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]
Etymology
The word is from the
Latin verb cluo, "(I) cleanse", thus the noun cloaca, "
sewer, drain".[4][5][6]
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".
Fish
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]
Mammals
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.
Monotremes
The
monotremes (egg-laying mammals) possess a true cloaca.[14]
In
marsupials, 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
vestibule or
urogenital 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.