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"Catapults" Give Chameleon Tongues Superspeed, Study Says

Bijal P. Trivedi
National Geographic Channel
May 19, 2004
 
Chameleons have an uncanny ability to shoot their long sticky tongues out of their mouths at mind-numbing speeds to catch fast-moving prey. But exactly how they propel their tongues with such power has never been convincingly explained.

Using high-speed video and x-ray film, two Dutch biologists calculated that the chameleon's tongue shoots out of its mouth at more than 26 body lengths per second—that's the equivalent of 13.4 miles (21.6 kilometers) an hour. It can snag prey located more than one and a half body lengths away.


The duo found that the chameleon's tongue accelerates from 0 to 20 feet per second (0 to six meters per second) in about 20 milliseconds—a rate so fast it defies the general principles of power production in muscles.

"If you do the calculation, you know that the muscle alone cannot be responsible for this rapid acceleration," said Jurriaan H. de Groot, a biomechanist at Leiden University in the Netherlands.

De Groot and project leader Johan L. van Leeuwen, a biologist at Wageningen University in the Netherlands, looked for a hidden structure in the tongue that could power this extreme athletic performance. They believed that there must be some sort of biological spring or catapult that launches the tongue on its culinary quest.

Muscles are limited in their power output. To hit speeds exceeding these limitations, many animals have coupled their muscles with biological "catapults." These catapults store energy and release it when triggered, allowing them to help produce much higher speeds than could be achieved by muscle alone.

Dissection of chameleon tongues revealed an elastic collagen tissue sandwiched between the tongue bone and the accelerator muscle. The collagen is wrapped in sheaths around the tongue bone at one end and to the accelerator muscle at the other. De Groot and van Leeuwen discovered that this collagen structure is the biological catapult that propels the tongue tip in much the same way a bow delivers an arrow.

Just as an arm muscle moves slowly to stretch a bow—storing power in the taut string—the chameleon's accelerator muscle also stores energy in the collagen tissue. When triggered, the concentric, overlapping sheaths of collagen telescope outward, allowing the adhesive tongue tip to extend rapidly toward the prey.

"The anatomy of the chameleon's tongue has been known for a long time, but no one understood how the various components were working," said Ulrike Müller, a biologist specializing in biomechanics at Wageningen University. Müller co-authored a commentary on the catapult discovery in the April 9 issue of the journal Science.

Until now many scientists had concocted fanciful explanations for the powerful tongue movements, like the presence of a "supermuscle" that had chemical and mechanical properties as yet undiscovered. Other explanations dating back 150 years suggested an influx of air or blood allowed the tongue to rocket out of the chameleon's mouth, Müller said.

The collagen catapult has a "weird helical [or spiral] structure" that allows it to store energy, said van Leeuwen. "So far we have not seen a parallel structure in biology or mechanics—it is a completely novel design." The unusual structure maybe one reason that the collagen tissue was not recognized as a spring.

The collagen catapult is beginning to receive attention from engineers who believe it may have a variety of medical applications.

Van Leeuwen's and de Groot's study appears in a recent issue of the journal Proceedings of the Royal Society of London: Biological Sciences.

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