How do guppies mate?

When a female guppy comes into heat, she forms unfertilized eggs inside her body, causing her abdomen to swell and a dark gravid spot to appear. Once the male receives the signal, his anal fin transforms into a gonopodium, and during the breeding period he continuously chases the female, inserting the gonopodium into her vent to release sperm that fertilizes the eggs, making her pregnant. Female guppies store the male’s sperm, so even without further mating they can give birth about once a month, producing 10-120 fry each time.