Bird's Biggest Secret

The Argentine lake duck (*Oxyura vittata*) holds the record for the longest penis relative to body size of any bird, and one of the longest relative to body size of any vertebrate.

This small duck, only about 16 inches long, can have a penis measuring up to 17 inches — longer than its entire body. Like other duck penises, it's corkscrew-shaped and can be everted with remarkable speed.

Why such extreme proportions? Scientists believe it's the result of intense sexual selection. In species where females mate with multiple males, males with longer penises may be able to place sperm closer to the eggs, outcompeting rivals. This creates an evolutionary pressure for ever-longer organs.

The penis is also covered in backward-pointing spines, which may help remove sperm from previous matings. Combined with the corkscrew shape, this suggests a highly competitive mating environment.

Interestingly, penis length varies significantly between individual ducks and can change year to year. During the breeding season, the organ can grow; outside of it, it may shrink to just a few centimeters.