The phallus that male waterfowl have evolved everts out of their bodies (in a clockwise coil) and aids in inseminating females without their cooperation. Most birds mate with the males balancing on top of the females and touching cloacas in a 'cloacal kiss' this makes forceful insemination very difficult. While most male birds have no external genitalia, male waterfowl ( Anatidae) have a phallus. It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curls up inside the cloaca. A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood. Among bird species with a penis are paleognathes ( tinamous and ratites) and Anatidae (ducks, geese and swans). Most male birds (e.g., roosters and turkeys) have a cloaca (also present on the female), but not a penis. In most species of animals in which there is an organ that might be described as a penis, it has no major function other than intromission, or at least conveying the sperm to the female, but in the placental mammals the penis bears the distal part of the urethra, which discharges both urine during urination and semen during copulation. Even within the Vertebrata there are morphological variants with specific terminology, such as hemipenes. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. The term penis applies to many intromittent organs, but not to all. Furthermore, penises are not necessarily homologous. Such organs occur in many animals, both vertebrate and invertebrate, but males do not bear a penis in every animal species. A penis ( / ˈ p iː n ɪ s/ PL penises or penes) is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation.

Size of a gorilla penis