animals helps us understand why conservation matters and why protecting wildlife is important for the future of the planet.

Image: Hippos are large land mammals. They share a common ancestor with whales and porpoises, and are believed to have evolved around 55 million years ago.

Interesting Animal Facts

1. Millions of Animal Species
   Scientists estimate there are more than 8.7 million species of animals on Earth, although only about 1.5 million species have been formally described.
2. Insects Are the Most Diverse Animals
   Insects make up the largest group of animals. Scientists have identified over one million insect species, which is more than half of all known animal species.
3. Mammals Are a Small Group
   There are about 6,500 species of mammals worldwide, a tiny fraction compared with insects and other invertebrates.
4. Birds Are Highly Diverse
   Scientists recognize roughly 11,000 species of birds, found on every continent and in nearly every type of habitat.
5. The Largest Animal on Earth
   The blue whale is the largest animal ever known to live on Earth. It can grow to over 100 feet (30 meters) long and weigh more than 200 tons.
6. The Fastest Land Animal
   The cheetah is the fastest land animal and can reach speeds of about 60–70 miles per hour (97–113 km/h) in short bursts.
7. The Largest Land Animal
   The African elephant is the largest land animal. Adult males can weigh over 13,000 pounds (6,000 kg).
8. The Longest Migration
   Some animals travel enormous distances. The Arctic tern makes the longest migration of any animal, traveling more than 40,000 miles (64,000 km) each year between the Arctic and Antarctica.