Penis

For other uses, see Penis (disambiguation).

This article is about penises of animals in general. For the human organ, see Human penis.

"Penile" redirects here. For the community in the United States, see Penile, Louisville.

"PENIS" redirects here. For the spectroscopy acronym, see Proton-enhanced nuclear induction spectroscopy.

Penises of minke whales on display at the Icelandic Phallological Museum

Penis (plural penises or penes) is a general term for the primary sexual organs that male and hermaphrodite organisms use in mating. Specifically in this sense the penis is the organ that introduces sperm into receptive mates (usually females and hermaphrodites respectively) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do not bear a penis in every animal species, and in those species in which the male does bear a so-called penis, the penes in the various species are not necessarily homologous. For example, the penis of a mammal is at most analogous to the penis of a male insect or barnacle.

The term penis applies to many reproductive intromittent organs, but not to all; for example the intromittent organ of most cephalopoda is the hectocotylus, a specialised 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 reasonably 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 as the occasion requires.

In different animals

See also: Category:Animal penises

Vertebrates

See also: Intromittent organ § Vertebrata

Mammals

Further information: Sexual reproduction in male mammals

Penis of an Asian elephant.

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 or os penis is present in most mammals but absent in humans 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 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.

Carnivorans

All members of Carnivora (except hyenas) have a baculum. In the Canoidea, the penis is highly specialized.

When a male spotted hyena copulates, his penis enters and exits the female's reproductive tract through her 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' upward angle. The pseudo-penis closely resembles the male hyena's penis, but can be distinguished from the males' 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. Male lions also have barbed penises. Male felids usually 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 forelegs when erect, with an average thickness of 20 mm (0.79 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. 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. 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.

Dogs

Main article: Canine penis

Canine reproduction is the process of sexual reproduction in domestic dogs, wolves, coyotes and other canine species.

Canine sexual anatomy and development

Male reproductive system

Canine male reproductive system

Erectile tissue

As with all mammals, a dog's penis is made up of three pieces of erectile tissue. These are the two corpora cavernosa and the singular corpus spongiosum which continues in the glans. A notable difference from the human penis is that the visible part during an erection consists entirely of the glans.

The retractor muscle is attached at the shaft of the penis. It is a paired smooth muscle that is used to retract the penis back into the sheath.

Glans

A dog's glans consists of two sections: Behind the lower, long part (pars longa glandis) lies the "knot" (Bulbus glandis) which expands only after penetrating the vagina and causes the male dog to remain inside the bitch ("Tie") for some time after ejaculation (typically between 15 and 30 min). This increases the chance of fertilisation and prevents, albeit for a short time, other suitors from mating with a particular female.

Behind the knot the penis is very flexible in the horizontal direction allowing the male to unmount while remaining tied.

Shaft

The shaft of a dogs penis is not visible, even during an erection; however its pathway can be felt starting at the knot passing between the hind legs and carrying on up to the anus.

Baculum and urethra

Baculum of a domestic dog; The arrow points to the bottom groove where the urethra is located. The tip is to the right.

Inside the corpus spongiosum lies the baculum. This allows the male dog to enter into the vagina before the erectile tissue is swollen. The urethra is located inside of a downward facing groove on the baculum and ends at the tip of the penis (urethral process).

During an erection a small dip just above the urethral process can be seen. This is because the skin at the tip of the penis is connected via cartilage to the baculum. When the erectile tissue swells, the size of the baculum and connective tissue remains constant, pulling back the skin at the tip.

Sheath

The penile sheath entirely surrounds the glans while not erect. The back part is intergrown with the abdominal skin. The front part, almost reaching to the navel, is free. The inner sheath, just like the glans, is covered with a mucous membrane and the outer sheath is covered with normal, hairy epidermis.

Development

In domestic dogs, sexual maturity (puberty) occurs between the ages of 6 and 12 months for both males and females, although this can be delayed until up to two years of age for some large breeds. Pregnancy is possible as soon as the first estrus cycle, but breeding is not recommended prior to the second cycle. As with other domesticated species, domestication has selectively bred for higher libido, and earlier and more frequent breeding cycles in dogs than in their ancestors.

The female reproductive cycle

Canine female reproductive tract

Female cycle

The average length of the reproductive cycle for females is 2–4 weeks. Females reach sexual maturity (puberty) between 8 and 18 months of age. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog.

1. Proestrus, in which eggs in the ovaries begin to mature and estrogen levels begin to rise, is the first stage of the reproductive cycle. During this stage females, though non-receptive, attract males. Initial changes include swelling of the vulva lips, which become pliable, small amounts of bloody vaginal discharge, frequent urination, and signs of restlessness. Proestrus generally lasts nine days.

2. Estrus follows, in which estrogen levels are high, mature eggs are released from both ovaries, and females become receptive both physically and mentally to copulation. Only during estrus will copulation result in pregnancy.

