Evolutionary Oddities: Duck Sex Organ, Lizard Tongue

By Hillary Mayell
for National Geographic News
October 23, 2001

In two new studies that highlight the curiosities of nature, scientists report on distinctive organs of chameleons and male Argentine lake ducks that give them an edge in very different functions: feeding and mating.

Chameleons can reel in food from a distance as far away as more than two and a half times their body lengths. The action is possible because the reptiles' tongues have powerful "super-contracting" muscles that are unique among back-boned animals, a team of researchers explains in the Journal of Experimental Biology. Another study reports on the exaggerated anatomy of the male Argentine lake duck, whose penis is about the same length as its body. The case is especially intriguing because very few species of birds have penises.

Researchers from the University of Alaska discovered that the penis of Oxyura vittata, when fully extended, measures about 17 inches (0.5 meters) long. When not in use, the corkscrew-shaped penis retracts into the duck's abdomen.

The trait is one that bears further study, say the researchers. It opens some interesting questions about the dynamics of male competition and sperm competition at a pretty high level in birds resulting in the anatomy evolving in this way, said Kevin McCracken, the lead author of a report on the finding published last month in Nature.

Unusual Anatomy

The Argentine lake duck is small, weighing a little more than a pound (640 grams) and extending about 16 inches (41 centimeters) long from head to tail. Its penis, at about 17 inches, is the longest of any bird known so far, said McCracken.

"Measurements that we had taken before were all from dissected birds, and we thought [the penis] was about 20 centimeters [8 inches]," he said. "But in April we were in Argentina collecting birds for another genetic study, and we found this bird running around in its natural form, with its penis hanging out, which was something we'd never seen before."

The Argentine lake duck is a stiff-tail duck; its tail feathers spike upward and its legs are set far back on its body. The bird is extremely clumsy on land and spends most of its time in the water.

Many species in the stiff-tail duck family have relatively long penises, said McCracken, but more along the lines of 8 inches (20 centimeters).

The base of the Argentine lake duck's penis is covered with coarse spines, while the tip is soft and brush-like. The researchers think a drake may use the brush-like tip as a sort of cleansing instrument before ejaculation to remove sperm in the females oviduct that was deposited by another suitor, thus increasing the mating drake's chances of paternity.

Similar sperm-removal behavior has also been seen in some fish and insect species.

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