SCIENCE NOTES

ANIMAL REPRODUCTION

1. Sexual reproduction and fertilisation.

SEXUAL REPRODUCTION IN ANIMALS

Sexual reproduction is the fertilisation of a female ovum by a male sperm.

The animals can be distinguished by their reproductive systems:

• Males have testicles, which produce sperm.

• Females have internal ovaries, which produce ova.

In some animal species, an individual can be both male and female. They have both testicles and ovaries. These animals are called hermaphrodites. Snails and earthworms are hermaphrodites.

         TYPES OF FERTILISATION

        

Internal fertilisation

Internal fertilisation occurs in all land animals, like mammals, birds, reptiles and insects. It also occurs in aquatic mammals, aquatic reptiles and in sharks.

During sexual union, or copulation, the male deposits his sperm inside the female's body.

External fertilisation

External fertilisation occurs in water. The female deposits ova in the water, and the male deposits sperm.

2. From zygote to birth.

OVIPAROUS ANIMALS

Animals that are born from an egg are oviparous animals.

The egg forms a protective shell around the zygote cell. It also contains the food that the zygote needs to develop into an embryo.

Some eggs are small and very simple. The eggs of fish and amphibians, for example, are made up of an embryo and a soft, gelatine shell.

Other eggs are larger and more complex.

The eggs of reptiles and birds are made up of a yolk, egg white and a hard shell.

VIVIPAROUS ANIMALS

In viviparous animals, the embryo develops (desarrollarse)  inside the mother until it is ready to be born. She provides the embryo with the food it needs to grow.

The time the embryo is inside the mother is called gestation. The length of gestation varies according to the species.

All mammals are viviparous.