Mammals are vertebrate animals that nourish their young with milk. All mammals are warm-blooded (and so are birds). At some stage in their development they all have hair, although this sometimes disappears before they are born.
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Apart from some of the simplest animals, reproduction requires a sperm to fuse with an egg cell. This fusion produces an embryo that will develop into a new animal. Some simple worms and single-celled animals can simply split into two. However, sexual reproduction is the most common means by which animals produce copies of themselves. Some animals, such as snails, contain male and female cells and can fertilize themselves. A few animals are all female and can reproduce without a male. These include aphids and greenfly.
Sexual reproduction is important because it mixes genes. This can prevent the deterioration of a species.
The fossil record and modern studies on genetics have shown how animals have evolved from the simplest worm-like organisms.
The development and relationship of animals can be drawn like a tree, with branches representing each of the major forms of life. This shows that vertebrates probably split off from primitive worm-like ancestors many millions of years ago. Invertebrates, such as insects and spiders, developed along quite a different line of evolution. Many branches of the evolutionary tree were unsuccessful and have died out, for example the dinosaurs.
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Mammals and birds are the main groups of warm-blooded animals. Some very active fish, such as tuna, are also warm-blooded. However, the mechanism that heats their bodies is different from that of mammals and birds.
What is the advantage of being warm-blooded?
The chemical reactions that power the bodies of animals work best at warmer temperatures.
Warm-blooded animals are able to live and move about at any temperature that they experience in their normal environment. They can do this because their internal temperature is always the same. Cold-blooded animals, however, become sluggish as the temperature around them drops, and many will hibernate or migrate in very cold weather.
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Vertebrates have a backbone that provides support for the muscles and protection for the spinal cord. The backbone is actually a series of small bones called vertebrae. They are jointed together and locked with rope-like ligaments to provide a flexible but extremely strong anchor for the back muscles. The spinal cord runs down a channel inside the vertebrae, providing protection from damage. In some primitive fish, such as sharks and rays, the spine is made of a tough rubbery material called cartilage.
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The animal kingdom is divided into two groups — vertebrates and invertebrates. The vertebrate group contains all animals with backbones (as well as some that have a primitive form of backbone).Vertebrates include fish, amphibians, reptiles, birds and mammals. Invertebrates have no backbone. They include worms, molluscs, corals, scorpions, spiders, insects and crustaceans such as crabs and lobsters. Scientists divide these two main groups into smaller groupings that show the relationship between all these animals.
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