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A Tongue That Doesn't Slip
18 October 2000 7:00 pm
A human tongue is remarkably useful for eating and talking, but it can't compete for sheer skill with that of a chameleon. With its so-called ballistic tongue, a chameleon can capture prey more than 15% its own body weight and more than a body-length away. The secret to this strength is suction, researchers reveal in the November issue of the Journal of Experimental Biology.
Chameleons are best known for their ability to change colors, which allows them to fade into their surroundings while they wait for prey. When an edible creature comes near, the chameleon shoots out its tongue and grabs it with deadly accuracy. Other lizards also capture prey with their large, sticky tongues, and scientists have determined that they primarily rely on surface tension--the force created when the wet tongue contacts the dry prey--to whisk victims back to the mouth.
But chameleons consume creatures too large for surface tension alone to provide the force necessary. "They would have to have a huge tongue--greater than the size of their head," says evolutionary biologist Anthony Herrel of the University of Antwerp in Belgium. Herrel, with colleagues from Northern Arizona University and the University of Antwerp, wondered what other forces might play a role. When they dissected tongues from several chameleon species, the researchers noticed that two muscles form a pouch at the tip of the tongue. Films of chameleons capturing crickets, grasshoppers, and even other lizards revealed that these muscles retract just before the tongue makes contact with the victim.
The team suspected that the tongue pouch was behaving like a suction cup. So they anesthetized chameleons, inserted a hollow glass tube into the pouch, and measured the force required to remove the tube when the pouch muscles were electrically stimulated. When they used a sealed tube around which the tongue could create suction, it took nearly 10 times as much force. As a final test, the team cut the nerve that controls the pouch muscles. Although the animals could extend their tongues normally, they could not latch onto prey and ended up knocking it away. (The nerve later regrew, and the animals were able to eat normally, says Herrel's colleague Kiisa Nishikawa.)
The team estimates that suction provides about 70% of the force chameleons use to capture large prey. Chameleons may be unique in their vacuum-generating tongues, says evolutionary biologist Kurt Schwenk of the University of Connecticut, Storrs, and the advantage has likely played a key role in their evolutionary success--the animals are plentiful across Africa and southern Asia. A better understanding of this unusual mechanism, he notes, should help scientists understand how it evolved.