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The biggest penis in the bird world

English name: Lake duck
Alternative names: Argentine Blue-billed Duck, Argentine blue-bill, Argentine Ruddy duck
Scientific name: Oxyura vittata


Phylum: Chordata (chordates)
Class: Aves (birds)
Order: Anseriformes (waterfowl)
Suborder: Anserides
Family: Anatidae
Subfamily: Oxyurinae (stiff-tailed ducks)
Genus: Oxyura
Species: vittatta

Range and habitat

Found in Chile and Argentina, sometimes migrating in winter to southern Brazil and Paraguay. Lake ducks are clumsy on land since their legs are set unusually far back, and rarely fly. They spend most of their time in water where they feed by diving.


Lake ducks are small, weighing around 640 grams (23 oz), and being about 40 cm (16 inches) in length. The male duck (or drake) has a black head, reddish-brown body, and blue bill (hence its alternative names). The female is dowdier, with a brown body, white throat, and a white horizontal stripe on the side of the head below the eyes.

Anatomy and reproduction

Like other ducks, in order for fertilization to occur, the male lake duck inserts his penis (sometimes called the male intromittent organ) into the female's oviduct, and sperm travels up the penis to fertilise the female. The lake duck's penis is corkscrew-shaped and retracts into the abdomen when not required.

Dr Kevin McCracken reported in 2000 that he and his team had discovered a lake duck with a penis that was 32.5 (13 inches) long when aroused and hanging loose, and 42.5 cm (17 inches) long when the corkscrew was fully unwound (McCracken 2000). This is even more impressive when you remember that (a) the duck is only 40 cm long, and (b) apart from ducks most birds don't have penises at all, reproducing by touching cloacas. The ostrich, which apart from ducks is one of the few birds to have a well-developed penis, has an organ about 20 cm (8 inches) long.

So why does it have such a long penis?

It has been suggested that ducks tend to have long penises because they copulate in water, a less precise procedure in which it is difficult to maintain genital contact; geese, which copulate on land, are far less well-endowed (McCracken 2000, p821). All members of the stiff-tailed duck family, which includes lake ducks, are very promiscuous and highly sexually competitive. This means that many males will try to have sex with each female. The typical penis length in the stiff-tailed duck family is about 8 inches, 20 cm.

Dr Kevin McCracken who first reported the penis length suggested that the length and ornamentation may be a result of sexual selection by females who want to mate with the bird with the biggest organ; group mating displays are an important part of the ducks' mating system and the drake commonly preens his penis after sex(McCracken 2000, McCracken 2001). However, this has been dismissed by bird expert Dr Raoul Mulder, who said, "I'm not fond of the 'nudge nudge, wink wink' comments by the authors." (Quoted by Salleh 2001)

The anatomy of the penis is interesting, suggesting it functions for more than just inserting sperm. The penis has coarse spines at the base and a soft brush-like tip; the brush at the end may be used by the duck to clear away other males' sperm from the female's oviduct, prior to ejaculating himself. This suggests that the penis may have evolved in response to sperm competition, where individuals compete to see who can get the most sperm into the female whilst trying to stop, remove or kill other males' sperm. This would help the male ensure that it was his sperm that fertilized the female, rather than that of previous visitors.

Most bird penises do not feature an internal sperm duct, with sperm travelling in grooves down the outside of the penis, and McCracken's team were unable to tell if the lake duck's sperm transmission was any different. McCracken concedes that there is much still to be discovered:

How much of his penis does the drake actually insert, and does the anatomy of the female’s oviducts make them unusually difficult to inseminate? (McCracken 2001, p128)

Sources and further reading (most include pictures of either the duck or the penis):

  • Arthur Grosset, 2000, "Lake Duck", Arthur Grosset's Birds, http://www.arthurgrosset.com/sabirds/lake%20duck.html
  • Harteman Wildfowl, undated, "Waterfowl (Anseriformes) of the world", Harteman Wildfowl, http://www.harteman.nl/omnibus/anseriformes/index2.html
  • Hillary Mayell, 2001, "Evolutionary Oddities: Duck Sex Organ, Lizard Tongue", National Geographic website, October 23, 2001, http://news.nationalgeographic.com/news/2001/10/1023_corkscrewduck.html
  • K. G. McCracken, 2000, "The 20-cm spiny penis of the Argentine Lake Duck (Oxyura vittata)", Auk 117(3):820-825, reproduced online at http://mercury.bio.uaf.edu/%7Ekevin_mccracken/reprints/auk-117-820.pdf
  • K. G. McCracken, 2001, "Are ducks impressed by drakes' display?", Nature 213 (September 13, 2001) p128, reproduced online at http://mercury.bio.uaf.edu/~kevin_mccracken/reprints/nature-413-128.pdf
  • Anna Salleh, 2001, "Found! The longest bird penis ever", ABC Online, September 14, 2001, http://www.abc.net.au/science/news/enviro/EnviroRepublish_366856.htm
  • Kelly Stewart, 1998, "Sperm Competition in a Nutshell", University of California, Davis website, http://www.anthro.ucdavis.edu/features/stp/stpballs.htm

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