Reproduction and responding to the environment
Sexual reproduction requires the combination of genetic material from two parents. This is achieved when two sex cells called gametes [gametes: the sex cells -sperm in males, ova (eggs) in females] fuse together. This process is called fertilisation and can happen either outside the female (external fertilisation) or inside (internal fertilisation).
The male sex cell is called a sperm. It is much smaller than an egg cell and has a head containing the nucleus, and a tail which enables it to swim. The female sex cell is called an egg. It contains the female nucleus anda store of food. Sperm are chemically attracted to swim to egg cells, and to combine with them.
Fertilisation in fish and most water dwelling animals is external. Most fish produce a large number of sex cells which are released into the water. Courtship rituals make sure that the male and female sex cells are released near each other, but even so the chances of fertilisation are very low, so that huge numbers of eggs and sperm are needed to ensure that enough young are produced.
Animals which live on land have internal fertilisation. This is necessary since sperm need fluid in which to swim to the egg. The chances of fertilisation are much greater since the sex cells are closer together when released, and this means that fewer sex cells need to be produced.
The number of eggs which any animal needs to produce for successful reproduction depends on the chance of fertilisation and the degree of parental care which the parents give.
Less care or a low chance of fertilisation mean that large numbers of eggs must be produced.
Look at the following slideshow which shows internal fertilisation.
Listen to the audio to find out more about reproduction and cloning.
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