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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),^[3] Anatidae (ducks, geese and swans),^[4] and a very few other species (including flamingoes^{[citation needed]} and chickens^[5]). 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.^[5] It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curls up inside the cloaca. The lake duck (also called Argentine blue-bill) has the largest penis in relation to body size of all vertebrates; while usually about half the body size (20 cm), a specimen with a penis 42.5 cm long is documented.

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.^[6] 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.^[6]

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.^[7][8] 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.

Male and female emus are similar in appearance,^[9] although the male's penis can become visible when it defecates.^[10]

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.^[11]

MammalsWikimedia Commons has media related to Mammal penis.

As with any other bodily attribute, the length and girth of the penis can be highly variable between mammals of different species.^[12][13] In many mammals, the size of a flaccid penis is smaller than its erect size. In the realm of absolute size, the smallest vertebrate penis belongs to the common shrew (5 mm or 0.2 inches).^{[citation needed]}

A bone called the baculum or os penis is present in most mammals but absent in humans, cattle and horses.

In mammals the penis is divided into three parts:^[14]

• 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 cm (1.5 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.^[15]

Hoofed mammals

When mating, the tip of a male pronghorn's penis is often the first part to touch the female pronghorn.^[16] The pronghorn's penis is about 5 inches long, and is shaped like an ice pick.^[17] The front of a pronghorn's glans penis is relatively flat, while the back is relatively thick.^[18] The male pronghorn usually ejaculates immediately after intromission.^[19][20]

The penis of a dromedary camel is covered by a triangular penile sheath opening backwards,^[21] and is about 60 cm (24 in) long.^[22][23] 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.^[24][25]

Wikimedia Commons has media related to Bull penis.

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.^[26][27][28]

Bulls, rams and boars have an S-shaped penis with a sigmoid flexure which straightens out during erection.^{[citation needed]} A boar's penis, which rotates rhythmically during copulation,^[29] is about 18 inches long, and ejaculates about a pint of semen.^[30] 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.^[31] Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the prepuce (foreskin, or sheath).

Tapirs have exceptionally long penises relative to their body size.^{[32][33][34][35]} The glans of the Malayan tapir resembles a mushroom, and is similar to the glans of the horse.^[36]

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.^[37] Some deer species spray urine on their bodies by urinating from an erect penis.^[38] One type of scent-marking behavior in elk is known as "thrash-urination,^[39][40] which typically involves palpitation of the erect penis.^[40][41][42] A male elk's urethra points upward so that urine is sprayed almost at a right angle to the penis.^[40] 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.^[43] Red deer stags often have erect penises during combat.^[44]

Carnivorans

All members of Carnivora (except hyenas) have a baculum.^[45] Canine penises have a structure at the base called the bulbus glandis.^[46][47]

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 testes. Once the female retracts her clitoris, the male enters the female by sliding beneath her, an operation facilitated by the penis's upward angle.^[48][49] 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.^[50] In male spotted hyenas, as well as females, the base of the glans is covered with penile spines.^[47][51][52]

Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines.^[53] 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.^[54][55] Male felids urinate backwards by curving the tip of the glans penisbackward.^[56][57] 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.^[58]

The male fossa has an unusually long penis and baculum (penis bone), reaching to between his front legs when erect, with an average thickness of 20 mm (0.8 in). The glans extends about halfway down the shaft and is spiny except at the tip. In comparison, the glans of felids is short and spiny, while that of viverrids is smooth and long.^[59] Fossa mating includes a copulatory tie, which may be enforced by the male's spiny penis. This unusually lengthy mating is due to the physical nature of the male's erect penis, which has backwards-pointing spines along most of its length.^[60] The male fossa has scent glands near the penis, with the penile glands emitting a strong odor.^[59]

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.^[61]

Raccoons have penis bones which bend at a 90 degree angle at the tip.^[62] The extrusibility of a raccoon's penis can be used to distinguish mature males from immature males.^[63][64]

Male walruses possess large penis bones, up to 63 cm (25 in) in length, the largest of any land mammal, both in absolute size and relative to body size.^[65][66]

The adult male American mink's penis is 2.2 in (5.6 cm) long, and is covered by a sheath. The baculum is well-developed, being triangular in cross section and curved at the tip