There are nearly 10,000 species of birds in the world, but only about 3% of them have functional penises. Most male birds rely on their cloaca, a multi-purpose opening used for both excretion and reproduction, to mate.
Bird species with penises include ducks, geese, and swans, large flightless birds such as ostriches and emus, and tinamous, a group of ground-dwelling birds closely related to the ratites. Some, like chickens, have a structure that resembles a penis but serves no functional purpose. Others possess a pseudo-penis, which will be discussed later.
Among the birds that do have penises, there is an extraordinary variety in size and shape, unmatched by any other class of vertebrates. For example, the lake duck is infamous for its corkscrew-shaped penis, which can be as long as its body when erect, while the ostrich has a peculiar conical penis that hangs to one side of its cloaca. Some birds even have feathers on their penises, while others are covered in spines or tiny filaments.
Like most animals, male birds pass on sexual characteristics to their offspring. These traits include plumage colour, singing ability, courtship displays, nest-building skills, and ornaments such as combs, wattles, long feathers, and air sacs. They also inherit the size and shape of their genitals. Male birds with more impressive physical traits are more likely to attract mates, ensuring they pass on their genes to the next generation.
The Fisherian runaway hypothesis, proposed by Sir Ronald Aylmer Fisher in the 1930s, suggests that certain physical traits become so appealing to females that they will only mate with males displaying the most pronounced versions of these traits. As a result, male offspring are more likely to inherit these traits, while female offspring develop a preference for them. This feedback loop drives the rapid evolution of exaggerated sexual dimorphism over time.
In the 1970s, two scientists introduced the “sexy son hypothesis” as an alternative explanation for why female birds select certain mates.
This theory suggests that females choose mates not because of traits that would make them good fathers or result in healthier offspring, but because those traits would produce physically attractive sons. These sons, in turn, would have better success attracting mates and producing offspring, thereby continuing the genetic line.
The unusual penises of some birds might be an example of this “runaway sexual selection,” where female preferences drive male anatomy to extremes, much like the extravagant tail feathers of a peacock.
However, this raises an intriguing question: if a large penis provides such an advantage in attracting mates, why do the vast majority of bird species lack a penis altogether? Particularly since birds still reproduce through internal fertilisation, and the absence of a penis makes the process significantly more challenging.
All birds carry the genes necessary to develop a penis, starting life with a structure known as a genital tubercle inside the egg. In mammals, this tubercle typically develops into either a penis or a clitoris. However, in most birds, the tubercle stops growing after just a few days. This happens because it undergoes apoptosis, a natural process of programmed cell death. Apoptosis is common in the animal kingdom; it’s how we get the spaces between our fingers and toes in the embryo, and how tadpoles lose their tails during metamorphosis.
In birds, the apoptosis of the genital tubercle is triggered by a gene called Bone Morphogenetic Protein 4 (Bmp4). Bmp4, which plays a role in the development of bones, muscles, cartilage, and limbs, is activated in most birds and prevents the tubercle from developing into a penis. As a result, these birds either lack a penis entirely or have one so small that it’s nonfunctional. In species like ducks, however, Bmp4 remains switched off, allowing the genital tubercle to grow into a functioning penis.
While this explains why most birds end up without a penis, it doesn’t clarify why this evolutionary trait was lost. Scientists are still unsure of the exact reason. Several theories exist. It could be that the absence of a penis was an unintended side effect of Bmp4 influencing other body developments, like feather growth or beak size. Another idea is that birds without a penis are lighter and more agile, potentially making it easier for them to fly, though this theory is complicated by the fact that ducks, which migrate long distances, still have penises. And most flightless birds, aside from the ratites, also lack penises.
One possible explanation is that female birds may have preferred a more compact penis, and, due to runaway evolution, males developed increasingly smaller penises until they eventually disappeared altogether. This idea makes sense when considering how ducks copulate. Emus, which have relatively small penises, form strong pair bonds, suggesting that females might be just as content with a less-endowed mate.
Birds’ penises differ from those of mammals and reptiles in that they become erect due to a rush of lymphatic fluid rather than blood flow. This characteristic is shared by all penis-having birds, but it also means they struggle to maintain an erection, as lymphatic fluid is kept under much lower pressure than blood. Consequently, copulation in birds with penises typically lasts only a few seconds, much like in birds that mate using their cloacas.
An interesting aspect of this system is that it evolved from the more efficient blood-based system, possibly as a response to female birds developing defences against the penis. By having a quick mating process, males still have a good chance of fertilising their mate, even if she resists. The lymphatic fluid may also help push semen towards the tip of the penis, further increasing the likelihood of successful fertilisation.