During proestrus and estrus, females may have a clear, blood tinged, or bloody discharge. Dogs during these stages are often informally referred to as being in heat. The length of these cycles varies greatly among breeds and even between individuals of the same breed. Proestrus and estrus can last anywhere from 5 days to 21 days.

3. Diestrus is the period following mating. Diestrus lasts approximately 56 to 60 days in a pregnant female, and 60 to 100 days in a non-pregnant female. During both of these periods, progesterone levels are high. Because the hormonal profile of a pregnant female and a female in diestrus are the same, sometimes a non-pregnant female will go through a period of pseudopregnancy. At that time she may gain weight, have mammary gland development, produce milk, and exhibit nesting behaviours.

4. Anestrus is the remaining period, the time of reproductive quiescence. The female has no attraction to mating. Anestrus generally lasts four to five months.

Copulation

"Copulatory tie" redirects here. For copulatory tying in non-canine animals, see Fossa (animal) § Breeding, Black-footed ferret § Reproduction and development, and Vasa parrot § Unusual characteristics. Penis captivus is a similar human phenomenon.

Wolves mating, Tama zoo

As with most tetrapods, canine copulation involves the male mounting the female from behind, a position that is colloquially referred to as "doggy style" but does not have a specifically known origin. When a male canine is interested in mounting a female, he will sniff the female's vulva, which, for females in the estrus stage, has been secreted with Methyl p-hydroxybenzoate. If the female is unreceptive, she may sit, lie down, snap, retreat, or otherwise be uncooperative. If the female is receptive, she will stand still and hold her tail to the side, a stance referred to as "flagging". The male will often continue examining the female's rear, before mounting her from behind while attempting penetration with his penis.

Unlike human sexual intercourse, where the male penis commonly becomes erect before entering the female, canine copulation involves the male first penetrating the female, after which swelling of the penis to erection occurs, which usually happens rapidly. At the time of penetration, the canine penis is not erect, and only able to penetrate the female because it includes a narrow bone called the "baculum", a feature of most placental mammals. When the male achieves penetration, he will usually hold the female tighter and thrust deeply. It is during this time that the male's penis expands and it is important that the bulbus gland is sufficiently far inside for the female to be able to trap it.

Knotting

Male canines are the only animals that have a locking bulbus glandis or "bulb", a spherical area of erectile tissue at the base of the penis. During copulation, and only after the male's penis is fully inside the female's vagina, the bulbus glandis becomes engorged with blood. When the female's vagina subsequently contracts, the penis becomes locked inside the female. This is known as "tying" or "knotting". While characteristic of mating in most canids, the copulatory tie has been reported to be absent or very brief (less than one minute) in the African wild dog, possibly due to the abundance of large predators in its environment.

When the penis is locked into the vagina by the bulbus glandis (when the stud is "tied"), thrusting behavior stops and the male will usually lift a leg and swing it over the female's back while turning around. The two stand with their hind ends touching and the penis locked inside the vagina while ejaculation occurs, decreasing leakage of semen from the vagina. After some time, typically between 5 and 20 minutes (but sometimes longer), the bulbus glandis disengorges, allowing the mates to separate. Virgin dogs can become quite distressed at finding themselves unable to separate during their first copulation, and may try to pull away or run. Dog breeders often suggest it is appropriate for handlers to attempt to calm the mating dogs if they show anxiety once this stage is reached. After mating, the male usually licks his penis and prepuce.

Gestation and litters

A pregnant mongrel

A feral dog from Sri Lanka nursing her puppies

Gestation in a dog is 63 days in length, if measured from the day of ovulation. Since it is difficult to determine the exact date of ovulation, errors are often made in calculating gestation period. Canine sperm can live for 10 to 11 days in the oviducts (fallopian tubes) so if a female is bred 10 days before the oocytes (eggs) can be fertilized, she will appear to have a gestation length of 70 days. If she is bred on the day the oocytes can be fertilized, her gestation length will appear to be 60 days long.

During gestation, many physiological changes are similar to other mammals like humans. This results in similar shifts in nutrients in the blood of dogs, especially affecting glucose, fatty acids (like DHA) and amino acid (like BCAA) levels.

A rule of thumb is that a mammal will produce half as many offspring as the number of teats on the mother. This rule is altered in domesticated animals since larger litters are often favoured for economic reasons and in dogs, particularly, the great range of sizes and shapes plays a role in how many healthy puppies a female can carry. A female dog usually has 10 teats, though this does not mean she can necessarily provide sufficient nutrition for 10 puppies in one litter.

An average litter consists of about five to six puppies, though this number may vary widely based on the breed of dog. Size of the breed is correlated with litter size. Miniature and small breeds average three to four puppies in each litter, with a maximum litter size of about 5–8. Large and giant breeds average 7 puppies per litter but can have a maximum litter size of about 15. In one study, the Rhodesian Ridgeback had the highest average litter size with 8.9 pups per litter while the Pomeranian and Toy Poodle had the lowest with 2.4 pups per litter.

