Published online 16 October 2000 | Nature | doi:10.1038/news001019-3

News

On the tip of a chameleon's tongue

Mischievous small boys in comics who fire toy arrows with suction cup ends at the bald heads of middle-aged park keepers have a lot in common with chameleons, says Henry Gee.

Chamaeleo calyptratus: it sucksChamaeleo calyptratus: it sucks

Mischievoussmall boys in comics who fire toy arrows with suction cup ends at the bald heads of middle-aged park keepers have a lot in common with chameleons, new research reveals.

Fired with sufficient force, suction arrows stick to any smooth surface. The same goes for a chamelon's characteristic long tongue. It shoots out to capture prey of all kinds -- with suction, Anthony Herrel of the University of Antwerp, Belgium, and colleagues have discovered.

Conventional wisdom has it that wet adhesion and surface roughness are responsible for the stickiness of the end of a chameleon's tongue. But stickiness cannot explain how chameleons can capture birds and lizards up to ten per cent of their own body mass. These are too large for stickiness to establish a firm grip, and also very smooth -- the head of a locust, for instance, is as bald as that of any cartoon park keeper.

Suction is the answer, Herrel's team announce in the Journal of Experimental Biology1.

Chameleons belong to a large group of primitive lizards, the Iguania. Iguanians use their tongues to capture prey but chameleons have taken this to an extreme, developing a ballistic mechanism in which the tongue is shot at prey at speeds too great for the human eye to follow.

Precisely what happens during the actual instant of prey capture has never been securely established. The heavy, gland-rich tongue tip had been thought to splay out on impact, enclosing the prey in adhesive, mucous glue.

Now, using old-fashioned anatomical dissection and high-tech, high-speed X-ray cinematography, Herrel's group have worked out the course of events during the eye blink in which chameleons capture their prey.

When the tongue is fired at the prey, the tongue tip is convex. During travel, and particularly just before impact, a pair of muscles called the pouch retractors inside the tongue, contract, turning the tip of the tongue into a concave suction cup.

At impact, the tip does not splay out. Instead, the 'lips' formed by the contraction of the pouch retractors fasten around the prey item, producing a suction effect.

Conventional adhesion and surface roughness are undoubtedly important, especially during the return phase in which the captured prey is conveyed to the mouth. But chameleons in which the nerves that work the pouch retractors had been surgically severed were unable to capture prey at all -- suggesting that suction is crucial to the success of the entire enterprise. 

  • References

    1. Herrel,A., Meyers, J. J., Aerts, P. & Nishikawa, K. C. The mechanics of prey prehension in chameleons, Journal of Experimental Biology 203, 3255 - 3263 2000. | PubMed | ISI |