Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For internal fertilization to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract.
Most taxa that reproduce by internal fertilization are gonochoric. In mammals, reptiles, and certain other groups of animals, this is done by copulation, an intromittent organ being introduced into the vagina or cloaca. In most birds, the cloacal kiss is used, the two animals pressing their cloacas together while transferring sperm. Salamanders, spiders, some insects and some molluscs undertake internal fertilization by transferring a spermatophore, a bundle of sperm, from the male to the female. Following fertilization, the embryos are laid as eggs in oviparous organisms, or continue to develop inside the reproductive tract of the mother to be born later as live young in viviparous organisms.
Evolution of internal fertilization
Internal fertilization evolved many times in animals. According to David B. Dusenbery all the features with internal fertilization were most likely a result from oogamy. It has been argued that internal fertilization evolve because of sexual selection through sperm competition.
In amphibians internal fertilization evolved from external fertilization.
Methods of internal fertilization
Fertilization which takes place inside the female body is called internal fertilization in animals is done through the following different ways: 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 amphibian tailed frog and some fish, the disappeared dinosaurs, as well as in other non-vertebrate animals. Cloacal kiss, which consists in that the two animals touch their cloacae together in order to transfer the sperm of the male to the female. It is used in most birds and in the tuatara, that do not have an intromittent organ. Via spermatophore, a sperm-containing cap placed by the male in the female's cloaca. Usually, the sperm is stored in spermathecae on the roof of the cloaca until it is needed at the time of oviposition. It is used by some salamander and newt species, by the arachnida, some insects and some mollusks. In sponges, sperm cells are released into the water to fertilize ova that are retained by the female. Some species of sponge participate in external fertilization where the ova is released.
Expulsion
At some point, the growing egg or offspring must be expelled. There are several possible modes of reproduction. These are traditionally classified as follows:
Oviparity, as in most invertebrates and reptiles, monotremes, dinosaurs and all birds which lay eggs that continue to develop after being laid, and hatch later. Viviparity, as in almost all mammals (such as whales, kangaroos and humans) which 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) of the species. Ovoviviparity, as in the garter snake, most vipers, and the Madagascar hissing cockroach, which have eggs (with shells) that hatch as they are laid, making it resemble live birth.
Advantages to Internal Fertilization
Internal fertilization allows for:
Female mate choice, which gives the female the ability to choose her partner before and after mating. The female cannot do this with external fertilization because she may have limited control of who is fertilizing her eggs, and when they are being fertilized. Making a decision for the conditions of reproduction, like location and time. In external fertilization a female can only choose the time in which she releases her eggs, but not when they are fertilized. This is similar, in ways, to cryptic female choice. Egg protection on dry land. While oviparous animals either have a jelly like ovum or a hard shell enclosing their egg, internally fertilizing animals grow their eggs and offspring inside themselves. This offers protection from predators and from dehydration on land. This allows for a higher chance of survival when there is a regulated temperature and protected area within the mother.