The number of puppies also varies with the mother's age. In smaller breeds, both young and old age are associated with smaller litter size. In larger breeds, only old age is associated with smaller litter size. Use of artificial insemination is also associated with smaller litter size, with frozen semen having a stronger effect than fresh semen.

The largest litter size to date was set by a Neapolitan Mastiff in Manea, England, on November 29, 2004; the litter was 24 puppies.

Some breeds have been developed to emphasize certain physical traits beyond the point at which they can safely bear litters on their own.

A large scale study in Norway showed that across all breeds, about 4% of pups will be stillborn and a further 4% will die within the first week (early neonatal mortality). Between 8 days and 8 weeks, 1% will die. Litter size, breed size and age of the female is associated with increased risk. High risk breeds for stillborn includes the Dogue de Bordeaux (14.2%), St. Bernard (12.3%), Chow Chow (12.1%), Pembroke Welsh Corgis (11.7%) and Dalmatian (10.6%). The Basenji, Italian Greyhound, Australian Terrier, Irish Soft Coated Wheaten Terrier and the Bichon Havanais had few to no stillborns (0-0.6%). High risk breeds for early neonatal mortality includes the Rhodesian Ridgeback (11.6%), Dogue de Bordeaux (10.4% ), Dalmatians (8.8%) and Icelandic Sheepdog (8.7%) while the Basenji and Tibetan Terrier had no early neonatal mortality and the Border Terrier and Danish-Swedish Farmdog had <1% early neonatal mortality.

Common causes of early neonatal mortality are bacteria infection, fetal asphyxia and fading puppy syndrome. Other causes may include elective euthanasia because of congenital defects or failure to meet breed standards.

Other multi-breed studies have put stillborn rates at 6.5–7% and early neonatal mortality at 11.5–19.8%.

Inbreeding depression

On the basis of an analysis of data on 42,855 Dachshund litters, it was found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression. Inbreeding depression is a reduction in progeny fitness due largely to the homozygous expression of deleterious recessive mutations.

The gray wolves (Canis lupus) of Isle Royale National Park, Michigan, US, were a small highly inbred population that was considered to be at the threshold of extinction in 2019. This wolf population had been experiencing severe inbreeding depression largely due to the homozygous expression of strongly deleterious recessive mutations. Another highly inbred Scandinavian population of wolves (Canis lupus) also suffered from inbreeding depression that was again attributed to the homozygous expression of deleterious recessive mutations.

Inbreeding avoidance

Because the African wild dog (Lycaon pictus) largely exists in fragmented small populations, its existence is endangered. Inbreeding avoidance via mate selection is characteristic of the species and has important potential consequences for population persistence. Inbreeding is rare within natal packs. Computer-population simulations indicate that all populations continuing to avoid incestuous mating will become extinct within 100 years due to the unavailability of unrelated mates. Thus the impact of reduced numbers of suitable unrelated mates will likely have a severe demographic impact on the future viability of small wild dog populations.

Red wolves primarily live in packs composed of a socially monogamous breeding pair and offspring of different ages. Using long-term data on red wolf individuals of known pedigree, it was found that inbreeding among first-degree relatives was rare. A likely mechanism for avoidance of inbreeding is independent dispersal trajectories from the natal pack. Many of the young wolves spend time alone or in small non-breeding packs composed of unrelated individuals. The union of two unrelated individuals in a new home range is the predominant pattern of breeding pair formation.

Among Ethiopian wolves, most females disperse from their natal pack at about two years of age, and some become "floaters" that may successfully immigrate into existing packs. Breeding pairs are most often unrelated to each other, suggesting that female-biased dispersal reduces inbreeding.

Grey wolves and Arctic foxes also exhibit inbreeding avoidance.

Inbreeding is ordinarily avoided because it leads to a reduction in progeny fitness (inbreeding depression) due largely to the homozygous expression of deleterious recessive alleles. Cross-fertilization between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.

Clinical issues

Female dogs are at risk for endometritis and pyometra in the postpartum period and after estrus or vaginitis. Signs and symptoms include fever, lethargy, loss of appetite, excessive thirst, restlessness, a foul smelling vaginal discharge which may or may not be bloody, infertility, or they may be asymptomatic. Uterine infections should be treated expeditiously if suspected. Contrary to common belief, uterine infections can strike any intact female, whether she has been bred or not, and whether it is her first season or not although it is more common as dogs become older.

Dog breeding

Main article: Dog breeding

These littermates were born to an Australian Shepherd mother.

Semen collection

An artificial vagina is prepared, which is a conical thin latex sleeve ending in a sterile collection tube. The inside of the latex sleeve is lightly lubricated. The male is allowed to sniff a female in estrus. Experienced studs cooperate readily in the process. New studs often require encouragement in the form of manual stimulation. Generally the male will mount the female, and the collector quickly directs the male's penis into the latex sleeve. The male ejaculates and the semen is collected in the tube. The semen is then drawn up into a long thin pipette.

