This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2013) (Learn how and when to remove this template message)
An intromittent organ is a general term for an external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most non-mammalian aquatic species fertilize their eggs externally, although there are exceptions. For many species in the animal kingdom, the male intromittent organ is a hallmark characteristic of internal fertilization.
Species with intromittent organsEdit
Male cephalopods have a specialized arm, the hectocotylus, which is inserted into the female's mantle cavity to deliver a spermatophore during copulation. In some species, the hectocotylus breaks off inside the female's mantle cavity; in others, it can be used repeatedly to copulate with different females.
In spiders the intromittent organs are the male pedipalps, even though these are not primarily sexual organs, but serve as indirect mating organs; in the male the pedipalps have hollow, clubbed tips, often of complex internal anatomy. The sexually mature male typically deposits his semen onto a specially woven silken mat, then sucks the emission into his pedipalps. In mating, he inserts the openings of the pedipalps in turn into the epigyne, the female external genital structure.
In most millipedes, sperm-transfer is performed by one or two pair of modified legs called gonopods, which are often on the seventh body segment. During mating, a male bends his body to collect a spermatophore from the genital pore of his third segment, and inserts it into the females body. Gonopods vary greatly among millipedes, and are often used to identify species.
Male insects possess an aedeagus, whose function is directly analogous to that of the vertebrate penis. Some insects also have claspers. Male moths have an additional organ called the juxta, which supports the aedeagus. These however are generalisations, and insect genitalia vary enormously in anatomy and in application. For example, some insects, most notoriously the Cimicidae and some Strepsiptera practise traumatic insemination, in which the intermittent organ pierces the abdominal wall and the semen is deposited into the hemocoel.
In male members of Chondrichthyes (sharks and rays), as well as now-extinct placoderms, the pelvic fins bear specialized claspers. During copulation, one clasper is inserted into the female's cloaca, and sperm is flushed by the male's body through a groove into the female.
In lizards and snakes, males possess paired hemipenes, each of which is usually grooved to allow sperm transport and spiny or rough at the tip to allow firm attachment to the female. To become erect, a hemipenis is evaginated (turned inside out) through muscle action and engorgement with blood. Only one is inserted into the female's cloaca at a time.
In some turtles, crocodiles, some birds, and in all mammals, males possess a penis centered along the midline of the body. During copulation it becomes erect due to engorgement with blood or lymph, though in many animals it also contains a stiff or even bony support structure. When not in use, its soft penile tissue is usually flaccid, and depending on the species, may be retracted into the body. The anatomy of the penis varies widely according to species.
All male mammals have a penis. Insectivores, bats, rodents, carnivorans, and most primates (but not humans) have a bone called the baculum or os penis that permanently stiffens the penis. During copulation, blood engorges the already-stiff penis resulting in a full erection.
Monotreme penes are variously unusual; the platypus has a penis with a two-lobed (bifid) tip though the whole shaft is inserted in mating, possibly to engage both of the uterine branches, but the echidna's penis actually has four heads, only two of which function at a time. Both monotremes and marsupial moles are the only mammals with internal penises, located on the cloacal wall instead of outside of it as in other mammals.
Most marsupial penises are variously forked or split into two in such ways that they resemble hemipenes; in different marsupial species their forms are characteristic enough to be taxonomically important.
Although birds reproduce through internal fertilization, 97% of males completely lack a functional intromittent organ. For the 3% of birds with an intromittent organ, copulation occurs through brief insertion of the male organ into the vagina before ejaculation. Alternatively, for the vast majority of birds—a group comprising nearly 10,000 species  —sperm transfer occurs by cloacal contact between the male and female, in a maneuver known as the “cloacal kiss”.
A functional intromittent organ is known to be present in most species of Paleognathae and Anseriformes. The Anseriformes (waterfowl) are a particularly interesting group to study given the high variability in intromittent organ morphology. Waterfowl intromittent organs range greatly in length, are often characterized by surface elaborations (both spines and grooves), and at times spiral counter-clockwise. Male ducks have a penis that is coiled along the ventral wall of the cloaca when flaccid and which may have an elaborate spiral shape when erect. Waterfowl intromittent organ variation is most likely due to an intersexual arms race resulting from a mating system in which forced extra-pair copulations are frequent.
