12 December 2011 Last updated at 09:16

Bloodless erections for big birds, say researchers

By Ella Davies Reporter, BBC Nature

Ostriches have bloodless erections, according to researchers.

The large birds were previously thought to have blood-based erection mechanisms similar to humans.

But scientists from Yale University, US, have now confirmed that the birds actually enlarge their penises with lymph fluid.

All other birds with a penis achieve erections in this way, leading scientists to believe the mechanism evolved in their ancient ancestors.

The findings are reported in the Journal of Zoology.

The majority of birds reproduce with a 'cloacal kiss' - touching together their cloaca for long enough for sperm to transfer from the males to the females.

The cloaca is a single opening through which urine and faeces are excreted but certain species, including ducks, geese, swans and flamingos also possess a penis.

In birds, this reproductive organ is unusual as it is enlarged by lymph: the fluid found in bodily tissues.

But the ratite family, from large ostriches to small kiwis, were thought to be the exception to this rule.

"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," said Dr Patricia Brennan who co-authored the study.

"Since all other birds with penises have lymphatic erection mechanisms, I always thought that it was strange that the ostrich would be blood vascular."

To solve the puzzle, Dr Brennan and her team closely examined the penis of a male ostrich and three male emus and found some key differences.

"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," she told BBC Nature.

"Ostriches do have blood vessels near the surface of the penis, that makes it look pink, but the inside of the penis fills up with lymph, not blood."

Low pressure

According to Dr Brennan, the evidence confirms that "the lymphatic erection mechanism evolved in the ancestor of birds".

But questions still remain concerning the evolution of this trait.

Similarities have been drawn in the past between bird and reptile penises but the latter use blood for erections, as do mammals.

"The reason why the change between blood vascular and lymphatic took place remains a mystery," said Dr Brennan.

"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," said Dr Brennan.

Some species of bird, such as ducks, are known for their "explosive" erections achieved when lymph fluid is forced into the penis to increase pressure for a short time.

But ornithological reproduction expert Prof Tim Birkhead from the University of Sheffield suggests that the structure of ostrich penises could make up for the shortcomings of the lymph system.

"Ostriches and rheas appear to have additional muscles that help to maintain a rigid phallus," he explained.