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, marsupials, amphibians, and monotremes.
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 ventral urogenital sinus which receives the urinary bladder, Wolffian ducts, and ureters, and into a dorsal part which receives the rectum. 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.
The word is from the Latin verb cluo, "to cleanse", thus the noun cloaca, "sewer, drain".
Cloaca of a female bird
Cloaca of a male bird
Birds reproduce using their cloaca; this occurs during a cloacal kiss in most birds. Birds that mate using this method touch their cloacae together, in some species for only a few seconds, sufficient time for sperm to be transferred from the male to the female. For some birds, such as ostriches, cassowaries, kiwi, geese, and some species of swans and ducks, the males do not use the cloaca for reproduction, but have a phallus. In those, the penis helps ensure water does not wash away the male's sperm during copulation.
One study has looked into birds that use their cloaca for cooling.
In marsupials (and a few birds), the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. This is one of the features of marsupials (and monotremes) that suggest their basal nature, as the amniotes from which mammals evolved possessed a cloaca, and the earliest animals to diverge into the mammalian class would most likely have had this feature, too.
Unlike other marsupials, marsupial moles have a true cloaca, a fact that has been used to argue against a marsupial identity for these mammals.
Most adult placental mammals have no remaining trace of the cloaca. In the embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the urethra and vagina, while in males it forms the entirety of the penile urethra. However, the tenrecs and golden moles, small placental mammals native to Africa, as well as some shrews retain a cloaca as adults.
Some aquatic turtle species can breathe underwater using a process known as cloacal respiration. In this process the turtles pump water into their cloacal orifice (labeled 1) by contracting muscles in their inguinal pocket. The water then travels to the cloacal bursae (labeled 2), which are a pair of internal pouch-like structures. The cloacal bursae are lined with long fimbriae (labeled 3), which is the site of gas exchange.
Some turtles, especially those specialized in diving, are highly reliant on cloacal respiration during dives. They accomplish this by having a pair of accessory air bladders connected to the cloaca which can absorb oxygen from the water. Various fish, as well as polychaete worms and even crabs, are specialized to take advantage of the constant flow of water through the cloacal respiratory tree of sea cucumbers while simultaneously gaining the protection of living within the sea cucumber itself. At night, many of these species emerge from the anus of the sea cucumber in search of food.
^Hoffman, Ty C. M.; Walsberg, Glenn E.; DeNardo, Dale F. (2007). "Cloacal evaporation: an important and previously undescribed mechanism for avian thermoregulation". The Journal of Experimental Biology. 210 (5): 741-9. doi:10.1242/jeb.02705. PMID17297135.
^Hager, Yfke (2007). "Cloacal Cooling". The Journal of Experimental Biology. 210 (5): i. doi:10.1242/jeb.02737.
^ 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. 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. Cite journal requires |journal= (help)[unreliable source?]
^Symonds, Matthew R. E. (February 2005). "Phylogeny and life histories of the 'Insectivora': controversies and consequences". Biological Reviews. 80 (1): 93-128. doi:10.1017/S1464793104006566.