Most species of bird copulate by touching together their cloaca, openings that facilitate both mating and excretion. Ducks, geese, swans, flamingos, ostriches, kiwis and emus are different; they all have penises that swell to erection. For most of them, the erection is produced by lymph fluid, but the ostrich has long been believed to have blood erections just like humans. Now scientists finally know for sure.
It all began with some 19th century data, suggesting that ostriches have a blood vascular erection mechanism, like humans. Until a recent study, published in the Journal of Zoology by Dr. Richard Prum of Yale University and Dr. Patricia Brennan of the University of Massachusetts Amherst, no research had been performed to suggest otherwise. So, Dr. Prum and Dr. Brennan set out to learn the truth. They obtained the carcasses of one ostrich and three emus and dissected the penises.
What they found was that both species of bird have spongy organs at the base of the penis, known as paralymphatic bodies, making them no different from all other studied species of birds. In other words: ostriches and emus have the lymph fluid based erections common to birds, rather than the blood vascular erections of reptiles and mammals.
Ostriches do have blood vessels in the penis, which is why they are pink and why one might think they contain blood, but these are small surface vessels that are unable to aid them in maintaining erections. Another reason why scientists believed that ostriches have blood vascular erections was the apparent rigidity of their erections, so unlike most bird species. Professor Tim Birkhead, an ornithological reproduction expert of the University of Sheffield, explained that according to Dr. Brennan’s and Dr. Prum’s findings “ostriches and rheas appear to have additional muscles that help to maintain a rigid phallus.”
Since all known phallus endowed birds have lymph erections, Dr. Brennan suggests that “the lymphatic erection mechanism evolved in the ancestor of birds” at least 130 million years ago as it separated from the reptiles and mammal-like reptiles which had blood vascular erection mechanisms.
The question now is, why would it be evolutionarily beneficial to have very short-lived erections? The answer, should they find it, may have some interesting implications for mammals as well, not the least for humans and the prevalence of premature ejaculations.