Internal fertilization

Internal fertilization is the union of an egg cell with a sperm during sexual reproduction inside the female body. For this to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract. In mammals, reptiles, some birds, some fish and certain other groups of animals, this is done by copulation, the penis or other intromittent organ being introduced into the vagina or cloaca.[1][2] In most birds, the cloacal kiss is used, the two animals pressing their cloacas together while transferring sperm.[3] 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 in viviparous organisms, continue to develop inside the reproductive tract of the mother to be born later as live young. In some animals like in sponges fertilization is internal

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:[4][5][6]

  • Copulation,[7] 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.[2][8]
  • 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.[3]
  • 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.[9][10]
  • In sponges, sperm cells are released into the water to fertilize ova that in some species are also released (external fertilization) and in others are retained by the "mother" (internal fertilization).[11]


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 insects and reptiles, monotremes, dinosaurs and all birds which lay eggs that continue to develop after being laid, and hatch later.[12]
  • 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.[13]
  • 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.[14]

See also

  • Insemination


  1. Libbie Henrietta Hyman (15 September 1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. ISBN 978-0-226-87013-7.
  2. Austin, Colin R. (1984). "Evolution of the copulatory apparatus". Bolletino di Zoologia. 51 (1–2): 249–269. doi:10.1080/11250008409439463.
  3. Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 396–399. ISBN 978-0-03-910284-5.
  4. Reichard, U.H. (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.
  5. Lipton, Judith Eve; Barash, David P. (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.
  6. Research conducted by Patricia Adair Gowaty. Reported by Morell, V. (1998). "Evolution of sex: A new look at monogamy". Science. 281 (5385): 1982–1983. doi:10.1126/science.281.5385.1982. PMID 9767050.
  7. Julian Lombardi (6 December 2012). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN 978-1-4615-4937-6.
  8. Diamond, Jared (1991). The Rise and Fall of the Third Chimpanzee. Radius. pp. 360 pages. ISBN 978-0091742683.
  9. Nina Wedell; Tom Tregenza; Leigh W. Simmons (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
  10. Peter D. Sozou; Robert M. Seymour (2005), "Costly but worthless gifts facilitate courtship", Proceedings of the Royal Society B, 272 (1575): 1877–1884, doi:10.1098/rspb.2005.3152, PMC 1559891, PMID 16191592
  11. Bergquist, Patricia R. (1978). Sponges. London: Hutchinson, .
  12. Thierry Lodé (2001). Les stratégies de reproduction des animaux (Reproduction Strategies in Animal Kingdom). Eds. Dunod Sciences. Paris.
  13. Blackburn, D. G. (2000). Classification of the reproductive patterns of amniotes.:" Herpetological Monographs", 371-377.
  14. Carrier, J.C.; Musick, J.A.; Heithaus, M.R., eds. (2012). Biology of Sharks and Their Relatives. CRC Press. pp. 296–301. ISBN 978-1439839249.
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