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.