Cross breeding

Designer breed dogs are mixed-breed dogs intentionally bred from parents of two established breeds. Studies have shown that cross-bred dogs have a number of desirable reproductive traits. Scott and Fuller found that cross-bred females were superior mothers compared to purebred females, producing more milk and giving better care. These advantages led to decreased mortality in the offspring; however, the qualities of such dogs are not predictable. For example, Labrador x Poodle ("Labradoodle") can inherit the coat of either a Labrador, a poodle, or a remix.

Spaying and neutering

Main article: Neutering

Spaying (females) and neutering (males) refers to the sterilization of animals—usually by castration (removal of the male's testicles) or ovariohysterectomy (removal of the female's ovaries and uterus)—to eliminate the ability to procreate, and reduce sex drive. Castration has also been known to reduce aggression in male dogs (in some cases), but spaying has been shown to occasionally increase aggression in female dogs.

Animal control agencies in the United States and the ASPCA advise that dogs not intended for further breeding should be spayed or neutered so that they do not have undesired puppies. Spaying and castrating can decrease the risk of hormone-driven diseases such as mammary cancer, as well as undesired hormone-driven behaviors. However, certain medical problems are more likely after neutering, such as urinary incontinence in females and prostate cancer in males.

Dogs shown in the conformation ring are not allowed to be either neutered or spayed. It disqualifies them from being shown as they must be intact and unaltered.

Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle.

Studies have shown that spaying or neutering may be associated with increasing some serious health and behavioural consequences while reducing others. The American Veterinary Association (AVMA) position provides no single recommendation to spay or neuter nor for one single age for spay or neuter that is more or less optimal than another. Rather, the AVMA position is that spay or neuter be determined on a case-by-case basis to assess risks for orthopedic disease, neoplasia, reproductive disease, longevity, and population control for each individual.

Altered females:

Increased aggression can be shown in altered females if they have previously displayed aggression prior to surgical alteration. In a study by O'Farrell and Peachy, female dogs less than 11 months of age that had previously shown signs of aggression are more likely to have an increase in aggression after being spayed. These increases in aggression may be due to the sudden change in hormone concentrations that are the result of alteration. While spaying female dogs does not "induce" aggression, it can increase aggression and facilitate indiscriminate appetite in young altered females and can include them rapidly eating meals or eating food-associated items such as trash.

Altered males:

In nearly 2/3 of the cases that involve inter-dog aggression, castration can help decrease aggression. Castration also decreases other male-typical behavioral traits such as mounting, roaming, and urine marking. But a few studies have shown that male behavioral issues of mounting, roaming and urine marking still exist in altered males. Some people have reported after altering their male dogs that behavior such as roaming, mounting and urine marking has not changed the behavior. Aggression may increase, as the decrease in testosterone may lead to emotional issues and become more likely to react aggressively when feeling under threat. Male puppies that are neutered between 7 and 10 weeks are three times less likely to display behavioral problems, compared to canines neutered at 6 months or older. Most dominantly aggressive dogs are male, which causes many people to neuter their male canine companions. Removing testosterone can decrease the intensity of a canine's reaction to stimulus. Testosterone does not cause a behavior to occur, but its absence may decrease the occurrence of a "bad" behavior.

See also

• Animal husbandry
• Canine reproductive behavior
• Canine transmissible venereal tumor
• Puppy mill
• Sexual behavior of coyotes
• Sexual behavior of golden jackals
• Sexual behavior of red foxes

References

• Siegal, Mordecai, ed. (1995). UCDavis Book of Dogs. HarperCollins. ISBN 978-0-06-270136-7.

Further reading

• Heide Schatten; Gheorghe M. Constantinescu (21 March 2008). Comparative Reproductive Biology. John Wiley & Sons. ISBN 978-0-470-39025-2.

External links

• Canine Gestation Calculator

Cetaceans

Cetaceans have fibroelastic penises, similar to those of Artiodactyla. The tip of the cetacean penis, which tapers toward the end, is called the pars intrapraeputialis or terminal cone. The blue whale has the largest penis of any organism on the planet, typically measuring 8–10 feet. Accurate measurements of the blue whale are difficult to take because the whale's erect length can only be observed during mating. The penis on a right whale can be up to 2.7 m (8.9 ft) – the testes, at up to 2 m (6.6 ft) in length, 78 cm (2.56 ft) in diameter, and weighing up to 525 lb (238 kg), are also by far the largest of any animal on Earth. Dolphins' reproductive organs are located on the underside of the body. Male dolphins have two slits, one concealing the penis and one further behind for the anus. On at least one occasion, a dolphin was known to tow bathers through the water by hooking his large penis around them. Between male bottlenose 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.

