The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent-marking behavior of some reptiles,[2] marsupials,[3] amphibians, andmonotremes.[4]
Cloaca of a female birdCloaca of a male birdAroseate spoonbill excreting urine in flight
Birds reproduce using their cloaca; this occurs during a cloacal kiss in most birds.[8] Birds that mate using this method touch their cloacae together, in some species for only a few seconds, sufficient time forsperm to be transferred from the male to the female.[9] Forpalaeognaths andwaterfowl, the males do not use the cloaca for reproduction, but have aphallus.[10]
One study[11] has looked into birds that use their cloaca for cooling.[12]
Amongfalconers, the word vent is also a verb meaning "to defecate".
Among fish, a true cloaca is present only inelasmobranchs (sharks and rays) andlobe-finned fishes. Inlampreys and in someray-finned fishes, part of the cloaca remains in the adult to receive the urinary and reproductive ducts, although the anus always opens separately. Inchimaeras and mostteleosts, however, all three openings are entirely separated.[13]
With a few exceptions noted below, mammals have no cloaca. Even in the marsupials that have one, the cloaca is partially subdivided into separate regions for theanus andurethra.
Diagrams to illustrate the changes in the cloaca in mammals during development.A, early embryonic stage, showing the cloaca receiving the urinary bladder, the rectum, and the Wolffian duct, as in non-therian vertebrates.B, later stage, showing the beginning of the fold which divides the cloaca into a ventralurogenital sinus which receives theurinary bladder, Wolffian ducts, andureters, and into a dorsal part which receives therectum.C, further progress of the fold, dividing the cloaca into urogenital sinus and rectum; the ureter has separated from the Wolffian duct and is shifting anteriorly.D, completion of the fold, showing complete separation of the cloaca into ventral urogenital sinus and dorsal rectum.[14]
Inmarsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.[13] This is one of the features of marsupials (and monotremes) that suggest their basal nature, as theamniotes from which mammals evolved had a cloaca, and probably so did the earliestmammals.
Most adultplacental mammals have no cloaca. In the embryo, theembryonic cloaca divides into a posterior region that becomes part of theanus, and an anterior region that develops depending on sex: in males, it forms thepenile urethra, while in females, it develops into thevestibule orurogenital sinus that receives the urethra and vagina.[13][19] However, some placental mammals retain a cloaca as adults: those are members of the orderAfrosoricida (small mammals native to Africa) as well aspikas,beavers, and someshrews.[20][21][22][23]
In reptiles, the cloaca consists of theurodeum,proctodeum, andcoprodeum.[24][25] Some species have modified cloacae for increased gas exchange (seereptile respiration andreptile reproduction). This is where reproductive activity occurs.[26]
Someturtles, especially those specialized in diving, are highly reliant on cloacalrespiration during dives.[27] They accomplish this by having a pair of accessory air bladders connected to the cloaca, which can absorb oxygen from the water.[28]
Sea cucumbers use cloacal respiration. The constant flow of water through it has allowed variousfish,polychaete worms and evencrabs to specialize to take advantage of it while living protected inside the cucumber. At night, many of these species emerge through the anus of the sea cucumber in search of food.[29]
^abcRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 396–399.ISBN978-0-03-910284-5.
^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.
^Chimento, Nicolás; Agnolin, Federico (22 December 2014),Morphological evidence supports Dryolestoid affinities for the living Australian marsupial mole Notoryctes, PeerJ PrePrints,doi:10.7287/peerj.preprints.755
^Jonathan Kingdon; David Happold; Thomas Butynski; Michael Hoffman; Meredith Happold; Jan Kalina (2020).Mammals of Africa: Volumes I-VI. Bloomsbury Publishing. p. 216.ISBN978-1-40818-996-2. Retrieved13 November 2024.
^Lukefahr, Steven D.; McNitt, James. I; Cheeke, Peter Robert; Patton, Nephi M. (2022).Rabbit Production. CABI. p. 3.ISBN978-1-78924-978-1. Retrieved27 December 2024.
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