Getting an erection is no problem for male ostriches – but maintaining it for more than a few seconds is a different matter. The birds may have inherited their underpowered genitalia from their dinosaur ancestors.
The penis is a vital piece of equipment for many vertebrates. Most male reptiles and mammals have them, and two of the most ancient groups of birds have them too, suggesting that dinosaurs, from which birds have descended, also possessed the male member. The majority of male birds don't have one, though: the reason why is something of a mystery.
The ostrich belongs to one of those two ancient, penis-bearing bird groups. It is a palaeognath, a group that also includes emus, kiwis and the near-flightless tinamous. The other endowed group is the Galloanserae, commonly known as fowl, which includes chickens, pheasants and ducks.
Many of these birds' penises are well understood, but the penises of ostriches and their relatives remained a mystery. So Richard Prum of Yale University and Patricia Brennan, now at the University of Massachusetts Amherst, obtained a dead ostrich and three emus to find out more.
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In both species they found spongy organs called paralymphatic bodies at the base of the penis. That tells us that the birds' penises fill up with lymphatic fluid to make an erection, like all other bird penises studied – and unlike reptiles and mammals, which use blood for the job.
That is bizarre, Brennan says, because the body keeps lymphatic fluid under lower pressure than blood. As a result, birds can't maintain an erection. "Copulation is very brief compared with mammals and reptiles," Brennan says. "It's a matter of a few seconds."
Brennan says she doesn't know why such an inefficient system would evolve, especially as it must have evolved from the more evolutionarily ancient blood-based system. However, all birds with penises produce erections this way, so the condition must already have existed in the last common ancestor of modern birds.
