Internal Fertilization

Internal fertilization in animals is done through copulation, which involves the insertion of the penis or other intromittent organ into the vagina (in most mammals) or to the cloaca in monotremes, most reptiles, some birds, the disappeared dinosaurs and some fish, as well as in other non-vertebrate animals. In some birds and in the tuatara, that don't have an intromittent organ, the internal fertilization is done by the means of the cloacal kiss, which consists in that the two animals touch their cloacae together in order to transfer the sperm from the male to the female.

At some point, the growing egg or offspring must be expelled. There are three general ways of doing this:

• Oviparous organisms, including most insects and reptiles, monotremes, dinosaurs and all birds lay eggs that continue to develop after being laid, and hatch later.

• Viviparous organisms, including almost all mammals (such as whales, kangaroos and humans) bear their young live. The developing young spend proportionately more time within the female's reproductive tract. The young are later released to survive on their own, with varying amounts of help from the parent (s) on the species.

• Ovoviviparous organisms, like the garter snake, most Vipers, and the Madagascar hissing cockroach, have eggs (with shells) that hatch as they are laid, making it resemble live birth.

Most species of land animals reproduce by internal fertilization. For example: All reptiles, such as the snake and turtle reproduce by internal fertilization. Males and females usually have an opening called the cloaca through which semen, urine and feces can be released. During mating, the male and female join their cloacas. The male releases semen into the female's cloaca. The spermatozoa then travel up a canal to reach the ova.