cloaca

A cloaca is a posterior body cavity that serves as a common exit for the digestive, urinary, and (in many species) reproductive tracts. It is found in birds, reptiles, amphibians, and monotremes, and in these groups it forms a single opening to the outside via the vent. In contrast, most mammals—including humans—have separate openings for the urogenital system and the digestive tract; monotremes are the notable exception within mammals, retaining a cloaca.

Anatomy and regional organization vary by species. In birds and many reptiles, the cloaca is divided into three functional parts: the coprodeal or coprodeum (receives feces), the urodeum (receives urine and the openings of reproductive ducts), and the proctodeum (the final chamber leading to the vent). The arrangement helps coordinate the excretion of solid and liquid waste and, in females and many species, the laying or passage of eggs. The external opening is termed the vent.

Functions and reproductive role are closely linked in these groups. The cloaca collects and expels waste, participates in waterproofing of excretions via uric acid in birds, and, in many species, also serves as the site of reproductive fluid exchange during mating (a behavior sometimes described as a cloacal kiss in birds). Eggs are fertilized and laid through the cloaca in many oviparous species.

Evolutionarily, the cloaca is considered ancestral to tetrapods. Most mammals have lost the cloacal condition, evolving separate anus, urinary, and genital openings; monotremes (such as the platypus and echidna) retain a cloaca. In clinical contexts, cloacal disorders can occur in birds and other cloacal-bearing animals, including prolapse or infections.