- Kardong, Kenneth V. (1995). Vertebrates: Comparative Anatomy, Function, Evolution. Dubuque, Iowa: Wm. C. Brown Publishers. pp. 567–570. ISBN 0-06-921991-5.
- Brennan, Patricia LR. "Studying genital coevolution to understand intromittent organ morphology." Integrative and comparative biology 56.4 (2016): 669-681.
- Kelly, Diane A., and Brandon C. Moore. "The morphological diversity of intromittent organs." Integrative and comparative biology 56.4 (2016): 630-634.
- Eberhard, W. G. (1985). Sexual selection and animal genitalia. Cambridge, MA: Harvard University Press.
- Iglesias, Fuentes, Villanueva (2014). Cephalopod Culture. Springer. p. 29. Retrieved 23 August 2015.
- "Reproductive behaviour in invertebrates". Encyclopædia Britannica. Retrieved 23 August 2015.
- Chapman, Simpson, Douglas. The Insects: Structure and Function. p. 304. Retrieved 23 August 2015.
- "Urogenital Anatomy of the Dogfish Shark". Maricopa Community Colleges. Retrieved 23 August 2015.
- Scott, Thomas. ABC Biologie. p. 554. Retrieved 23 August 2015.
- "Hemipenes of snakes and lizards". University of Colorado Boulder Museum of Natural History. Archived from the original on 26 August 2015. Retrieved 23 August 2015.
- Annual Report of the Board of Regents of the Smithsonian Institution. 1900. p. 700. Retrieved 23 August 2015.
- "Baculum (penile bone) in mammals". Map of Life. Retrieved 23 August 2015.
- Grant, Tom (1995). The Platypus: A Unique Mammal. UNSW Press. p. 36. Retrieved 23 August 2015.
- "Echidna ejaculation is a little one-sided". Scientopia. Retrieved 23 August 2015.
- Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
- Riedelsheimer, B., Unterberger, P., Künzle, H. and U. Welsch. 2007. Histological study of the cloacal region and associated structures in the hedgehog tenrec Echinops telfairi. Mammalian Biology 72(6): 330-341.
- Hugh Tyndale-Biscoe; Marilyn Renfree (30 January 1987). Reproductive Physiology of Marsupials. Cambridge University Press. ISBN 978-0-521-33792-2.
- C. Hugh Tyndale-Biscoe (2005). Life of Marsupials. Csiro Publishing. ISBN 978-0-643-09199-3.
- Sadleir, Richard (1973). The Reproduction of Vertebrates. p. 120. ISBN 978-0-12-614250-1. Retrieved 23 August 2015.
- Montgomerie, R. (2010). "Sexual Conflict and the Intromittent Organs of Male Birds". In J. L. Leonard and A. Córdoba-Aguilar. The Evolution of Primary Sexual Characters in Animals. New York: Oxford University Press. pp. 453–470.
- Briskie, J. V.; R. Montgomerie (1997). "Sexual Selection and the Intromittent Organ of Birds". Journal of Avian Biology. 28: 73–86. doi:10.2307/3677097.
- Herrera, A. M.; S. G. Shuster; C. L. Perriton; M. J. Cohn (2013). "Developmental Basis of Phallus Reduction During Bird Evolution". Current Biology. 23 (12): 1065–1074. doi:10.1016/j.cub.2013.04.062. PMID 23746636.
- Brennan, Patricia LR, and Richard O. Prum. "The erection mechanism of the ratite penis." Journal of Zoology 286.2 (2012): 140-144.
- Birkhead, P. L. R.; R. O. Prum; K. G. McCracken; M. D. Sorenson; R. E. Wilson; T. R. Birkhead (2007). Pizzari, Tom, ed. "Coevolution of Male and Female Genital Morphology in Waterfowl". PLOS ONE. 2 (5): e418. Bibcode:2007PLoSO...2..418B. doi:10.1371/journal.pone.0000418. PMC . PMID 17476339.
- Adler, M. (2010). "Sexual Conflict in Waterfowl: Why do Females Resist Extrapair Copulations?". Behavioral Ecology. 21: 182–192. doi:10.1093/beheco/arp160.