North American scientists have discovered the longest bird penis ever - a 42.5cm organ belonging to a duck.

Dr Kevin McCracken of the University of Alaska, Fairbanks, and colleagues, report in this week's Nature that they have found a specimen of the Argentine lake duck (Oxyura vittata) that has a penis as long as its body - nearly half a metre long.

This has extended an earlier estimate of the length of the duck's corkscrew-shaped penis, which was 20cm.

"It's a fascinating bit of anatomy they've discovered here. It really is unusual," commented bird mating expert Dr Raoul Mulder from the University of Melbourne.

Most male birds don't have penises - they copulate by briefly touching genital openings, called a 'cloacal kiss'," he explained.

Dr Mulder said a number of duck species and ostriches have penises, but this was definitely the longest ever encountered.

Dr McCracken and colleagues speculate that the giant penis may be an example of 'runaway' sexual selection, where female preference drives male anatomy to ever-greater extremes, as in the peacock's tail.

Dr Mulder believes this may be a bit of anthropomorphising, however.

"I'm not fond of the 'nudge nudge, wink wink' comments by the authors," he said.

The authors write that this species is "promiscuous and boisterous in their sexual activity", which means that there is likely to be stiff competition by drakes to be the father of ducklings.

One finding in support of this idea, they suggest, is that the drake's penis has a brush-like tip, which they say the males probably use to scrub the sperm of previous mates from the female's oviduct.

Mulder agreed this was likely, but felt another suggestion by the authors - that males might use an everted penis as a "display" to attract females - was not.

"They are being rather speculative, and it seems not much is known about this bird and we need to observe its behaviour in the wild before we can understand what function this penis might have," he said.

The authors acknowledge that many questions remain: How much of the penis does the drake actually insert? And does the anatomy of the female make them unusually difficult to inseminate?