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. P. endoi, P. paterculus, and P. abramus have more similar bacula, but that of P. abramus is more curved, the shaft and the tip are more robust in P. paterculus, and the proximal (near) end is more robust in P. endoi. In males, penis length is 9.6 to 11.8 mm (0.38 to 0.46 in) and baculum length is 8.8 to 10.0 mm (0.35 to 0.39 in).

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 7.3 mm (0.29 in) long and 4.6 mm (0.18 in) broad, and the baculum (penis bone) is 6.6 mm (0.26 in) 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. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character the marsh rice rat shares only with Oligoryzomys and Oryzomys couesi among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process (the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except O. couesi and Holochilus brasiliensis. The baculum is deeper than it is wide.

In Transandinomys talamancae, the outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue.

Some features of the accessory glands in the male genital region vary among oryzomyines. In Transandinomys talamancae, 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 Pseudorhyzomys, 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. The two lateral digits are not supported by mounds of the baculum (penis bone). There is only one spine on the papilla (nipple-like projection) on the upper side of the penis. On the urethral process, located in the crater at the end of the penis, a fleshy process at the side, the lateral lobule, is present. The preputial glands (glands in front of the genitals) are large. The lack of lateral bacular mounds, presence of a lateral lobule, and size of the preputial glands are all unique traits among the oryzomyines.

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. Most of the glans is covered with spines, except for an area near the tip.

The glans penis of a male cape ground squirrel is large with a prominent baculum.

Unlike the penises of other squirrel species, a red squirrel's penis is long, thin, and narrow, 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.

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 prosimians has provided information about their taxonomy. The penis of the ring-tailed lemur is nearly cylindrical in shape and is covered in small spines, as well as having two pairs of larger spines on both sides. Males have a relatively small baculum (penis bone) compared to their size. The scrotum, penis, and prepuce are usually coated with a foul-smelling secretion.

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 impeding danger and a threat to predators. Genital display among male squirrel monkeys is an important social signal in relation to group hierarchy; it is derived from sexual behavior but is used for social communication. It involves the animal spreading his thighs and having an erect penis.

Humans

Main article: Human penis

The human penis is an external sexual 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 /miːˈeɪtəs/, 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 male 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: removal of part or all of the foreskin for various cultural, religious, and more rarely medical reasons. There is controversy surrounding circumcision.

While results vary across studies, the consensus is that the average erect human penis is approximately 12.9–15 cm (5.1–5.9 in) in length with 95% of adult males falling within the interval 10.7–19.1 cm (4.2–7.5 in). Neither patient age nor size of the flaccid penis accurately predicted erectile length.

Horses

See also: Horse sheath cleaning and Stallion § Reproductive anatomy

Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the prepuce (foreskin, or sheath). The retractor penis muscle is relatively underdeveloped. Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis. When not erect, the penis is housed within the prepuce, 50 centimetres (20 in) long and 2.5 to 6 centimetres (0.98 to 2.36 in) in diameter with the distal end 15 to 20 centimetres (5.9 to 7.9 in). The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. The urethra opens within the urethral fossa, a small pouch at the distal end of the glans. A structure called the urethral process projects beyond the glans.

Other mammals

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 5 inches 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. r

An adult elephant has the largest penis of any land animal at 6 feet (1.8 m) on average. An elephant's penis can reach a length of 100 cm (39 in) and a diameter of 16 cm (6 in) at the base. It is S-shaped when fully erect and has a Y-shaped orifice. 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.

In giant anteaters, the (retracted) penis and testes are located internally between the rectum and urinary bladder.

The foreskin of a capybara is attached to the anus in an unusual way, forming an anogenital invagination.

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 7–35 minutes, averaging 11–15 minutes.

When C. villosus is sexually aroused with an erect penis, species determination is easier. Its penis can be as long as 35 mm, 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. During its slow wave sleep phase, penile protrusion makes some very complex movements. The penis during this phase is not erect, but remains outside of its receptacle. During paradoxical sleep, no erections occur, and the penile muscles share the characteristics of the rest of the body.

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.

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.

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. Bulls are occasionally affected by a condition known as "corkscrew penis". The penis of a mature bull is about 3–4 cm in diameter.

Most marsupials, except for the two largest species of kangaroos, have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas. Neither marsupials nor monotremes possess a baculum. When not erect, it is retracted into the body in an S-shaped curve. The shape of the glans varies among species. A male koala's foreskin contains naturally occurring bacteria that play an important role in fertilization. The penises of marsupials curve forward when erect.

Bulls, rams and boars have an S-shaped penis with a sigmoid flexure which straightens out during erection. A boar's penis, which rotates rhythmically during copulation, is about 18 inches long, and ejaculates about a pint of semen.

In the realm of absolute size, the smallest vertebrate penis belongs to the common shrew (5 mm or 0.2 inches).

