Internal fertilization

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

Methods of internal fertilization[edit]

Fertilization which takes place inside the female body is called internal fertilization in animals is done through the following different ways:[5][6][7]


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 from Internal Fertilization[edit]

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.[1]
  • Making a decision for the conditions of reproduction, like location and time.[16] 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.[17] 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.[18] This allows for a higher chance of survival when there is a regulated temperature and protected area within the mother.

Disadvantages to Internal Fertilization[edit]

  • Gestation can and will add additional risks for the mother.[19] The additional risks from gestation come from extra energy demands.
  • Along with internal fertilization comes sexual 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 peak fertility. While animals which externally fertilize are able to release egg and sperm, usually into the water, not needing a specific partner to reproduce.[19]
  • 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.[20]


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’s in the female’s mouth.[21] 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.[22]


Most amphibians have external fertilization but there is an exception to some like salamanders which mostly have internal fertilization. Male salamanders don't have a penis to insert and deposit sperm inside females. Their alternate for that consists of an encased capsule of sperm and nutrients called a spermatophore. The male will deposit 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.[21] 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.[23]


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. [21] Being the only vertebrate taxon with internal fertilization, the bird’s intromittent phallus has been lost in most species. The phallus is a penis and being an evolution without this makes them have to be internally fertilized.[24]

See also[edit]


  1. ^ a b Alonzo SH, Stiver KA, Marsh-Rollo SE (August 2016). "Ovarian fluid allows directional cryptic female choice despite external fertilization". Nature Communications. 7 (1): 12452. Bibcode:2016NatCo...712452A. doi:10.1038/ncomms12452. PMC 4990696. PMID 27529581.
  2. ^ Hyman LH (15 September 1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. ISBN 978-0-226-87013-7.
  3. ^ a b Austin CR (1984). "Evolution of the copulatory apparatus". Bolletino di Zoologia. 51 (1–2): 249–269. doi:10.1080/11250008409439463.
  4. ^ a b Romer AS, Parsons TS (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 396–399. ISBN 978-0-03-910284-5.
  5. ^ Reichard UH (2002). "Monogamy—A variable relationship" (PDF). Max Planck Research. 3: 62–7. Archived from the original (PDF) on 14 May 2011. Retrieved 24 April 2013.
  6. ^ Lipton JE, Barash DP (2001). The Myth of Monogamy: Fidelity and Infidelity in Animals and People. San Francisco: W.H. Freeman and Company. ISBN 978-0-7167-4004-9.
  7. ^ 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.
  8. ^ Lombardi J (6 December 2012). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN 978-1-4615-4937-6.
  9. ^ Diamond J (1991). The Rise and Fall of the Third Chimpanzee. Radius. pp. 360 pages. ISBN 978-0091742683.
  10. ^ 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.
  11. ^ 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.
  12. ^ Bergquist PR (1978). Sponges. London: Hutchinson. ISBN 9780520036581.
  13. ^ Lodé T (2001). Les stratégies de reproduction des animaux [Reproduction Strategies in the Animal Kingdom] (in French). Paris: Dunod Sciences.
  14. ^ Blackburn DG (January 2000). "Classification of the reproductive patterns of amniotes". Herpetological Monographs. 14: 371–7. doi:10.2307/1467051. JSTOR 1467051.
  15. ^ Carrier JC, Musick JA, Heithaus MR, eds. (2012). Biology of Sharks and Their Relatives. CRC Press. pp. 296–301. ISBN 978-1439839249.
  16. ^ Yokoe M, Takayama-Watanabe E, Saito Y, Kutsuzawa M, Fujita K, Ochi H, et al. (2016-08-31). Klymkowsky M (ed.). "A Novel Cysteine Knot Protein for Enhancing Sperm Motility That Might Facilitate the Evolution of Internal Fertilization in Amphibians". PLOS ONE. 11 (8): e0160445. Bibcode:2016PLoSO..1160445Y. doi:10.1371/journal.pone.0160445. PMC 5007030. PMID 27579691.
  17. ^ 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.
  18. ^ "43.2A: External and Internal Fertilization". Biology LibreTexts. 2018-07-17. Retrieved 2020-11-09.
  19. ^ 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.
  20. ^ 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.
  21. ^ a b c Cotner, Sehoya; Wassenberg, Deena. The Evolution and Biology of Sex.
  22. ^ Engel, Kathrin M, Dzyuba, Viktoriya, Ninhaus-Silveira, Alexandre, Veríssimo-Silveira, Rosicleire, Dannenberger, Dirk, Schiller, Jürgen, . . . Dzyuba, Borys. (2020). Sperm Lipid Composition in Early Diverged Fish Species: Internal vs. External Mode of Fertilization. Biomolecules (Basel, Switzerland), 10(2), 172.
  23. ^ Yokoe, Misato, Takayama-Watanabe, Eriko, Saito, Yoko, Kutsuzawa, Megumi, Fujita, Kosuke, Ochi, Haruki, . . . Watanabe, Akihiko. (2016). A Novel Cysteine Knot Protein for Enhancing Sperm Motility That Might Facilitate the Evolution of Internal Fertilization in Amphibians. PLOS ONE, 11(8), E0160445.
  24. ^ Brennan, Patricia L. R, Birkhead, Tim R, Zyskowski, Kristof, Van Der Waag, Jessica, & Prum, Richard O. (2008). Independent evolutionary reductions of the phallus in basal birds. Journal of Avian Biology, 39(5), 487-492.