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.[1] 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.[2]: 124–125  In mammals, reptiles, and certain other groups of animals, this is done by copulation, an intromittent organ being introduced into the vagina or cloaca.[3][4] In most birds, the cloacal kiss is used, the two animals pressing their cloacas together while transferring sperm.[5] 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

Main article: Evolution of sexual reproduction

Internal fertilization evolved many times in animals.[2]: 2  According to David B. Dusenbery all the features with internal fertilization were most likely a result from oogamy.[6] It has been argued that internal fertilization evolve because of sexual selection through sperm competition.[7]

In amphibians, internal fertilization evolved from external fertilization.[8]

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:[9][10][11]


Main article: Modes of reproduction

At some point, the growing egg or offspring must be expelled. There are several possible modes of reproduction. These are traditionally classified as follows:

Advantages to internal fertilization

Internal fertilization allows for:

Disadvantages to internal fertilization


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.[25] 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.[26]


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 a spermatophore. 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.[25] Over time amphibians have been found evolving to increasing internal fertilization.[citation needed] 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.[20]


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. [25] 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.[27]

See also


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  2. ^ a b Leonard, Janet; Cordoba-Aguilar, Alex (2010-07-16). The Evolution of Primary Sexual Characters in Animals. Oxford University Press. ISBN 978-0-19-971703-3.
  3. ^ Hyman LH (15 September 1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. ISBN 978-0-226-87013-7.
  4. ^ a b Austin CR (1984). "Evolution of the copulatory apparatus". Bolletino di Zoologia. 51 (1–2): 249–269. doi:10.1080/11250008409439463.
  5. ^ a b Romer AS, Parsons TS (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 396–399. ISBN 978-0-03-910284-5.
  6. ^ Dusenbery, David B. (2009). Living at Micro Scale: The Unexpected Physics of Being Small. Harvard University Press. p. 326. ISBN 978-0-674-06021-0.
  7. ^ Smith, Robert L. (2012-12-02). Sperm Competition and the Evolution of Animal Mating systems. Elsevier. pp. 7–8. ISBN 978-0-323-14313-4.
  8. ^ Sawada, Hitoshi; Inoue, Naokazu; Iwano, Megumi (2014-02-07). Sexual Reproduction in Animals and Plants. Springer. pp. 97–98. ISBN 978-4-431-54589-7.
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  11. ^ Morell V (September 1998). "A new look at monogamy". Science. 281 (5385): 1982–3. doi:10.1126/science.281.5385.1982. PMID 9767050. S2CID 31391458.
  12. ^ Lombardi J (6 December 2012). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN 978-1-4615-4937-6.
  13. ^ Diamond J (1991). The Rise and Fall of the Third Chimpanzee. Radius. pp. 360 pages. ISBN 978-0091742683.
  14. ^ Wedell N, Tregenza T, Simmons LW (July 2008). "Nuptial gifts fail to resolve a sexual conflict in an insect". BMC Evolutionary Biology. 8: 204. doi:10.1186/1471-2148-8-204. PMC 2491630. PMID 18627603.
  15. ^ Sozou PD, Seymour RM (September 2005). "Costly but worthless gifts facilitate courtship". Proceedings. Biological Sciences. 272 (1575): 1877–84. doi:10.1098/rspb.2005.3152. PMC 1559891. PMID 16191592.
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  17. ^ Lodé T (2001). Les stratégies de reproduction des animaux [Reproduction Strategies in the Animal Kingdom] (in French). Paris: Dunod Sciences.
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  19. ^ Carrier JC, Musick JA, Heithaus MR, eds. (2012). Biology of Sharks and Their Relatives. CRC Press. pp. 296–301. ISBN 978-1439839249.
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  21. ^ Altig R, McDiarmid RW (December 2007). "Morphological diversity and evolution of egg and clutch structure in amphibians". Herpetological Monographs. 21 (1): 1–32. doi:10.1655/06-005.1. S2CID 55728625.
  22. ^ "43.2A: External and Internal Fertilization". Biology LibreTexts. 2018-07-17. Retrieved 2020-11-09.
  23. ^ a b Wallen K, Zehr JL (February 2004). "Hormones and history: the evolution and development of primate female sexuality". Journal of Sex Research. 41 (1): 101–12. doi:10.1080/00224490409552218. PMC 1255935. PMID 15216429.
  24. ^ Parker, G. A. (1970). "Sperm Competition and Its Evolutionary Consequences in the Insects". Biological Reviews. 45 (4): 525–567. doi:10.1111/j.1469-185X.1970.tb01176.x. ISSN 1469-185X. S2CID 85156929.
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