Male echidnas have a 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 7 centimeters long when erect, and its shaft is covered with penile spines. The male short-beaked echidna has a highly unusual penis with four knobs on the tip, which is nearly a quarter of his body length when erect. Each side of the bilaterally symmetrical, rosette-like, four-headed penis [similar to that of reptiles and 7 centimetres (2.8 in) in length] is used alternately, with the other half being shut down between ejaculations.

Other vertebrates

Mallard pseudo-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), Anatidae (ducks, geese and swans), and a very few other species (including flamingoes and chickens). 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. The 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.

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 more commonly is 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.

Although most male birds have no penis, ducks have a long corkscrew penis, and the females have a long corkscrew vagina, which spirals in the opposite direction. The males often try to force copulation, but the complex mating geometry allows the females to retain control—most forced copulations do not result in successful fertilisation.

Male and female emus are similar in appearance, although the male's penis can become visible when it 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.

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.

In some fishes, the gonopodium, andropodium, and claspers are intromittent organs (to introduce sperm into the female) developed from modified fins.

Invertebrates

Main article: Intromittent organ § Invertebrates

The spine-covered penis of Callosobruchus analis, a Bean weevil.

The record for the largest penis to body size ratio is held by the barnacle. The barnacle's penis can grow to up to forty times its own body length. This enables them to reach the nearest female.

A number of invertebrate species have independently evolved the mating technique of traumatic insemination where the penis penetrates the female's abdomen and deposits sperm in the wound it produces. This has been most fully studied in bed bugs.

The penis in most male Coleoidea is a long and muscular end of the gonoduct used to transfer spermatophores to a modified arm called a hectocotylus. That, in turn, is used to transfer the spermatophores to the female. In species where the hectocotylus is missing, the penis is long and able to extend beyond the mantle cavity and transfers the spermatophores directly to the female. Deep water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile barnacles Penis elongation in Onykia ingens may result in a penis that is as long as the mantle, head and arms combined. Giant squid of the genus Architeuthis are unusual in that they possess both a large penis and modified arm tips, although it is uncertain whether the latter are used for spermatophore transfer.

Insects

In male insects, the structure analogous to a penis is known as aedeagus. The male copulatory organ of various lower invertebrate animals is often called the cirrus.

The lesser water boatman's mating call, generated by rubbing the penis against the abdomen, is the loudest sound, relative to body size, in the animal kingdom.

Etymology

The word "penis" is taken from the Latin word for "tail." Some derive that from Indo-European *pesnis, and the Greek word πέος = "penis" from Indo-European *pesos. Prior to the adoption of the Latin word in English the penis was referred to as a "yard". The Oxford English Dictionary cites an example of the word yard used in this sense from 1379, and notes that in his Physical Dictionary of 1684, Steven Blankaart defined the word penis as "the Yard, made up of two nervous Bodies, the Channel, Nut, Skin, and Fore-skin, etc."

As with nearly any aspect of the body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock". See WikiSaurus:penis for a list of alternative words for penis.

The Latin word "phallus" (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe representations, pictorial or carved, of the penis.

Pizzle, an archaic English word for penis, of Low German or Dutch origin, is now used to denote the penis of a non-human animal.

The adjectival form of the word penis is penile. This adjective is commonly used in describing various accessory structures of male copulatory organs found in many kinds of invertebrate animals.

Human use of animal penises

Pizzles are represented in heraldry, where the adjective pizzled (or vilené) indicates that part of an animate charge's anatomy, especially if coloured differently.

See also

• Buried penis
• Castration anxiety
• Diphallia
• Dildo
• Erogenous zone
• Koteka
• Micropenis
• Penis enlargement
• Penis envy
• Penis removal
• Phallic architecture
• Preputioplasty
• Pubic hair
• Stamen
• Stunt cock
• Testicle

Bibliography

• Brehm, Alfred Edmund (1895). "Brehm's Life of Animals". Chicago: A. N. Marquis & Company. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help)
• Heptner, V. G.; Sludskii, A. A. (2002). Mammals of the Soviet Union. Vol. II, part 1b, Carnivores (Mustelidae and Procyonidae). Washington, D.C. : Smithsonian Institution Libraries and National Science Foundation. ISBN 90-04-08876-8. {{cite book}}: Invalid |ref=harv (help)

• Dr. Barbara N. Horowitz; Kathryn Bowers (12 June 2012). Zoobiquity: What Animals Can Teach Us About Being Human. Doubleday Canada. ISBN 978-0-385-67061-6. Retrieved 25 July 2013.

• Jonathan Kingdon; David Happold; Thomas Butynski (23 May 2013). Mammals of Africa. A&C Black. ISBN 978-1-4081-8996-2. Retrieved 20 July 2013. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Stewart Keith Eltringham (1979). The ecology and conservation of large African mammals. Macmillan. ISBN 978-0-333-23580-5. Retrieved 20 July 2013.

