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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.

The unusually long penis is probably part of a reproductive strategy designed to increase the male Argentine duck's success in a highly competitive environment. Stiff-tail ducks are promiscuous, said McCracken, and Argentine lake ducks are particularly so.

Although information on their mating habits is limited, researchers know that the courtship displays to attract females—and fend off other males—are elaborate, vigorous, and typically performed in the presence of other ducks. There is very little pair-bonding between males and females, and the female ducks are often resistant to mating.

"Our best guess is that the birds use [the long penis] as a kind of lasso," McCracken said. "The males have to chase the females, and even during copulation the females are trying to escape."

McCracken and his colleagues don't discount the idea that sexual selection—female preference for a longer penis—has played a role in the development of the Argentine duck's very long penis. They think it is more likely, however, the result of limited access to females, intense sperm competition, and the need for male dominance over females.

Special Tongue Muscles

Anthony Herrel of the University of Antwerp, Belgium, is co-author of the report on the chameleon's tongue and its special muscles. He believes the organ may have become specialized so that chameleons, who sit and wait for food, can maximize limited mealtime opportunities by capturing not only insects but also prey as large as birds.

In studies of two chameleon species, Chameleo calyptratus and Chameleo oustaletti, Herrel and his colleagues positioned crickets at various distances from the chameleons' jaws. The chameleons were about 12 centimeters (4.7 inches) long and the prey was placed from 5 to 30 centimeters away.

The chameleons generated enough strength in their tongues to snatch the prey and snap their tongues back, which would be quite a feat for normal muscles. The action was possible, the researchers found, because the chameleons' tongue muscles have unusual filaments that allow "super-contraction."

The unique muscle mechanism has been observed in insects, but this is the first time it's been found in a vertebrate, according to the researchers.

A chameleon's tongue doesn't poke from the reptile's mouth but is ejected with a strong force as though spring-loaded. Because the action is less powerful when the prey is closer, chameleons sometimes back up before releasing the tongue to generate the intense force needed to snatch a meal.
 

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