Animals of the World

Animals make up the biological kingdom Animalia, a vast group of multicellular organisms that obtain energy by consuming other organisms rather than producing their own food. They range from microscopic invertebrates to the largest whales, and inhabit nearly every environment on Earth, from deep oceans and polar ice to deserts and rainforests. Zoologists estimate that well over a million animal species have been described, with many more still unidentified, especially among insects and other small invertebrates.

To make sense of this diversity, biologists group animals using a hierarchy of categories — kingdom, phylum, class, order, family, genus and species. A first broad division separates vertebrates, which possess a backbone, from the far more numerous invertebrates, which do not. Vertebrates include fish, amphibians, reptiles, birds and mammals, while invertebrates include arthropods such as insects, spiders and crustaceans, along with molluscs, worms and many other phyla. Some classifications also recognise single-celled groups such as the Alveolata when discussing the broader tree of life.

Animals show enormous variation in body plan, diet and behaviour. Some, such as antelopes, alpacas and many birds like the auk, are familiar grazers and foragers, while predators such as the alligator or the green anaconda occupy the top of their food chains. Studying animals involves anatomy, physiology, ecology and evolution, and increasingly the conservation of species whose habitats are shrinking. Africa, with its large mammals and rich birdlife, is one of many regions where wildlife illustrates how varied animal life can be.

Frequently asked questions

What is the difference between vertebrates and invertebrates?

Vertebrates have an internal backbone and skeleton; invertebrates lack a backbone. Invertebrates, such as insects and worms, vastly outnumber vertebrate species.

How many animal species are there?

More than a million species have been formally described, but the true number is far higher because many invertebrates remain undiscovered.

What does the kingdom Animalia include?

It includes all multicellular animals, from sponges and insects to fish, birds and mammals, that consume organic material for energy.