Intromittent organ

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An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation. [1] [2] 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. [3]

Contents

Species with intromittent organs

Invertebrates

Molluscs

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. [4]

Arachnids

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 semen from the palpal bulb 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.[ citation needed ]

In Solifugae, sperm transfer is also indirect; the male deposits a spermatophore on the ground, picks it up in his chelicerae, then inserts it into the female's genital opening.

In Opiliones (harvestmen), males have a structure called a penis, which is not present in other arachnids.

Millipedes

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 female's body. Gonopods vary greatly among millipedes, and are often used to identify species. [5]

Insects

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 intromittent organ pierces the abdominal wall and the semen is deposited into the hemocoel. [6]

Vertebrates

Fish

Gonopodium of a black molly (Poecilia sphenops). Black Molly 3 Male.jpg
Gonopodium of a black molly (Poecilia sphenops).

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. [7]

Members of Poeciliidae are small fishes that give birth to live young. In males, the anal fin is shaped into a grooved, rod-shaped organ called a gonopodium used to deliver sperm to females. [8]

Tetrapods

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. [9] 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. [10] In reptiles, the phallus has an open sulcus instead of a closed urethral tube. [11]

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. However, the penis evolved only once in the evolutionary history of amniotes. [12] In male caecilians, the intromittent organ is called the phallodeum. [13] [14]

Mammals
Baculum of a dog (Canis lupus familiaris). Armand de Montlezun Baculum Pyrenees.jpg
Baculum of a dog (Canis lupus familiaris).

All male mammals have a penis. Eulipotyphlans, 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. [15]

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. [16] [17] 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. [18] [19]

Most marsupial penises are variously forked or split into two in such ways that they resemble hemipenes; [20] [21] in different marsupial species their forms are characteristic enough to be taxonomically important. [22] [23]

Birds

Although birds reproduce through internal fertilization, 97% of males completely lack a functional intromittent organ. [24] For the 3% of birds with an intromittent organ, copulation occurs through brief insertion of the male organ into the vagina before ejaculation. [25] Alternatively, for the vast majority of birds—a group comprising nearly 10,000 species [26] —sperm transfer occurs by cloacal contact between the male and female, in a maneuver known as the "cloacal kiss". [25] Birds are one of the only groups which reproduce through internal fertilization but have repeatedly lost the intromittent organ. [27]

Male ostriches have a conical shaped penis that is wider at the base. [28]

A functional intromittent organ is known to be present in most species of Paleognathae and Anseriformes . [24] The Anseriformes (waterfowl) are a particularly interesting group to study given the high variability in intromittent organ morphology. [24] Waterfowl intromittent organs range greatly in length, are often characterized by surface elaborations (both spines and grooves), and at times spiral counter-clockwise. [29] 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. [30]

Related Research Articles

<span class="mw-page-title-main">Sex organ</span> Biological part involved in sexual reproduction

A sex organ, also known as a reproductive organ, is a part of an organism that is involved in sexual reproduction. Sex organs constitute the primary sex characteristics of an organism. Sex organs are responsible for producing and transporting gametes, as well as facilitating fertilization and supporting the development and birth of offspring. Sex organs are found in many species of animals and plants, with their features varying depending on the species.

<span class="mw-page-title-main">Mating</span> Process of pairing in biology

In biology, mating is the pairing of either opposite-sex or hermaphroditic organisms for the purposes of sexual reproduction. Fertilization is the fusion of two gametes. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization. Mating may also lead to external fertilization, as seen in amphibians, fishes and plants. For most species, mating is between two individuals of opposite sexes. However, for some hermaphroditic species, copulation is not required because the parent organism is capable of self-fertilization (autogamy); for example, banana slugs.

<span class="mw-page-title-main">Cloaca</span> Posterior opening in zoology

A cloaca, pl.: cloacae, or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts of many vertebrate animals. All amphibians, reptiles, birds, and a few mammals have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have separate orifices for evacuation and reproduction. Excretory openings with analogous purpose in some invertebrates are also sometimes called cloacae. Mating through the cloaca is called cloacal copulation and cloacal kissing.

<span class="mw-page-title-main">Sperm competition</span> Reproductive process

Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means each individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase male's chance of reproductive success. Sperm competition results in a sexual conflict between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by mate guarding or by physically removing another male's sperm prior to mating with a female. For an example, see Gryllus bimaculatus.

<span class="mw-page-title-main">Male reproductive system</span> Reproductive system of the human male

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body, and within the pelvis.

<span class="mw-page-title-main">Internal fertilization</span> Union of an egg and sperm to form a zygote within the female body

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. For internal fertilization to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract.