• R. L. Kotpal (2010). Modern Text Book Of Zoology Vertebrates. Rastogi Publications. ISBN 978-81-7133-891-7. Retrieved 23 July 2013.

• Virginia Douglass Hayssen; Ari Van Tienhoven (1993). Asdell's Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data. Cornell University Press. ISBN 978-0-8014-1753-5. Retrieved 23 July 2013.

• Eugene Burns (1953). The sex life of wild animals: a North American study. Rinehart. Retrieved 23 July 2013.

• Robert H. Horwich (June 1972). The ontogeny of social behavior in the gray squirrel (Sciurus carolinensis). P. Parey. ISBN 978-3-489-68036-9. Retrieved 23 July 2013.

• Meredith F. Small (1993). Female Choices: Sexual Behavior of Female Primates. Cornell University Press. ISBN 978-0-8014-8305-9. Retrieved 23 July 2013.

• Donald F. Hoffmeister (2002). Mammals of Illinois. University of Illinois Press. ISBN 978-0-252-07083-9. Retrieved 22 July 2013.

• J. D. Skinner; Christian T. Chimimba (15 November 2005). The Mammals of the Southern African Sub-region. Cambridge University Press. ISBN 978-0-521-84418-5. Retrieved 19 July 2013.

• Trevor Carnaby (22 January 2007). Beat About the Bush: Mammals. Jacana Media. ISBN 978-1-77009-240-2. Retrieved 19 July 2013.

• Hartley H. Jackson; Hartley Harrad Thompson Jackson (January 1961). Mammals of Wisconsin. Univ of Wisconsin Press. ISBN 978-0-299-02150-4. Retrieved 22 July 2013.

• Mammal Species of the World: A Taxonomic and Geographic Reference - Google Books

• Shirley C. Strum; Linda Marie Fedigan (15 August 2000). Primate Encounters: Models of Science, Gender, and Society. University of Chicago Press. ISBN 978-0-226-77754-2. Retrieved 22 July 2013.

• Colin Russell Austin; Roger Valentine Short (21 March 1985). Reproduction in Mammals: Volume 4, Reproductive Fitness. Cambridge University Press. ISBN 978-0-521-31984-3. Retrieved 22 July 2013.

• Jonathan Kingdon (January 1984). East African Mammals: An Atlas of Evolution in Africa. Vol. I. University of Chicago Press. ISBN 978-0-226-43718-7. Retrieved 22 July 2013.

• Kenneth D. Rose; J. David Archibald (22 February 2005). The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. JHU Press. ISBN 978-0-8018-8022-3. Retrieved 22 July 2013.

• Robin Sturtz; Lori Asprea (30 July 2012). Anatomy and Physiology for Veterinary Technicians andn Nurses: A Clinical Approach. John Wiley & Sons. ISBN 978-1-118-40585-7. Retrieved 22 July 2013.

• Steven D. Lukefahr, Ph.D.; Peter R. Cheeke, Ph.D.; Nephi M. Patton, Ph.D. (2013). Rabbit Production. CABI. ISBN 978-1-78064-012-9. Retrieved 20 July 2013.

• Don E. Wilson; DeeAnn M. Reeder (16 November 2005). Mammal Species of the World: A Taxonomic and Geographic Reference. JHU Press. ISBN 978-0-8018-8221-0. Retrieved 20 July 2013.

• William J. Krause (1 March 2008). An Atlas of Opossum Organogenesis. Universal-Publishers. ISBN 978-1-58112-969-4. Retrieved 20 July 2013.

• B. J. Verts; Leslie N. Carraway (1998). Land Mammals of Oregon. University of California Press. pp. 41–. ISBN 978-0-520-21199-5. Retrieved 20 July 2013.

• Amita Sarkar (1 January 2003). Sexual Behaviour In Animals. Discovery Publishing House. ISBN 978-81-7141-746-9. Retrieved 20 July 2013.'

• Donald W. Linzey (28 December 2011). Vertebrate Biology. JHU Press. ISBN 978-1-4214-0040-2. Retrieved 20 July 2013.

• Donald W. Linzey (28 December 2011). Vertebrate Biology. JHU Press. ISBN 978-1-4214-0040-2. Retrieved 20 July 2013.

• Dev Raj Khanna; P. R. Yadav (1 January 2005). Biology Of Mammals. Discovery Publishing House. ISBN 978-81-7141-934-0. Retrieved 20 July 2013.

• Uldis Roze (2009). The North American Porcupine. Cornell University Press. ISBN 978-0-8014-4646-7. Retrieved 25 July 2013.

• M Horace Hayes; Peter D. Rossdale (March 1988). Veterinary Notes for Horse Owners: An Illustrated Manual of Horse Medicine and Surgery. Simon & Schuster. ISBN 978-0-671-76561-3. Retrieved 1 July 2013.

