A cloaca ( ), : cloacae ( or ), or vent, is the rear Body orifice that serves as the only opening for the digestive (rectum), reproductive, and urination (if present) of many vertebrate animals. All , , birds, cartilaginous fish
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,[Harris, R. L., Elissa Cameron, Davies, N. W., & Nicol, S. C. (2016). Chemical cues, hibernation and reproduction in female short-beaked echidnas (Tachyglossus aculeatus setosus): implications for sexual conflict. In Chemical Signals in Vertebrates 13 (pp. 145–166). Springer, Cham.]
Etymology
The word is from the
Latin verb
cluo, "(I) cleanse", thus the noun
cloaca, "
sanitary sewer, drain".
[Cassell's Latin Dictionary, Marchant, J.R.V, & Charles, Joseph F., (Eds.), Revised Edition, 1928, p.103]
Birds
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
and
waterfowl, the males do not use the cloaca for reproduction, but have a
Bird penis.
One study
Among Falconry, the word vent is also a verb meaning "to defecate".
Fish
Among fish, a true cloaca is present only in
(sharks and rays) and
. In
and in some
, part of the cloaca remains in the adult to receive the urinary and reproductive ducts, although the anus always opens separately. In
and most
, however, all three openings are entirely separated.
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
(egg-laying mammals) possess a true cloaca.
Marsupials
In
, 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 from which mammals evolved had a cloaca, and probably so did the earliest mammalia.
]
Unlike other marsupials, have a true cloaca.
Placentals
Most adult 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 Vulval vestibule or urogenital sinus that receives the urethra and vagina.
Being placental animals, humans have an embryonic cloaca which divides into separate tracts during the development of the urinary and reproductive organs. However, a few human congenital disorders result in persons being born with a cloaca, including persistent cloaca and sirenomelia (mermaid syndrome).
Reptiles
In reptiles, the cloaca consists of the urodeum, proctodeum, and coprodeum.
Cloacal respiration in animals
Some , especially those specialized in diving, are highly reliant on cloacal respiration during dives.[ The Straight Dope - Is it true turtles breathe through their butts?]
Sea cucumbers use cloacal respiration. The constant flow of water through it has allowed various fish, polychaete worms and even 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.[ Aquarium Invertebrates by Rob Toonen, Ph.D.]
See also
