Ostriches 'have bloodless erections'

All other birds with a penis achieve erections in this way, according to the researchers who believe the mechanism evolved in their ancient ancestors, the 'Journal of Zoology' reported in its latest edition.

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 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 blood vascular erection mechanism," team leader Dr Patricia Brennan was quoted by the 'BBC' as saying.

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. 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," she said