<span class="mw-page-title-main">Hemipenis</span> Male sex organ in squamate reptiles

A hemipenis is one of a pair of intromittent organs of male squamates. Hemipenes are usually held inverted within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. They come in a variety of shapes, depending on species, with ornamentation such as spikes.

<span class="mw-page-title-main">Aedeagus</span> Reproductive organ of male arthropods

An aedeagus is a reproductive organ of male arthropods through which they secrete sperm from the testes during copulation with a female. It can be thought of as the insect equivalent of a mammal's penis, though the comparison is fairly loose given the greater complexity of insect reproduction. The term is derived from Ancient Greek αἰδοῖα (aidoia) 'private parts' and ἀγός (agos) 'leader'.

<span class="mw-page-title-main">Gonopod</span> Specialized reproductive appendages in arthropods

Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. In males, they facilitate the transfer of sperm from male to female during mating, and thus are a type of intromittent organ. In crustaceans and millipedes, gonopods are modified walking or swimming legs. Gonopods may be highly decorated with elaborate structures which may play roles in sperm competition, and can be used to differentiate and identify closely related species. Gonopods generally occur in one or more pairs, as opposed to the single (un-paired) reproductive organs such as the aedeagus of insects or the penis of harvestmen.

A pseudo-penis is any structure found on an animal that, while superficially appearing to be a penis, is derived from a different developmental path.

<span class="mw-page-title-main">Sexual conflict</span> Term in evolutionary biology

Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.

<span class="mw-page-title-main">Penis</span> Primary sexual organ of male animals

A penis is a male sex organ that is used to inseminate female or hermaphrodite animals during copulation. Such organs occur in both vertebrates and invertebrates, including humans, but not in all male animals.

<span class="mw-page-title-main">Scrotum</span> Sac of skin that protects the testicles

In most terrestrial mammals, the scrotum or scrotal sac is a part of the external male genitalia located at the base of the penis. It consists of a sac of skin containing the external spermatic fascia, testicles, epididymides, and vasa deferentia. The scrotum will usually tighten when exposed to cold temperatures.

The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.

<span class="mw-page-title-main">Mating plug</span> Gelatinous secretion used in the mating of some species

A mating plug, also known as a copulation plug, sperm plug, vaginal plug, or sphragis, is a gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract, such as the vagina, and later hardens into a plug or glues the tract together. While females can expel the plugs afterwards, the male's sperm still gets a time advantage in getting to the egg, which is often the deciding factor in fertilization.

<span class="mw-page-title-main">Reproductive system of gastropods</span>

The reproductive system of gastropods varies greatly from one group to another within this very large and diverse taxonomic class of animals. Their reproductive strategies also vary greatly.

<span class="mw-page-title-main">Mammalian reproduction</span> Most mammals are viviparous, giving birth to live young

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.

Sexual coercion among animals is the use of violence, threats, harassment, and other tactics to help them forcefully copulate. Such behavior has been compared to sexual assault, including rape, among humans.

Cryptic female choice is a form of mate choice which occurs both in pre and post copulatory circumstances when females in certain species use physical or chemical mechanisms to control a male's success of fertilizing their ova or ovum; i.e. by selecting whether sperm are successful in fertilizing their eggs or not. It occurs in internally-fertilizing species and involves differential use of sperm by females when sperm are available in the reproductive tract.

<span class="mw-page-title-main">Copulation (zoology)</span> Animal sexual reproductive act in which a male introduces sperm into the females body

In zoology, copulation is animal sexual behavior in which a male introduces sperm into the female's body, especially directly into her reproductive tract. This is an aspect of mating. Many aquatic animals use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates occurs via cloacal copulation, known as cloacal kiss, while most mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.

