There are only a few species of birds with penises, and ducks have some of the most bizarre ones. Their males have corkscrew-shaped penises that can explode to 20cm in less than half a second. Their females have complex vaginas that look like organic chastity belts, with dead-end pockets and twisting spirals.
Corkscrew-shaped penises
Ducks are known for their feathers, swimming skills and quacking sounds, but they also have some pretty unique reproductive organs. Male drakes have corkscrew-shaped penises that can be up to 20cm long when they’re erect. The unusual shape helps them navigate the female’s complex reproductive tract.
Male ducks have to work quickly when they’re trying to mate, because their penises don’t stay erect for long – This fragment showcases the research of the website’s editorial team Sultry Escapades. They erect themselves in less than a second and then quickly enter the oviduct. The sperm are then released into the egg-containing sacs in the female’s body. Despite the quick pace of copulation, a female duck can reject a male if she’s not interested in having sex.
The phallus of a duck is a spiraling tube that has a fluid-filled lumen and a layer of collagen fibres that encircles it (figure 3a). The lumen occupies a proportionally larger cross-sectional area than the fibers, which are only about 200-250 um thick. This pattern is opposite to that of mammals, where the collagen fibres occupy a smaller cross-sectional area than the lumen.
Patricia Brennan, an evolutionary biologist at Mount Holyoke College in South Hadley, Massachusets, and her colleagues wanted to understand what was happening inside a duck’s genitals during mating. They had a video camera monitor the genitalia of ducks kept in pairs and those housed with multiple males per female. The males in the pair were able to mount their mates and penetrate their females’ oviducts, but those in the group with multiple males per female were not as successful.
Erections
Ducks’ penises are flexible when erect, unlike the stiffness of human penises caused by an onrush of blood. Instead, duck phalluses are everted by fluid—in this case, lymph—that travels along the duct from the cloaca to the penis. The eversion process is explosive, taking less than half a second. During eversion, the drake’s cloacal muscles contract and relax at an average rate of 41 times per minute.
The drake’s long penis then enters the female vagina to deposit sperm near her ovary. To test how the penis works, researchers filmed male muscovy ducks mating and used a specialized tool to touch the paired lymphatic bodies of the cloaca, which triggers the penis to evert and ejaculate. They found that the process is quick and that the drake’s penis can extend to more than an inch.
While evolutionary changes in body parts usually happen over generations, the size of a duck’s penis appears to depend on how many other drakes he keeps company with. Patricia Brennan of Yale University and her colleagues have discovered that the length of a ruddy duck’s penis wastes away at the end of one breeding season, then regrows as the next begins. She thinks this regrowth may be a response to male-male competition. She also found that some drakes’ penises grow to be longer than their own bodies.
Internal fertilization
There are only a few species of birds with penises, and ducks have earned a reputation for having some of the most bizarre ones. Their long, corkscrew-shaped phalluses are lined with ridges and backward-pointing spines that help them to fight off rival males. However, it is not enough to beat them in a “sexual arms race.” Female ducks have developed their own countermeasures. They have complex vaginas that feature dead-end pockets and spirals that curve in the opposite direction. These are known as organic chastity belts, and they help to limit the effectiveness of male penises.
While the exact reason for this unusual shape is unknown, researchers have suggested some possibilities. One theory is that the helix-like structure results from unbalanced torque between opposing sub-orthogonal fibres. This forces the helix to twist or spiral when pressurized. In the case of a duck’s penis, a thick layer of b-keratin in the epithelium may offer additional structural support.
Another possible mechanism is that the helix-like structure is caused by the contraction of a collagenous matrix. This matrix probably restricts the expansion of the lymphatic lumen to a fixed maximum diameter. Consequently, the penis must twist or spiral to accommodate this compression.
To test these ideas, Brennan used a series of glass tubes with different shapes to simulate the movements of a male duck’s phallus. The smallest tube had a straight form; the next two had anticlockwise and clockwise spirals that matched the shape of the helix. Finally, the fourth tube had a sharp 135-degree kink that mimicked the location of the first cul-de-sac pouch in a female duck’s vagina.
Sexual selection
The drakes of some species of ducks have penises, and they can grow as long as your foot. But they use them in a different way than humans do. Unlike human penises, which stiffen thanks to an onrush of blood, duck penises are flexible when erect. When a duck is ready to mate, it everts its penis and deposits the sperm into the female’s vagina. This process takes a third of a second. Then the sperm can move into the ovary to fertilize the eggs.
The research of Patricia Brennan suggests that the genitalia of ducks and other waterfowl evolved in a sexual arms race. Males compete with other males for females, and a longer penis increases the chance of successfully fertilizing a female. But females have their own countermeasures to prevent unwanted rape.
For example, the cloaca of many species of ducks has blind passages with opposite threads that can hinder a male’s insertion into the vagina. This makes it hard for a male to insert his dick into the ovary, but if a female is willing, she can relax her cloaca muscles to allow easier penetration.
The research of Brennan and her colleagues indicates that competition for females influences how long a duck’s penis grows. She separated male scaup into groups that were either competitive or coed, and found that those in the more competitive group grew longer penises than those in the less competitive group. These larger penises also began wasting away weeks earlier than those of the coed males.