Tim Birkhead and Nicola Hemmings explain how fertilisation, in some animals, requires more than one sperm

The Biologist 63(2) p20-23

The classic view of sexual reproduction is of a single sperm penetrating an ovum to initiate a new life. This perception is reinforced by images of intra-cytoplasmic sperm injection (ICSI) so often seen on television whenever there is any mention of human reproduction.

It is true that in humans and all other therian mammals examined, normally only a single sperm enters the ovum. But this is not the case in other taxa, including certain insects, arachnids, elasmobranches (sharks and rays), urodeles (newts and salamanders), reptiles and birds, where several sperm typically penetrate the ovum...

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