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Mating and Fertilization

Bringing the sex cells together

The sperm cell carries half of the information needed for reproduction and the egg cell carries the other half. To produce an offspring the sperm and egg have to meet. This means that the male and female animals have to come together. This is called mating.

Coral, Aquarium, Paris © Shirley Burchill

Because the sperms can swim on their own, aquatic animals such as fish can simply release the eggs and sperms into the water. Amphibians, which return to water to reproduce, also release their eggs and sperms directly into the water. The sperm cell is microscopic and only lives for a short time, so the eggs and sperms need to be released close together.

Facts iconJohnny's Fact File No.26
The male animal is often smaller than its female. The marine worm females are 100 million times heavier than their males. The females are 100 cm in length and their males are 10 mm long!
Ascaris is a roundworm parasite which lives in the gut of many animals, including man. The males are between 12 cm and 17 cm long but their females are 25 cm long.

Fish, such as herring, which live in the open sea will shoal at certain times of the year. Shoaling occurs when the males and females gather close together, usually near the surface of the sea. The females release their eggs and the males release their sperms at the same time and in the same place. Therefore, the sperms do not have far to swim.

Fish, such as salmon, trout and sticklebacks, build their nests in the mud and gravel on the bottom of shallow rivers and lakes. The male usually builds the nest and then he guards it. When a female enters his territory he will try to encourage her to lay her eggs in his nest. This sometimes involves quite a complicated mating display. If the female decides to lay her eggs in his nest he will release his sperms over the eggs.


Drawing of frogs mating © Shirley Burchill

Animals which live on land have a problem sperm cells cannot walk or fly! For the amphibians the answer to this problem is to return to the water to mate. Frogs and toads will migrate to the ponds and lakes where they were born. They do this usually in Spring. Sometimes they cross roads on their migration route in such large numbers that they can cause accidents! In France the motorway which runs through the Sologne, south of Orleans, has special tunnels constructed underneath it. These tunnels allow the amphibians, which are very numerous in this region, to cross underneath the road safely. These tunnels are called crapauducts!

When the frogs arrive at the water the males and females call to one another. This helps the amphibians to identify one another, as many different species may be mating at the same time and in the same place. The male holds the female round the waist so that as she releases her eggs he will release his sperms close to them in the water. The mating calls are very important for some species because the males will try to mate with any object floating in the water, including fish or pieces of wood!

Mating on Land

Peacock showing tail display © Shirley Burchill

Vertebrate animals such as reptiles, birds and mammals, and invertebrates, such as spiders and insects, do not return to water to mate, so the male must place the sperm cells directly inside the female's body. The male often has a special organ to place the sperms inside the female and the female has a special opening to receive the sperms. This kind of mating is called copulation and it has two advantages. First, the sperms are released in a liquid called semen which is made by the male. The sperms can swim in this liquid towards the eggs inside the female. The second advantage is that the sperms cells inside the female are more certain to meet the egg cells. Animals which mate in this way do not produce so many eggs because only a few of them are lost. Copulation is so effective that many aquatic animals use it too, even though they are surrounded by water in which the sperms could swim.

Getting closer to the female can have its disadvantages. If the animal is a carnivore, such as a spider, the female may think that the male is a meal to be eaten rather than a mate! The result is that land animals have developed complicated mating behaviour which involves dances, smells and often very colourful mating displays. It is usually the male which is colourful in the animal world, even amongst the herbivores. This is very apparent amongst birds; the peacock, for example, has brilliant tail feathers. The female (peahen) by comparison is quite drab.


Drawing of an egg about to be fertilized © Shirley Burchill

An egg cell surrounded by sperm. Only one sperm cell will enter the egg and fertilize it.

When a sperm cell meets an egg cell, the head of the sperm fuses with the egg. The flagellum of the sperm is lost, its work is finished. The information in the nucleus of the male's sperm can combine with the information in the nucleus of the female's egg to produce an offspring. This important event is called fertilization. If fertilization takes place inside the female's body - as it does in mammals or birds - it is called internal fertilization. If it happens outside the female, in water, it is called external fertilization.


Only one sperm cell will fertilize one egg cell successfully. It is possible, however, for two sperms to fertilize two eggs. This will result in two offspring. This is not at all unusual in the animal world. Foxes give birth to litters of between four and eight puppies. A litter is a group of babies born from the same mother at the same time.

The table below gives some examples of different mammals and their average litter size


Litter Size











Red Fox




Black rat


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