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,[1] marsupials,[2] amphibians, and monotremes.[3]
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.[7] 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.[8] 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.[9]
One study[10] has looked into birds that use their cloaca for cooling.[11]
The cloaca in birds may also be referred to as the vent. Among falconers, the word vent is also a verb meaning "to defecate".
Among fish, a true cloaca is present only in elasmobranchs (sharks and rays) and lobe-finned fishes. In lampreys and in some ray-finned fishes, part of the cloaca remains in the adult to receive the urinary and reproductive ducts, although the anus always opens separately. In chimaeras and most teleosts, however, all three openings are entirely separated.[12]
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 the anus and urethra.
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.[13]
Inmarsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.[12] This is one of the features of marsupials (and monotremes) that suggest their basal nature, as the amniotes from which mammals evolved had a cloaca, and probably so did the earliest mammals.
Most adult placental mammals have no cloaca. In the embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that develops depending on sex: in males, it forms the penile urethra, while in females, it develops into the vestibuleorurogenital sinus that receives the urethra and vagina.[12][18] However, some placental mammals retain a cloaca as adults: those are the tenrecs and golden moles (small mammals native to Africa), as well as some shrews.[19]
In reptiles, the cloaca consists of the urodeum, proctodeum, and coprodeum.[20][21] Some species have modified cloacae for increased gas exchange (see reptile respiration and reptile reproduction). This is where reproductive activity occurs.[22]
Some turtles, especially those specialized in diving, are highly reliant on cloacal respiration during dives.[23] They accomplish this by having a pair of accessory air bladders connected to the cloaca, which can absorb oxygen from the water.[24]
Sea cucumbers use cloacal respiration. The constant flow of water through it has allowed various fish, polychaete worms and even crabs 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.[25]
^ abcRomer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 396–399. ISBN978-0-03-910284-5.
^Libbie Henrietta Hyman, A laboratory manual for comparative vertebrate anatomy. 1922 (1920s)
^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
^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. PMID15727040. S2CID21132866.
