There are nearly 10,000 species of birds in the world, but only around 3% of them have functional penises. Most male birds use their cloaca, a multi-purpose orifice used for both digestive excretion and reproduction, to mate.
Species of birds that have a penis include ducks, geese, and swans, large flightless birds such as ostriches and emus, and tinamous, a family of ground-dwelling birds closely related to the ratites. Others, like chickens, have something which looks a bit like a penis, but doesn’t actually do anything useful. And some birds have a pseudo-penis, more of which later.
Despite these shortcomings, of the birds that do have penises, they exhibit the widest variation in phallic size and shape of any class of vertebrate, from the infamous lake duck’s corkscrew penis that can be as long as its body when erect, to the ostrich’s bizarre-looking conical penis, that hangs to one side of its cloaca. Some birds have feathers on their penis, while others are covered in spines and small filaments.
Like most animals, male birds pass on sexual characteristics to their sons such as plumage colour, singing, displaying, and nest-building abilities, and ornaments, including combs, wattles, long feathers, and air sacs, as well as the size and shape of their genitals.
Male birds possessing better physical traits are more likely to attract females with which to mate with and pass on their genes to the next generation of the species.
According to the Fisherian runaway hypothesis, proposed by Sir Ronald Aylmer Fisher in the 1930s, some physical traits are so attractive to females that they will only mate with males who possess the strongest examples. Over time, male offspring are more likely to have these traits, and female offspring are more likely to have a preference for them, which leads to the relatively rapid evolution of exaggerated sexual dimorphism.
Another explanation put forward by two scientists in the 1970s is the sexy son hypothesis. This states that a female chooses a mate, not because of traits that would make him a good father or because he would produce healthier offspring of both sexes, but simply because he will provide her with physically attractive sons. When it comes to their turn to reproduce, they will have more luck attracting females than their less handsome rivals, and therefore have a better chance of producing offspring and continuing the genetic line.
Birds’ weird penises may therefore be an example of this runaway sexual selection where female preference drives male anatomy to extremes, as in the peacock’s flamboyant tail.
But if a large penis gives birds an advantage when it comes to attracting a mate, why do the majority of birds not have a penis at all? Particularly when they still reproduce by internal fertilisation, and the lack of a penis makes things that much trickier.
All the genes responsible for growing a penis exist in birds, and they start life inside the egg with something called a genital tubercle. But whereas in mammals this develops into either a penis or a clitoris, for the majority of birds, the tubercle stops growing after just a few days.
This is because it experiences something called apoptosis, which is the programmed death of its cells. This cell-suicide is quite normal; the gaps between our fingers and toes, for example, occur due to apoptosis when we were an embryo, and tadpoles lose their tails by apoptosis during their metamorphosis into frogs.
In birds, apoptosis of the genital tubercle is triggered by a gene called Bone morphogenetic protein 4, or Bmp4, a protein involved in bone, muscle, cartilage, and limb development. In the majority of birds, Bmp4 gets switched on and blocks the genital tubercle from developing into a penis so they either lack one altogether or it’s so small as to be rendered useless.
In mammals the genital tubercle formed in utero will grow into either a penis or a clitoris, so if in some birds it develops into a penis you might expect that the female of the species develops a clitoris.
But it turns out that only a couple of known species of birds possess anything resembling a clitoris: cassowaries, and red-billed buffalo weavers. And even then, things aren’t quite what they seem.
Male cassowaries have a pseudo-penis which is not connected to the reproduction system but is used to help push sperm from his cloaca into the female’s. Female cassowaries also have this pseudo-penis type appendage, but it would be equally valid to refer to it as a clitoris, which means that male cassowaries don’t have penises at all but “male clitorises”.
Red-billed buffalo weavers, renowned in the avian kingdom for their marathon sex sessions, have a pseudo-penis, about 1.5 cm in length that they rub against the female during copulation. It does not contain blood vessels or spermatozoa and it appears its function is solely to give pleasure to the female. Red-billed buffalo weavers who live in colonies have longer “male clitorises” which would suggest that its evolution can be explained by competition from other males.
In ducks and other birds which grow penises, Bmp4 remains switched off and the genital tubercle develops into a functioning penis.
But although this explains how most birds end up penis-less, it doesn’t explain why. And scientists are still not sure of the answer to that conundrum.
There are several plausible explanations. It could be that the loss of the penis was an unfortunate side-effect of Bmp4 triggering other changes to body parts such as the development of feathers, or the variations in beak size. A bird without a penis carries less weight and therefore would be able to fly more easily, although this theory is unlikely when you consider that fact that ducks make some of the longest migrations of all species of birds. And most flightless birds or birds who fly very little, other than the ratites, have non-existent penises.
One theory is that female birds actually prefer a more compact penis, but due to runaway evolution, males took it to the extreme and ended up with no penis at all. If you’ve ever seen ducks copulate, then this theory makes complete sense.
Emus which have relatively small penises form pair bonds which could imply that females are happier to stick around a less-endowed mate.
Birds’ penises differ from mammals’ and reptiles’ penises in that they become erect from a huge rush of lymphatic fluid, rather than blood flow. All penis-having birds share this trait but it also means they have difficulty maintaining an erection, because lymphatic fluid is kept under much lower pressure in the body than blood. As a result, copulation in birds with penises lasts just a few seconds, as it does in birds who use their cloacas to mate.
What’s strange is this inefficient system evolved from the more-efficient blood-based system, perhaps as a reaction to female birds evolving defences against the penis. If a male can be in and out in just a few seconds he still has a good chance of fertilising his mate, no matter how hard she puts up a fight. Lymph fluid may also help push semen towards the tip of the penis, again increasing the chance of a successful fertilisation.