References

  1. ↑ Brennan, Patricia L. R. (October 2016). "Studying Genital Coevolution to Understand Intromittent Organ Morphology". Integrative and Comparative Biology. 56 (4): 669–681. doi: 10.1093/icb/icw018 . PMID   27252198.
  2. ↑ Kelly, Diane A.; Moore, Brandon C. (October 2016). "The Morphological Diversity of Intromittent Organs". Integrative and Comparative Biology. 56 (4): 630–634. doi: 10.1093/icb/icw103 . PMID   27471228.
  3. ↑ Eberhard, W. G. (1985). Sexual selection and animal genitalia. Cambridge, MA: Harvard University Press.
  4. ↑ José Iglesias; Lidia Fuentes; Roger Villanueva, eds. (2014). Cephalopod Culture. Springer. p. 29. ISBN   9789401786485 . Retrieved 23 August 2015.
  5. ↑ "Reproductive behaviour in invertebrates". Encyclopædia Britannica. Retrieved 23 August 2015.
  6. ↑ Chapman, R.F.; Douglas, Angela E. (2013). The Insects: Structure and Function. Cambridge University Press. p. 304. ISBN   9780521113892 . Retrieved 23 August 2015.
  7. ↑ "Urogenital Anatomy of the Dogfish Shark". Maricopa Community Colleges. Retrieved 23 August 2015.
  8. ↑ Scott, Thomas (1996). ABC Biologie. Walter de Gruyter. p. 554. ISBN   9783110106619 . Retrieved 23 August 2015.
  9. ↑ "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.
  10. ↑ Annual Report of the Board of Regents of the Smithsonian Institution. 1900. p. 700. Retrieved 23 August 2015.
  11. ↑ Gredler, Marissa L., et al. "Evolution of external genitalia: insights from reptilian development." Sexual Development 8.5 (2014): 311-326.
  12. ↑ Brennan, Patricia L.R. (January 2016). "Evolution: One Penis After All". Current Biology. 26 (1): R29–R31. Bibcode:2016CBio...26..R29B. doi: 10.1016/j.cub.2015.11.024 . ISSN   0960-9822. PMID   26766229.
  13. ↑ Barrie G M Jamieson (5 January 2006). Reproductive Biology and Phylogeny of Gymnophiona: Caecilians. CRC Press. ISBN   978-1-4822-8014-2.
  14. ↑ William E. Duellman; Linda Trueb (February 1994). Biology of Amphibians. JHU Press. ISBN   978-0-8018-4780-6.
  15. ↑ "Baculum (penile bone) in mammals". Map of Life. Retrieved 23 August 2015.
  16. ↑ Grant, Tom (1995). The Platypus: A Unique Mammal. UNSW Press. p. 36. ISBN   9780868401430 . Retrieved 23 August 2015.
  17. ↑ Johnston, S.D.; Smith, B.; Pyne, M.; Stenzel, D.; Holt, W.V. (2007). "One-Sided Ejaculation of Echidna Sperm Bundles". The American Naturalist. 170 (6): E162–E164. doi: 10.1086/522847 . PMID   18171162.
  18. ↑ Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
  19. ↑ Riedelsheimer, B.; Unterberger, Pia; Künzle, H.; Welsch, U. (November 2007). "Histological study of the cloacal region and associated structures in the hedgehog tenrec Echinops telfairi". Mammalian Biology. 72 (6): 330–341. Bibcode:2007MamBi..72..330R. doi:10.1016/j.mambio.2006.10.012.
  20. ↑ Hugh Tyndale-Biscoe; Marilyn Renfree (30 January 1987). Reproductive Physiology of Marsupials. Cambridge University Press. ISBN   978-0-521-33792-2.
  21. ↑ C. Hugh Tyndale-Biscoe (2005). Life of Marsupials. Csiro Publishing. ISBN   978-0-643-09199-3.
  22. ↑ Menna Jones; Chris R. Dickman; Mike Archer; Michael Archer (2003). Predators with Pouches: The Biology of Carnivorous Marsupials. Csiro Publishing. ISBN   978-0-643-06634-2.
  23. ↑ Sadleir, Richard (1973). The Reproduction of Vertebrates. Elsevier. p. 120. ISBN   978-0-12-614250-1 . Retrieved 23 August 2015.
  24. 1 2 3 Montgomerie, R. (2010). "Sexual Conflict and the Intromittent Organs of Male Birds". In J. L. Leonard and A. Córdoba-Aguilar (ed.). The Evolution of Primary Sexual Characters in Animals. New York: Oxford University Press. pp. 453–470. ISBN   9780199886753.
  25. 1 2 Briskie, J. V.; R. Montgomerie (1997). "Sexual Selection and the Intromittent Organ of Birds". Journal of Avian Biology. 28 (1): 73–86. doi:10.2307/3677097. JSTOR   3677097.
  26. ↑ 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. Bibcode:2013CBio...23.1065H. doi: 10.1016/j.cub.2013.04.062 . PMID   23746636.
  27. ↑ Briskie, James V.; Montgomerie, Robert (1997). "Sexual Selection and the Intromittent Organ of Birds". Journal of Avian Biology. 28 (1): 73–86. doi:10.2307/3677097. JSTOR   3677097.
  28. ↑ Brennan, P.L.R.; Prum, R.O.; Hayssen, Virginia (February 2012). "The erection mechanism of the ratite penis" (PDF). Journal of Zoology. 286 (2): 140–144. doi:10.1111/j.1469-7998.2011.00858.x.
  29. ↑ 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   1855079 . PMID   17476339.
  30. ↑ Adler, M. (2010). "Sexual Conflict in Waterfowl: Why do Females Resist Extrapair Copulations?". Behavioral Ecology. 21: 182–192. doi: 10.1093/beheco/arp160 .
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