Penis

In many animals, a penis (; pl.: penises or penes) is the male sexual organ used to inseminate females (or hermaphrodites) during copulation. Such organs occur in both vertebrates and invertebrates, but males do not bear a penis in every animal species. Furthermore, penises are not necessarily homologous. The term penis applies to many intromittent organs, but not to all. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. Even within the Vertebrata, there are morphological variants with specific terminology, such as hemipenes. 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.

Vertebrates The last common ancestor of all living amniotes (mammals, birds and reptiles) likely possessed a penis.

Birds

Most male birds (e.g., roosters and turkeys) have a cloaca (also present on the female), but not a penis. Among bird species with a penis are paleognathes (tinamous and ratites) and Anatidae (ducks, geese and swans). 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. It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curls up inside the cloaca. While most male birds have no external genitalia, male waterfowl (Anatidae) have a phallus. 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. The phallus that male waterfowl have evolved everts out of their bodies (in a clockwise coil) and aids in inseminating females without their cooperation. The male waterfowl evolution of a phallus to forcefully copulate with females has led to counteradaptations in females in the form of vaginal structures called dead end sacs and clockwise coils. These structures make it harder for males to achieve intromission. The clockwise coils are significant because the male phallus everts out of their body in a counter-clockwise spiral; therefore, a clockwise vaginal structure would impede forceful copulation. Studies have shown that the longer a male's phallus is, the more elaborate the vaginal structures were.

The lake duck is notable for possessing, in relation to body length, the longest penis of all vertebrates; the penis, which is typically coiled up in flaccid state, can reach about the same length as the animal himself when fully erect, but is more commonly about half the bird's length. It is theorized that the remarkable size of their spiny penises with bristled tips may have evolved in response to competitive pressure in these highly promiscuous birds, removing sperm from previous matings in the manner of a bottle brush. The lake duck has a corkscrew shaped penis. Male and female emus are similar in appearance, although the male's penis can become visible when he defecates. The male tinamou has a corkscrew shaped penis, similar to those of the ratites and to the hemipenis of some reptiles. Females have a small phallic organ in the cloaca which becomes larger during the breeding season.

Mammals

As with any other bodily attribute, the length and girth of the penis can be highly variable between mammals of different species. In many mammals, the size of a flaccid penis is smaller than its erect size. A bone called the baculum is present in most mammals but absent in humans, cattle and horses. In mammals, the penis is divided into three parts:

Roots (crura): these begin at the caudal border of the pelvic ischial arch. Body: the part of the penis extending from the roots. Glans: the free end of the penis. The internal structures of the penis consist mainly of cavernous, erectile tissue, which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae). Some mammals have a lot of erectile tissue relative to connective tissue, for example horses. Because of this a horse's penis can enlarge more than a bull's penis. The urethra is on the ventral side of the body of the penis. As a general rule, a mammal's penis is proportional to its body size, but this varies greatly between species – even between closely related ones. For example, an adult gorilla's erect penis is about 4.5 cm (1.8 in) in length; an adult chimpanzee, significantly smaller (in body size) than a gorilla, has a penis size about double that of the gorilla. In comparison, the human penis is larger than that of any other primate, both in proportion to body size and in absolute terms.

Artiodactyls The penises of even-toed ungulates are curved in an S-shape when not erect. In bulls, rams and boars, the sigmoid flexure of the penis straightens out during erection. When mating, the tip of a male pronghorn's penis is often the first part to touch the female pronghorn. The pronghorn's penis is about 13 cm (5 in) long, and is shaped like an ice pick. The front of a pronghorn's glans penis is relatively flat, while the back is relatively thick. The male pronghorn usually ejaculates immediately after intromission. The penis of a dromedary camel is covered by a triangular penile sheath opening backwards, and is about 60 cm (24 in) long. The camelmen often aid the male to enter his penis into the female's vulva, though the male is considered able to do it on his own. Copulation time ranges from 7 to 35 minutes, averaging 11–15 minutes.

Bulls have a fibro-elastic penis. Given the small amount of erectile tissue, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure. The male genitalia of mouse deer are similar to those of pigs. A boar's penis, which rotates rhythmically during copulation, is about 46 cm (18 in) long, and ejaculates about a pint of semen. Wild boars have a roughly egg-sized sack near the opening of the penis, which collects urine and emits a sharp odour. The purpose of this is not fully understood.

Deer

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