Internal fertilization

Internal fertilizationis the union of aneggandsperm cellduringsexual reproductioninside 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.

Mosttaxathat reproduce by internal fertilization aregonochoric.Inmammals,reptiles,and certain other vertebrates, this is done byejaculationofsementhrough anintromittent organinto the female'svaginaorcloacaduringcopulation.In most birds, thecloacal kissis used, the two animals pressing their cloacas together while transferring sperm.Salamanders,spiders,someinsectsand somemolluscsundertake internal fertilization by transferring aspermatophore,a bundle of sperm, from the male to the female. Following fertilization, the embryos are laid as eggs inoviparousorganisms, or continue to develop inside the reproductive tract of the mother to be born later as live young inviviparousorganisms.

Evolution of internal fertilization

Main article:Evolution of sexual reproduction

Internal fertilization evolved many times inanimals.According toDavid B. Dusenberyall the features with internal fertilization were most likely a result fromoogamy.It has been argued that internal fertilization evolve because ofsexual selectionthroughsperm competition.

Inamphibians,internal fertilization evolved fromexternal fertilization.

Methods of internal fertilization

Fertilizationwhich takes place inside the female body is called internal fertilization inanimalsis done through the following different ways:

• Copulation,which involves the insertion of thepenisor otherintromittent organinto thevagina(in mostmammals)or to thecloacainmonotremes,mostreptiles,somebirds,theamphibiantailed frogand somefish,the disappeareddinosaurs,as well as in othernon-vertebrate animals.
• Cloacal kiss,which consists in that the two animals touch their cloacae together in order to transfer thespermof the male to the female. It is used in most birds and in thetuatara,that do not have an intromittent organ.
• Viaspermatophore,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 somesalamander and newt species,by theArachnida,some insects and somemollusks.
• Insponges,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

Main article:Modes of reproduction

At some point, the growingeggoroffspringmust be expelled. There are several possiblemodes of reproduction.These are traditionally classified as follows:

• Oviparity,as in mostinvertebratesand 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 aswhales,kangaroosandhumans) 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 theparent(s) of thespecies.
• Ovoviviparity,as in thegarter snake,mostvipers,and theMadagascar hissing cockroach,which have eggs (withshells) that hatch as they are laid, making it resemble live birth.

Advantages to internal fertilization

Internal fertilization allows for:

• Femalemate 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, tocryptic 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.

Disadvantages to internal fertilization

• Gestationcan and will add additional risks for the mother.The additional risks from gestation come from extra energy demands.
• Along with internal fertilization comessexual reproduction,in most cases. Sexual reproduction comes with some risks as well. The risks with sexual reproduction are with intercourse, it is infrequent and only works well during peakfertility.While animals which externally fertilize are able to release egg and sperm, usually into the water, not needing a specific partner to reproduce.
• Fewer offspring are produced through internal fertilization in comparison to external fertilization. This is both because the mother cannot hold and grow as many offspring as eggs, and the mother cannot provide and obtain enough resources for a larger amount of offspring.

Fish

Some species of fish like guppies have the ability to internally fertilize, this process happens by the male inserting a tubular fin into the female's reproductive opening and then will deposit sperm into her reproductive tract. There are other species of fish that are mouthbrooders which means that one fish puts the eggs in its mouth for incubation. A certain type of fish that is a mouthbrooder is called cichlids and many of them are maternal mouthbrooders. The process for this is the female would lay the egg and pick it up in her mouth. Then the males will encourage the female to open her mouth so they can fertilize the eggs while it is in the female's mouth.Internal fertilization in cartilaginous fishes contains the same evolutionary origin as reptiles, birds, and mammals that internally fertilize. Also in these internally fertilizing fish while the sperm is transferred to the reproductive tract there is no noticeable change in tonality.

Amphibians

Most amphibians have external fertilization but there is an exception to some like salamanders which mostly have internal fertilization. Salamanders do not use intercourse for sexual reproduction due to their lack of external penis. Rather, the male salamander produces an encased capsule of sperm and nutrients called aspermatophore.The male deposits a spermatophore on the ground and the female will pick it up with her cloaca (a combined urinary and genital opening) and fertilize her eggs with it.Over time amphibians have been found evolving to increasing internal fertilization.Within amphibians, it is common for high vertebrates to internally fertilize because of the transition from water to land during vertebrate evolution. There is an advantage for the amphibians who are internally fertilizing allowing for the selection of a time and place for reproduction.

Birds

Most birds do not have penises, but achieve internal fertilization via cloacal contact (or "cloaca kiss" ). In these birds, males and females contact their cloacas together, typically briefly, and transfer sperm to the female. However, water fowls such as ducks and geese have penises and are able to use them for internal fertilization.While birds have internal fertilization, most species no longer have phallus structures. This makes them the only vertebrate taxon to fall into both categories of lacking the phallus but participating in internal fertilization.

See also

• Insemination
• Fertilization