animal

Member of the kingdom Animalia, one of the major categories of living things, the science of which is zoology. Animals are all multicellular heterotrophs (they obtain their energy from organic substances produced by other organisms); they have eukaryotic cells (the genetic material is contained within a distinct nucleus) which are bounded by a thin cell membrane rather than the thick cell wall of plants. Most animals are capable of moving around for at least part of their life cycle.

In the past, it was common to include the single-celled protozoa with the animals, but these are now classified as protists, together with single-celled plants. Thus all animals are multicellular. The oldest land animals known date back 440 million years. Their remains were found 1990 in a sandstone deposit near Ludlow, Shropshire, UK, and included fragments of two centipedes a few centimetres long and a primitive spider measuring about one millimetre.

Feeding types Animals can be divided into three feeding types: herbivores eat plants and plant products, carnivores eat other animals, and omnivores eat both. Since few animals can digest cellulose, herbivores must have a mechanism for dealing with it. Some have symbiotic bacteria or protozoa in their guts that digest the cellulose for them. Others grind the plant's digestible protoplasm out of its cellulose-walled cells; the elephant, for example, uses its large, flattened teeth for this purpose. Carnivores are adapted for hunting and eating flesh, with well-developed sense organs and fast reflexes, and weapons such as sharp fangs, claws, and stings. Omnivores eat whatever they can find, and often scavenge among the remains of carnivores' prey; because of the diversity of their diet, they have more versatile teeth and guts than herbivores or carnivores. Many animals are adapted for a parasitic way of life, living on other animals or plants, and feeding solely by absorbing fluids from their hosts. Some animals absorb food directly into their body cells; others have a digestive system in which food is prepared for absorption by body tissues.

Movement The mechanisms by which living organisms move vary from the flowing protoplasm of an amoeba to the complex muscle-and-bone movements of a bird's wing in flight. Wriggling, crawling, swimming, and the internal flow of fluids are often assisted by minute cellular organelles, cilia, which beat in a rhythmic pattern. Most animals have contractile tissue to assist with movement; the increased complexity of muscle systems in higher animals, coupled with a stiff skeleton, allows greater flexibility of movement.

Circulation In order to contract, muscles must have energy, which is obtained by breaking down sugars produced by the digestion of food, usually by combining them with oxygen (see respiration). In simple animals, the sugars and oxygen diffuse through the body to the contractile tissue; in higher animals they are carried by a blood circulatory system, which is pumped by the heart, to the muscles. In insects, the oxygen is carried through tiny branching air-filled tubes, called trachea.

Excretion The circulatory system also carries waste products away from the cells. These are separated from the blood (in the kidneys of higher animals), and are excreted together with undigested food. The excretory systems of aquatic animals are also responsible for maintaining a correct fluid balance within the animals. This is particularly important in freshwater creatures, which tend to absorb water by osmosis.

Nervous system The movement and behaviour of animals is controlled by the nervous system. Impulses pass from centres of control – often grouped to form a brain – to the various muscles and glands. As animals become more advanced, their brains become more complex, and their nervous control more complete. Behaviour varies from the simplest reflex action to the highly complex learned and intelligent behaviour of the higher primates. In many animals special behaviour patterns form an integral part of courtship.

Colour and camouflage Most animals are coloured in some way. Often the colour is defensive, camouflaging the animal or disguising it as another that may taste bad or be venomous. Colour can also act as a threat to enemies or competitors, and plays an important part in recognition during courtship. Some animals, such as chameleons, can alter their colour to blend with the environment by changing the size of different-coloured patches in the skin.