Ostriches and their bloodless erections solve a centuries-old mystery

Sometimes, scientists ask deep, probing questions that cut to the very core of the universe's greatest mysteries. And sometimes, they wonder about how ostriches' and emus' penises work. And sometimes, that apparently prurient research can lead to new insights into how birds and reptiles evolved apart.

There are two major types of bird sex. The most common is called a cloacal kiss, in which the male and female touch their cloaca - the opening through which all the bird's bodily fluids are excreted - together long enough for sperm to transfer across. Some birds, including ducks, geese, and swans, do have a penis, but it's enlarged not by blood like in mammals, but by lymph, the bodily fluid found in the body's tissues.

The one big enigma has been the rattite family, which includes ostriches, emus, kiwis, and a few other species. Researchers in the 1800s had reported that male ostriches had blood-based erections — and yes, it's probably best to avoid imagining how precisely 19th century researchers tested that hypothesis — creating a longstanding mystery about what made ostrich penises different from those of other birds.

Now Yale biologists have examined the penises of an ostrich and three emus, and found that those 19th century biologists might not have been entirely accurate. Study co-author Dr. Patricia Brennan details their findings:

Earlier reports form the late 19th Century had suggested that the ostrich had a blood vascular erection mechanism, while no data existed for the emu or rhea. Since all other birds with penises have lymphatic erection mechanisms, I always thought that it was strange that the ostrich would be blood vascular. The penis of the ostrich is fundamentally very different from emu and rhea because it is made out of a dense collagen matrix, but the lymphatic machinery is all there. The penis of the ostrich is fundamentally very different from emu and rhea because it is made out of a dense collagen matrix, but the lymphatic machinery is all there.

This means that all known birds with penises rely on the lymphatic mechanism, which in turn suggests the mechanism evolved in the common ancestor of birds. Still, putting together an exact evolutionary tree for erections is difficult — reptiles and birds have some key similarities, but reptiles use blood like mammals. This suggests the blood-based mechanism is the more ancient system, and something happened with birds' evolutionary ancestor to prompt the change to the lymphatic mechanism. Brennan explains:

"The reason why the change between blood vascular and lymphatic took place remains a mystery. The lymphatic system is a low pressure system, so this means that erection cannot be maintained, and this has some important implications for how birds actually copulate."

It seems — and I don't know why I ever doubted this — that there's still a lot to learn about ostrich penises.

Via BBC News. Image by left-hand on Flickr.