Penis
In many animals, a penis is the male sexual organ used to inseminate females 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 likely possessed a penis.Birds
Most male birds have a cloaca, but not a penis. Among bird species with a penis are paleognathes and Anatidae. 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 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 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 : 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.
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 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 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 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
A stag's penis forms an S-shaped curve when it is not erect, and is retracted into its sheath by the retractor penis muscle. Some deer species spray urine on their bodies by urinating from an erect penis. One type of scent-marking behavior in elk is known as "thrash-urination, which typically involves palpitation of the erect penis. A male elk's urethra points upward so that urine is sprayed almost at a right angle to the penis. A sambar stag will mark himself by spraying urine directly in the face with a highly mobile penis, which is often erect during its rutting activities. Red deer stags often have erect penises during combat.Cetaceans
are located inside the body. Male cetaceans have two slits, the genital groove concealing the penis and one further behind for the anus. Cetaceans have fibroelastic penises, similar to those of Artiodactyla. The tapering tip of the cetacean penis is called the pars intrapraeputialis or terminal cone. The blue whale has the largest penis of any organism on the planet, typically measuring. Accurate measurements are difficult to take because its erect length can only be observed during mating, which occurs underwater. The penis on a right whale can be up to – the testicles, at up to in length, in diameter, and weighing up to, are also by far the largest of any animal on Earth.On at least one occasion, a dolphin towed bathers through the water by hooking his erect penis around them. Between captive male dolphins—including bottlenose dolphins and Amazon river dolphins—homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting the penis in the other's genital slit and sometimes anus.
Perissodactyls
s have a vascular penis. When non-erect, it is quite flaccid and contained within the prepuce.Tapirs have exceptionally long penises relative to their body size. The glans of the Malayan tapir resembles a mushroom, and is similar to the glans of the horse. The penis of the Sumatran rhinoceros contains two lateral lobes and a structure called the processus glandis.
Carnivores
All members of Carnivora have a baculum. Canine penises have a structure at the base called the bulbus glandis.During copulation, the spotted hyena inserts his penis through the female's pseudo-penis instead of directly through the vagina, which is blocked by the false scrotum and testicles. Once the female retracts her clitoris, the male enters the female by sliding beneath her, an operation facilitated by the penis' upward angle. The pseudo-penis closely resembles the male hyena's penis, but can be distinguished from the male's genitalia by its greater thickness and more rounded glans. In male spotted hyenas, as well as females, the base of the glans is covered with penile spines.
Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which is a trigger for ovulation. Lions also have barbed penises. Male felids urinate backwards by curving the tip of the glans penis backward. When male cheetahs urine-mark their territories, they stand one meter away from a tree or rock surface with the tail raised, pointing the penis either horizontally backward or 60° upward.
The male fossa has an unusually long penis and baculum, reaching to between his front legs when erect with backwards-pointing spines along most of its length. The male fossa has scent glands near the penis, with the penile glands emitting a strong odor.
The beech marten's penis is larger than the pine marten's, with the bacula of young beech martens often outsizing those of old pine martens.
Raccoons have penis bones which bend at a 90 degree angle at the tip. The extrusibility of a raccoon's penis can be used to distinguish mature males from immature males.
Male walruses possess the largest penis bones of any land mammal, both in absolute size and relative to body size.
The adult male American mink's penis is long, and is covered by a sheath. The baculum is well-developed, being triangular in cross section and curved at the tip.