The last common ancestor of all living
amniotes (mammals, birds and reptiles) likely possessed a penis.
Birds penis 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. 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. 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:
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.
Red deer stags often have erect penises during combat.
Cetaceans s on display at the
Icelandic Phallological Museum Cetaceans' reproductive organs are located inside the body. Male
cetaceans (whales, dolphins, and porpoises) 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 Stallions (male horses)
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. 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 (
Procyon lotor) All members of
Carnivora (except
hyenas) have a baculum.
Canine penises have a structure at the base called the
bulbus glandis. 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 (penis bone), 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.
Bats Males of
Racey's pipistrelle bat have a long, straight penis with a notch between the shaft and the narrow, egg-shaped
glans penis. Near the top, the penis is haired, but the base is almost naked. In the
baculum (penis bone), the shaft is long and narrow and slightly curved. The length of the penis and baculum distinguish
P. raceyi from all comparably sized African and Malagasy vespertilionids. In males, penis length is and baculum length is . Copulation by male
greater short-nosed fruit bats is dorsoventral and the females lick the shaft or the base of the male's penis, but not the glans which has already penetrated the vagina. While the females do this, the penis is not withdrawn and research has shown a positive relationship between length of the time that the penis is licked and the duration of copulation. Post copulation genital grooming has also been observed.
Rodents The glans penis of the
marsh rice rat is long and robust, averaging long and broad, and the baculum (penis bone) is long. As is characteristic of
Sigmodontinae, the marsh rice rat has a complex penis, with the distal (far) end of the baculum ending in three digits. The central digit is notably larger than those at the sides. a single pair of
preputial glands is present at the penis. As is usual for sigmodontines, there are two pairs of
ventral prostate glands and a single pair of
anterior and
dorsal prostate glands. Part of the end of the
vesicular gland is irregularly folded, not smooth as in most oryzomyines. In
Pseudoryzomys, the
baculum (penis bone) displays large protuberances at the sides. In the
cartilaginous part of the baculum, the central digit is smaller than those at the sides. In
Drymoreomys, there are three digits at the tip of the penis, of which the central one is the largest. In
Thomasomys ucucha, the
glans penis is rounded, short, and small and is superficially divided into left and right halves by a trough at the top and a ridge at the bottom. The
glans penis of a male
cape ground squirrel is large with a prominent baculum. Unlike other squirrel species,
red squirrels have long, thin, and narrow penises, without a prominent baculum.
Winkelmann's mouse can easily be distinguished from its close relatives by the shape of its penis, which has a partially corrugated
glans. The foreskin of a
capybara is attached to the anus in an unusual way, forming an anogenital invagination.
Primates It has been postulated that the shape of the human penis may have been
selected by
sperm competition. The shape could have favored displacement of
seminal fluids implanted within the female reproductive tract by rival males: the
thrusting action which occurs during sexual intercourse can mechanically remove seminal fluid out of the
cervix area from a previous mating. The penile morphology of some types of
strepsirrhine primates has provided information about their taxonomy. Male
galago species possess very distinctive penile morphology that can be used to classify species. The
northern greater galago penis is on average in length, with doubled headed or even tridentate spines pointing towards the body. They are less densely packed than in
Otolemur crassicaudatus. The adult male of each vervet monkey species has a pale blue
scrotum and a red penis, and male
proboscis monkeys have a red penis with a black scrotum. Male
baboons and
squirrel monkeys sometimes gesture with an erect penis as both a warning of impending danger and a threat to predators. In male
squirrel monkeys, this gesture is used for social communication.
Humans The human penis is an external sex organ of
male humans. It is a
reproductive,
intromittent organ that additionally serves as the
urinal duct. The main parts are the root of the penis (radix): It is the attached part, consisting of the
bulb of penis in the middle and the
crus of penis, one on either side of the bulb; the body of the penis (corpus); and the
epithelium of the penis consists of the shaft
skin, the
foreskin, and the
preputial mucosa on the inside of the foreskin and covering the
glans penis. The human penis is made up of three columns of
tissue: two
corpora cavernosa lie next to each other on the
dorsal side and one
corpus spongiosum lies between them on the
ventral side. The
urethra, which is the last part of the
urinary tract, traverses the corpus spongiosum, and its opening, known as the
meatus, lies on the tip of the glans penis. It is a passage both for
urine and for the
ejaculation of
semen. In males, the expulsion of
urine from the body is done through the penis. The urethra drains the bladder through the
prostate gland, where it is joined by the
ejaculatory duct, and then onward to the penis. An erection is the stiffening and rising of the penis, which occurs during
sexual arousal, though it can also happen in non-sexual situations. Ejaculation is the ejecting of
semen from the penis and is usually accompanied by
orgasm. A series of muscular contractions delivers semen, containing male
gametes known as sperm cells or
spermatozoa, from the penis. The most common form of genital alteration is
circumcision, the removal of part or all of the foreskin for various cultural, religious, and more rarely medical reasons. There is controversy surrounding circumcision. , a
systematic review of 15,521 men, who were measured by health professionals rather than themselves, concluded that the average length of an
erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches).
Marsupials Most marsupials, except for the two largest species of kangaroos and
marsupial moles (assuming the latter are true marsupials),
have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.
Monotremes Monotremes and
marsupial moles are the only mammals in which the penis is located inside the cloaca. Male
echidnas have a bilaterally symmetrical, rosette-like, four-headed penis. During mating, the heads on one side "shut down" and do not grow in size; the other two are used to release semen into the female's two-branched reproductive tract. The heads used are swapped each time the mammal copulates. When not in use, the penis is retracted inside a preputial sac in the cloaca. The male echidna's penis is long when erect, and its shaft is covered with
penile spines. The penis is nearly a quarter of his body length when erect.
Others The penis of the
bush hyrax is complex and distinct from that of the other hyrax genera. It has a short, thin appendage within a cup-like glans penis and measures greater than when erect. Additionally, it has been observed that the bush hyrax also has a greater distance between the anus and preputial opening in comparison to other hyraxes. An adult
elephant has the largest penis of any land animal. An elephant's penis can reach a length of and a diameter of at the base. During
musth, a male elephant may urinate with his penis still in the
sheath, which causes the urine to spray on the
hind legs. An elephant's penis is very mobile, being able to move independently of the male's pelvis, and the penis curves forward and upward prior to
mounting another elephant. When the male armadillo
Chaetophractus villosus is
sexually aroused, species determination is easier. Its penis can be as long as , and usually remains completely withdrawn inside a skin receptacle. Scientists conducting studies on the
C. villosus penis muscles revealed this species' very long penis exhibits variability. During its waking hours, it remains hidden beneath a skin receptacle, until it becomes erect and it projects outside in a
rostral direction.
Fish and reptiles Male
turtles and
crocodiles have a penis, while male specimens of the reptile order
Squamata have two paired organs called
hemipenes.
Tuataras must use their cloacae for reproduction. Due to
evolutionary convergence, turtle and mammal penises have a similar structure. In some fish, the
gonopodium,
andropodium, and
claspers are intromittent organs (to introduce sperm into the female) developed from modified fins. ==Invertebrates==