Cloaca

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 palaeognaths and waterfowl, the males do not use the cloaca for reproduction, but have a phallus.

One study has looked into birds that use their cloaca for cooling.

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.

Synapsids including 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.

Non-mammalian synapsids

A cloacal impression in a trace fossil was created by a non-mammalian synapsid, likely a caseid, has been reported from the early Permian of Germany. This indicates the cloaca was located at the base of the tail and was directed downwards.

Monotremes

The monotremes (egg-laying mammals) possess a true cloaca.

Marsupials

In marsupials, 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 had a cloaca, and probably so did the earliest mammals.

Unlike other marsupials, marsupial moles have a true cloaca.

Placental 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. However, some placental mammals retain a cloaca as adults: those are members of the order Afrosoricida (small mammals native to Africa) as well as pikas, beavers, and some shrews.

Being placental mammals, 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. Some species have modified cloacae for increased gas exchange (see reptile respiration and reptile reproduction). This is where reproductive activity occurs.

Cloacal respiration in animals

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.

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.