SEX LIFE — Birds also have penises

Usually people believe that birds do not have penises, which is not true. Fact is 3 per cent of 10,000 species existing globally do have penises to facilitate internal fertilization. Ratites (large flightless birds of the infraclass Palaeognathae) like ostriches, rheas and cassowaries are huge birds, they cannot mate in the way most birds do, so they have evolved true penises.

Some large waterfowls, like geese and swans are highly monogamous and their males have small phalluses, whereas other species that are quite small but more promiscuous have more elaborate genitalia. 

Male Argentine Lake Duck with 42.5 cm penis (Pic - K McCracken - Nature)

Male Argentine Lake Duck with 42.5 cm penis (Pic – K McCracken – Nature)

World’s Longest penis

For instance, Argentine blue-bill (Oxyura vittata) also known as Lake Duck, Argentine lake duck or Argentine ruddy duck, is a small South American stiff-tailed duck, but holds the Guinness World Record for largest avian reproductive organ. It has a remarkable 42.5 cm (17 inches) long penis, when fully erect, at a body length of 20 cm! (Could you imagine a 6 ft (1.8 m) tall man having a 13.3 ft (4 m) long penis?). It is the longest penis of all vertebrates, in relation to body length. When not in use, the organ is typically coiled up in flaccid state.

Rape is not an easy job

Till sometime back, it was believed that penises in birds were just the result of male competition to fertilize females, but recent findings have changed the story. While most birds have vaginas (oviducts) like simple tubes, waterfowls have just as ornate as male genitalia. Despite the fact that most waterfowls form monogamous pairs, forced copulations by other males — the avian equivalent of rape — are common in many species. According to ornithologists about a third of duck-sex is forced. However, actual fertilization occurs in only about 2 per cent of forced sex.

Male waterfowls (Anatidae), family of birds that includes ducks, geese and swans, have long corkscrew penises, while their females have long corkscrew vaginas, which spiral in the opposite direction and also have many “dead ends” (lateral sacs) and other countermeasures, seemingly designed to impede successful fertilization. Since males frequently try to force copulation, the complex mating geometry allows the females to retain control; therefore most forced copulations do not result in successful fertilization.

Male Mallard (Family Anatidae) with visible pseudo-penis (Attribution - Glen Bowman & CC BY 2.0)

Male Mallard (Family Anatidae) with visible pseudo-penis (Attribution – Glen Bowman & CC BY 2.0)

According to the lead researcher Patricia Brennan, behavioral ecologist from Yale University, the evolution of phalluses in male waterfowls to forcefully copulate with females has led to counter adaptations in females in the form of vaginal structures called ‘dead end sacs’ and ‘clockwise coils’. If male forces copulation, in other words tries to “rape”, it is most likely that his phallus would enter one of the dead end sacs and would not progress further into the oviduct where it would deposit sperm more effectively.

These structures make it harder for males to achieve intromission. The clockwise coils of vagina are significant because the male phallus everts out of their body in a counter-clockwise spiral; therefore, clockwise vaginal structure would further impede forceful copulation.

Studies have shown that the longer a male’s phallus is, the more elaborate the vaginal structures were. The complexity of sacs and spirals in the waterfowl vagina has been found to be linked to the male penis length amongst the 14 species of ducks and geese investigated by the team, pointing to a war between the sexes in controlling mating.

The researchers speculate that when a female agrees with a chosen male partner, her cooperation could help the penis bypass the complicated defenses.

Why waterfowls have penises

The reason why members of family Anatidae have penises is that they cannot go for “cloacal kiss” as there is danger of water-seeping inside, which will interfere with the act of insemination. 

Bird penises are different in structure from mammalian organ. Usually partially feathered and are an erectile expansion of the cloacal wall, they are erected by lymph and not by blood as in mammals. Ducks’ penises are very bizarre: beside a great variation a half-inch (1.2 cm) to 17 inches long, they are extremely variable in shape, from smooth to covered with spines, grooves and variably curled.

Chickens still have penises, but barely—they’re tiny nubs that are no good for penetrating anything.

Copulating by cloacal kiss

97 percent of bird species that do not posses penises copulate very differently compared to their compatriots with sex organs. To mate, the males belonging to this massive majority press their cloaca, a multi-tasking orifice possessed by both sexes of all the species, against female’s, so that his sperm can flow into her body. Scientists have given this act a poetic name: the cloacal kiss.

