The salamander’s ballistic tongue

In Arabic, there is a phrase that is often applied to cocky youngsters: lisanak taweel ( “your tongue is long”). But even the cockiest young Arab is no match for the plethodontids, or lungless salamanders – in 1997, zoologist Stephan Deban, who was then at the University of California, Berkeley, and his colleagues showed that salamanders of the genus Hydromantes, which are 2.5 inches long, have a tongue that measures 2 inches – that is, 80% of their body length.

The salamander can launch its tongue like a projectile, and is highly accurate with its aim. When protruded from the mouth, the tongue leaves the salamander’s body completely; the sticky pad at the end of the tongue adheres to the small insects on which the salamander preys, which are pulled into the mouth when the tongue is reeled back in. Protraction and retraction of the tongue occurs within 20 milliseconds (twenty thousandths of a second).

The salamander is able to do this because of an elaborate tongue skeleton and specialized tongue muscles. Whereas the tongue skeleton of other salamanders is located in the floor of the mouth, that of plethodontid salamanders has elongated cartilage elements which extend back over the shoulders. The skeleton contains a tuning fork-shaped cartilage structure which is derived from the visceral skeleton that supported the gill arches of the salamander’s ancient ancestor. The forked ends of this structure taper to a thin point at the back of the mouth; the tapered ends are surrounded by specialized ring-shaped tongue protractor muscles, which are so large that they are visible as bulges in the side of the salamander’s body.

The tongue projector muscles exert a squeezing as well as a pulling force on the tongue skeleton. When they contract around the tapered ends of the forked cartilage, the muscles force the tongue from the mouth at a high speed, in the same way that a watermelon seed can be projected by squeezing it between the fingers. The tongue leaves the mouth at such a high speed that it is propelled by its own momentum. As the tongue is being protracted, the fork stretches and folds up partially and, when it leaves the body, collapses completely along the midline of the tongue. When the tongue is not in use, the forked bones remain open and unfolded in the mouth.

The salamander also has tongue retractor muscles, which are anchored between the salamander’s hind limbs in the pelvis, and which extend throughout to the tongue’s tip. When the tongue is at rest, the retractor muscles are slack, and remain looped and folded over themselves within the salamander’s mouth. When the tongue is protruded, the retractor muscles stretch like a rubber band, then quickly reel the tongue back into the mouth.

Lungless salamanders are the only known vertebrates with the ability to shoot part of the visceral skeleton entirely out of the body. And now, new work from Deban’s lab shows that the tongue projector muscles (the subarcualis rectus, or SAR muscles) generate the largest force exerted by any vertebrate. The findings are published in the Journal of Experimental Biology.

Deban’s team used several different species of lungless salamanders in the experiments, which involved using a electromyograph to record the electrical activity of the SAR muscles during tongue projection. Small incisions were made in the salamanders’ skin so that electrodes could be placed on the surface of the projector muscles. It was found that the muscles can generate a power output of up to 436 Watts per kilogram. Electrical activity was recorded in the projector muscles just 117 milliseconds, on average, before the tongue left the mouth. High-speed cameras were then used to film the kinematics of tongue protraction. Analysis of the footage showed that, in the fastest species, the average velocity of tongue protraction was 4.6 metres per second, and the average acceleration was 1740 metres per second per second.

The salamander’s tongue protrusion mechanism requires three components: a motor to generate the mechanical work, a spring to store the energy, and a latch to control the release of the spring. In the salamander, the SAR muscles constitute the motor. But the speed with which the tongue is projected cannot be produced by the power of the muscles alone, and the other components are as yet unidentified. Deban speculates that elastic collagen fibres which line the insides of the SAR muscles constitute the springs of the protrusion mechanism, and believes that the short time delay between activation of the SAR muscles and tongue protraction is sufficient for the energy generated by the muscles to be transmitted to, and stored in, the collagen fibres.

References:

Deban, S. M., et al. (2007). Extremely high-power tongue projection in plethodontid salamanders. J. Exp. Biol. 210: 655-667.

Deban, S. M., et al. (1997). Salamander with a ballistic tongue. Nature 389: 27-28.

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