Mammals
With a few exceptions noted below, mammals have no cloaca. Even in those that have one, the cloaca is partially sub-divided into separate regions for the anus and urethra.
- Monotremes and marsupials
The monotremes (egg-laying mammals) possess a true cloaca.
- Marsupials
In marsupials (and a few birds), the genital tract is separate from the anus, but a trace of 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 likely have had this feature too.
- Placentals
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, retain a cloaca as adults.
- Humans
Being placental animals, humans only have an embryonic cloaca, which is split up 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).
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