During breeding season, high doses of hormones produced by pituitary gland cause dramatic physiological changes in birds. In males of some species these changes cause testes to grow up to 500 times their off season size and weigh up to 400 times more, sometimes comprising 10 per cent of the bird’s body mass. Females’ ovaries and oviducts also swell.

House Sparrows mating (GFDL)

House Sparrows mating (GFDL)

Cloaca (in vertebrates, common chamber and outlet into which the intestinal, urinary, and genital tracts open) in both sexes bulges so much that it takes shape of external lips. For mating to be successful, males and females must connect their cloacas so that sperms can be passed on from male to female. Since the cloacas are situated under the tail, the male mounts the female, she moves her tail out of the way and twists her abdomen sideways, male curves his tail and abdomen downwards and they both invert their cloacas to achieve copulation or the “cloacal kiss”.

Usually the whole process of copulation is quite brief, but in some species it is so quick that it is hard to believe that sperms have actually transferred. On the other hand there are species that take so long that they don’t seem to be interested in ending the act. A pair of fiery-throated hummingbirds (Panterpe insignis) was seen engaged in cloacal kissing for at least 50 minutes.

Although one successful insemination is indeed enough to fertilize the whole clutch, but the birds don’t seem to leave any doubt, they usually go for it frequently during the entire fertile period. Probably the record holder is northern goshawks (Accipiter gentilis); they have been observed making love 600 times for a single clutch of three to four eggs.

Vanishing bird penis

The question of the vanishing bird penis is an important question — not just for understanding the evolution of birds, but for clues it may offer to little-understood human genetic disorders.

Dr. Cohn and his colleagues have figured out how our feathered friends have performed this vanishing act. They started with detailed anatomical studies of the embryos of two species without penises, chicken and quails, and three species with them — ducks, geese and emus.

They found that during the initial stage of development, the male embryos of all five species developed identically. Within the first few days of growth, they developed a bump, called the genital tubercle. In the ducks, geese and emus, the tubercle continued to grow until it became a full-fledged penis, but in chicken and quails it stopped growing after a couple of days. It then shrank almost entirely away. When Dr. Cohn and his team looked closely at the vanishing tissue, they found that the cells were dying off.

After detailed investigation the experts identified genes that are responsible for these different results. When the tubercle started to grow, the same set of genes switched on in all the bird species. But in chicken and quails, the cells at the tip of the tubercle started making a protein called Bmp.

To confirm that the Bmp protein was the real cause of the penises’ disappearance, scientists loaded beads with Bmp proteins and implanted them in the genital tubercles of ducks. Instead of growing normally, their penises ended up as withered vestiges.

After this the team performed the opposite experiment. They loaded beads with a protein called Noggin, which blocks Bmp proteins. When they inserted the Noggin-laced beads into the tubercles of roosters, the cells stopped dying. Instead, the tubercle continued to grow. After tens of millions of years, the scientists had resurrected the bird penis, if only briefly.

In their report, published in the journal Current Biology, Dr. Cohn and his colleagues suggest that bird penises may have started to shrink as a side effect of some other evolutionary change. Scientists know that Bmp proteins aren’t important only for building penises; they help build the skeleton and other tissues as well. Scientists have found that changes to Bmp proteins have been pivotal in the evolution of other parts of birds, like feathers and beaks. As Bmp proteins took on a new role elsewhere in the bodies of birds, they may have also stunted the penis.

For some reason, male birds with smaller penises had more offspring than other birds. Why is still a matter of debate. Some scientists have suggested that males with smaller penises were less likely to acquire sexually transmitted diseases. Others have proposed that smaller penises were lighter, and thus made flying easier. Or perhaps females preferred males with smaller penises because they could gain more control over which males sired their offspring.


The male tinamou (order Tinamiformes) has corkscrew shaped penis, similar to those of the ratites and to the hemipenis (one of a pair of intromittent organs of male squamates (snakes, lizards and worm lizards)). Females have a small phallic organ in the cloaca which becomes larger during the breeding season.

Order Tinamiformes, comprises of a single family Tinamidae with two distinct subfamilies, containing 47 species of found in Mexico, Central America and South America. 

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