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Reproduction is one of the most characteristic features of living organisms. Life would not exist on Earth if plants and animals did not reproduce to make their offspring. By reproducing, a living organism can be sure that there is another individual of its kind to take its place when it dies. In this way a species of organism guarantees its survival.
A species (pl. species) is a particular type of organism. For example, a horse is a species and a zebra is another species. A species which cannot reproduce enough offspring will disappear for ever from the face of the Earth - it will become extinct. This has happened many times in the past. The best known example of animals which have become extinct is the dinosaurs. The dinosaurs were a group of reptiles which mysteriously became extinct 60 million years ago. Fortunately there has always been another type of living organism to replace those that become extinct. In the case of the dinosaurs they left the Earth to the group of animals that we belong to, the mammals.
How Animals Reproduce
Animals can be grouped into those which give birth to living offspring and those which lay eggs that eventually hatch into offspring. Those animals which give birth to live offspring are called live-bearing or viviparous. Those animals which lay eggs are called egg-laying or oviparous. The difference is in the place where the offspring develops before it is born.
Below is a table of the vertebrates which shows which groups are viviparous and which groups are oviparous.
Amongst the invertebrates there are many which are oviparous but a few are viviparous such as sea anemones and aphids.
Sexes and Sex Cells
A species of animal usually exists in two types or sexes called males and females. Each sex has its role to play in reproduction.
When an animal gives birth or lays eggs we notice that it is always the female which does this. In some cases, such as the sea horse and the midwife toad, the male appears to give birth to the offspring. In these exceptions the male has been given the eggs to look after but it is still the female animal that lays the eggs in the first place.
To produce an offspring two special cells are needed. A cell is a microscopic part of an organism's body. The bodies of animals are made of millions of cells. There are many different sorts of cells in a body, each having specialized functions. Some are found in the blood transporting oxygen, others are found in the walls of our stomach producing juices to digest our food. To reproduce, animals make special sex cells. In the male animal these sex cells are called sperm cells or sperms and in the female animal they are called egg cells or eggs.
Sperm cells are very small but they are very specialized too. The sperm cells of different species of animal are all about the same size, about 60µm long (µm = micrometres ; this is a thousandth of a millimetre). Over 11 000 sperm cells could fit on a pin head! All sperms have a head and a tail called a flagellum. They use the flagellum to swim through liquids. The head of the sperm is very important because it contains the nucleus.
The nucleus of the cell is the control centre. In sex cells the nucleus carries half of the information needed for reproduction (like the plans needed to construct a house).
Sperm cells are made in very large numbers by special organs in the male's body called testes (sing. testis). In most animals the testes are carried inside the male's abdomen but in mammals they are kept in a sack of skin called sperm duct the scrotum outside the abdomen.
Egg cells are produced by female animals in special organs called ovaries (sing. ovary). These are found inside the abdomen of all female animals.
Egg cells are much bigger and simpler than sperm cells in their structure. They do, however, vary a lot from one species of animal to another. For example the egg of a human is only 0,1 mm wide but the egg of a chicken is 20 mm wide. Even so the human egg cell is still nearly 50 times wider than a sperm cell. The reason for the difference in size is that the egg has a large food supply stored inside it called yolk. Yolk is the yellow part of a chicken's egg. The egg also has its own nucleus which carries the other half of the information needed for reproduction.
Because eggs are bigger than sperms they are not produced in such large numbers. Even so animals which do not take care of their offspring, such as fishes and frogs, will lay a large number of eggs. Animals which take care of their offspring, such as mammals and birds, will produce less eggs.