• "The Stallion: Breeding Soundness Examination & Reproductive Anatomy". University of Wisconsin-Madison. Archived from the original on 2007-07-16. Retrieved 7 July 2007.

• Heide Schatten; Gheorghe M. Constantinescu (21 March 2008). Comparative Reproductive Biology. John Wiley & Sons. ISBN 978-0-470-39025-2. Retrieved 23 July 2013.

• James Warren Evans (15 February 1990). The Horse. W. H. Freeman. ISBN 978-0-7167-1811-6. Retrieved 23 July 2013.

• Equine Research (2004). Horse Conformation: Structure, Soundness, and Performance. Lyons Press. ISBN 978-1-59228-487-0. Retrieved 23 July 2013.

• Donald F. Walker; John T. Vaughan (1 June 1980). Bovine and equine urogenital surgery. Lea & Febiger. ISBN 978-0-8121-0284-0. Retrieved 23 July 2013.

• Horst Erich König; Hans-Georg Liebich (2007). Veterinary Anatomy of Domestic Mammals: Textbook and Colour Atlas. Schattauer Verlag. ISBN 978-3-7945-2485-3. Retrieved 23 July 2013.

• Modern Horse Breeding: A Guide for Owners - Susan McBane. Books.google.com. Retrieved 2013-06-30.

• Equine Clinical Medicine, Surgery and Reproduction - Graham Munroe BVSc (Hons) PhD Cert EO DESM Dip ECVS FRCVS, Scott Weese DVM DVSc DipACVIM. Books.google.com. 2011-03-15. Retrieved 2013-06-30.

• Fertility and Obstetrics in the Horse - Gary England. Books.google.com. 2008-04-15. Retrieved 2013-06-30.

• Equine Science, 4th ed. - Rick Parker. Books.google.com. Retrieved 2013-06-30.

• McCurnin's Clinical Textbook for Veterinary Technicians - Joanna M Bassert, Dennis M McCurnin. Books.google.com. Retrieved 2013-06-30.

• Klaus Dieter Budras; W. O. Sack (1 March 2012). Anatomy of the Horse. Manson Publishing. ISBN 978-3-8426-8368-6. Retrieved 1 July 2013.

• Rowen D. Frandson; W. Lee Wilke; Anna Dee Fails (30 June 2009). Anatomy and Physiology of Farm Animals. John Wiley & Sons. ISBN 978-0-8138-1394-3. Retrieved 1 July 2013.

• Natural History Bulletin of the Siam Society. 1975. Retrieved 5 July 2013.

• Bryce Towsley (13 August 2008). Big Bucks The Benoit Way: Secrets From America's First Family of Whitetail Hunting. Krause Publications. pp. 57–. ISBN 978-1-4402-2631-1. Retrieved 5 July 2013.

• Benjamin B. Beck; Christen M. Wemmer (1983). The Biology and management of an extinct species: Père David's deer. Noyes Publications. ISBN 978-0-8155-0938-7. Retrieved 5 July 2013.

• Valerius Geist (1993). Elk Country. T&N Children's Publishing. ISBN 978-1-55971-208-8. Retrieved 5 July 2013.

• Journal of the Mammalogical Society of Japan. The Society. 1986. Retrieved 5 July 2013.

• Lawrence Mark Elbroch; Michael Raymond Kresky; Jonah Wy Evans (7 April 2012). Field Guide to Animal Tracks and Scat of California. University of California Press. ISBN 978-0-520-95164-8. Retrieved 5 July 2013.

• Jones, Menna E.; Dickman, Chris R.; Archer, Mike; Archer, Michael (2003). Predators With Pouches: The Biology of Carnivorous Marsupials. Csiro Publishing. ISBN 9780643066342. Retrieved 5 May 2013.

• Flannery, Tim (2008). Chasing Kangaroos: A Continent, a Scientist, and a Search for the World's Most Extraordinary Creature. Grove/Atlantic, Incorporated. pp. 60–. ISBN 9780802143716. Retrieved 5 May 2013.

• Bernard Stonehouse; Desmond Gilmore (1977). The Biology of marsupials. University Park Press. ISBN 978-0-8391-0852-8. Retrieved 25 July 2013.

• Lynda Staker (2006). The Complete Guide to the Care of Macropods: A Comprehensive Guide to the Handrearing, Rehabilitation and Captive Management of Kangaroo Species. Lynda Staker. ISBN 978-0-9775751-0-7. Retrieved 19 July 2013.

• The Biology of Marsupials. Books.google.com. Retrieved 2013-06-30.

• Life of Marsupials - C. Hugh Tyndale-Biscoe. Books.google.com. Retrieved 2013-06-30.

• Anna King (2001). "Discoveries about Marsupial Reproduction". Iowa State University Biology Dept. Retrieved 2012-11-22.

External links

• Media related to Penis at Wikimedia Commons
• Kinsey Institute on the penis