FunNature & AnimalWhy is a duck's penis spiral?

Why is a duck's penis spiral?

The penis is a sexual organ of males
in many animal species, which fulfills the function of facilitating or carrying out copulation. In reality, different groups of animals have evolved independently acquiring this structure, of very different origin and conformation. In this way, the penis of a mammal, the hemipenes of a snake, the pterygopodia of a shark, the hectocotyl of an octopus, and the exaggeratedly long gonopod of barnacles—as they are called in each group—are analogous anatomical structures, fulfilling the same function, but have different origins .

However, some groups of animals do not have a penis , and copulation is carried out by making contact between the cloacae of the male and the female. It is the case of birds. It is estimated that only 3% of bird species have a penis , and in such a privileged list only ducks, geese and swans, ostriches and related species, and some species of tinamous are found.

Of these, the case of the duck is perhaps the most amazing. And not only because of its enormous size —it can be longer than the bird itself— or because of its curious corkscrew shape , but also because of the speed at which it can expand.

explosive eversion

In a state of rest, a duck’s penis is not noticeable. It hides inside a body cavity near the cloaca. When the time comes , the duck’s penis fills with hemolymph —unlike the human’s, which fills with blood—and expands , coming out of its cavity and uncoiling . This process is called eversion , and it occurs at great speed, reaching its maximum length in 360 thousandths of a second , approximately the duration of a human blink.

Even at its maximum level of erection the penis remains flexible. This is because the collagen matrix is extraordinarily thin. The instant the eversion ends, when it has reached its maximum, he immediately ejaculates . No wait.

The inverted spiral of the female

In the reproduction of ducks, another curious condition occurs: while the penis has a spiral shape in a clockwise direction , the vagina is also spiral, but in an anticlockwise direction . In addition, the female organ has secondary blind tubes and forms a sharp angle to the opening of the cloaca.

All these complexities had to have some kind of evolutionary explanation . In 2009, a research group led by Patricia Brennan, from Yale University, decided to investigate the background of this phenomenon, and tested to what extent the shape of the vagina could be an impediment to a duck’s penis .

They designed four tubes with which they would test the penetrating ability of the spiral penis of a Muscovy duck—also called a mute duck, because of its almost inaudible squawk. One of them was straight, another traced a spiral in the same direction as the penis; another traced an inverted spiral, like that of a paw vagina, and the other formed an angle of 135º that simulated that of the females.

They observed that for the ducks, it was not difficult to penetrate and ejaculate into the straight tube or the spiral tube in a clockwise direction. However, in tubes that emulated the female’s anatomy, they failed to evert the penis completely , and those that did were significantly slower—between half a second and nearly a second. And although, in most cases, even without complete eversion, there was ejaculation , it happened closer to the entrance the less eversion there was.

forced coevolution

The reasons behind this battle of spirals and inverted spirals are indeed evolutionary. The morphology of the penis and vagina of ducks presents an evolutionary race of the same type as that presented by predators and prey. And it is that the male ducks reproduce, sometimes, forcing the females.

The vagina thus evolved by forming physical barriers that prevented penile eversion and thus complete penetration. The evolutionary novelties in the morphology of the vagina would be restricting the forced intromissions , and preventing the sperm from reaching the deep tract, where they fulfill their mission.

Evolution has thus provided the female with tools to protect herself from males who try to reproduce without consent , reducing the probability of having offspring as a result of this type of forced copulation and avoiding further damage. In addition, it makes this act painful and traumatic for the male . On the other hand, the male evolved acquiring a longer and more flexible penis, to avoid suffering so much damage in copulation and maximize its success.

Observations confirm that when females are sexually receptive they adopt a posture that reduces the angle of entry. In addition, in these situations, females relax their muscles so that the walls widen, forming a straighter tube with their vagina.

When the female gives her consent, and decides that only yes is yes , she allows the male to access copulation without pain and with much more success .


Birkhead, T. R. et al. 2009. Elaborate vaginas and long phalli: Post-copulatory sexual selection in birds. Biologist, 56, 33-39.
Brennan, P. L. R. et al. 2010. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia. Proceedings of theRoyal Society B: Biological Sciences, 277(1686), 1309-1314.
DOI: 10.1098/rspb.2009.2139

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