WHAT IS AN ANIMAL? THE SIX KEY ANIMAL FEATURES
Animal life encompasses an astonishing diversity of form and behavior. Many animals seem familiar to us because we are animals too. We instinctively understand the needs of an animal such as a cat, for example, and how it responds to its environment.
But some animals have ways of life that are harder to comprehend, and creatures such as corals may seem to behave more like plants.
What is it that links corals with the cat, and with us?
Why are the animals, and not some other form of life?
SIX KINGDOMS OF LIFE
Scientists currently divide life on Earth into six “kingdoms.” Three of these—the archaea, bacteria, and protists—are mainly made up of microscopic organisms, so we are rarely aware of them, even though we could not exist without them.
The other three are the fungi, plants, and animals. The differences between fungi and plants are not very obvious, but most animals are easy to recognize by the way they move and react to their environment.
The simplest life forms are single cells—tiny fluid-filled sacs that absorb energy to fuel their reproduction.
Similar to archaea, but with, different chemistry, bacteria live everywhere. Some cause disease; others keep us alive.
More complex than bacteria, these mainly single-celled organisms contain structures that have different functions.
Made up of many cells that resemble those of protists, plants typically use solar energy to fuel their growth.
Single-celled or multicelled fungi typically obtain energy by feeding on dead plant and animal material.
All animals are multicelled. Most get their energy by eating and digesting plants, fungi, or other animals.
SIX KEY ANIMAL FEATURES
Ranging from microscopic worms to colossal whales, animals are amazingly diverse in shape, size, and anatomy. Despite this, they all share some key features, including the most basic structure of their bodies, the way they fuel their growth and reproduction, their ability to sense their environment, and their mobility.
♦ Multicellular bodies
Archaea, bacteria, and most protists consist of microscopic single cells containing the complex chemicals needed for life. All animals have bodies built up from many of these cells, which are typically organized into different types of tissue and organs. Even this water flea (a type of freshwater crustacean) has specialized organs, though it is only up to 1⁄4 in (5 mm) long.
♦ Energy and food
Living things need energy to function. Plants use the energy of sunlight to make tissues that store the energy. Animals eat these tissues, or those of other living things, and process them to release the energy and essential chemicals they need to fuel and build their own bodies.
All animals eat living organisms, or the remains of dead ones. Most have ways of processing them in digestive organs that break down the tissues, turning them into nutrients. Some aquatic animals filter food particles from the water, but most have mouths that they use to seize and swallow food. Animals have acute senses and most are mobile. Both these traits help them to find food and, if necessary, catch it.
♦ Gas exchange
Animals need oxygen to release energy from food, in a process that produces carbon dioxide. The bodies of insects contain tubes that pipe air to their muscles and organs, which absorb the vital oxygen from the air and release waste carbon dioxide.
In most other animals this exchange of gases takes place in the gills or lungs, which have a rich blood supply to carry the gases to and from all parts of their bodies.
Nearly all animals have networks of nerve cells in their skin that respond to touch. More advanced animals have specialized sense organs that detect light, heat, scent, taste, sound, pressure, and even electrical activity.
Their brains can memorize the patterns of these stimuli, enabling the animals to learn by experience and identify them again. Most of the sense organs of a typical animal, such as this eagle, are concentrated on its head, near its mouth and brain.
♦ Getting around
The most obvious feature of animals is their mobility. Some animals such as mussels and barnacles spend their adult lives attached to rocks and may not move visibly. But they do open and close their shells and may pump water through their bodies. Most other animals are able to crawl, swim, walk, run, and even fly. This enables them to find food, escape enemies, and seek out breeding partners.