Multilingual - Brief Article

Natural History, Oct, 1999 by Carl Zimmer

Reptiles and amphibians show there is more than one way to stick out your tongue.

When we humans stick out our tongues, we lengthen them by about 25 percent. That's a paltry feat compared with what some other mammals can do. Anteaters and pangolins, for instance, can double the length of their tongues to snap up insects; much of their success comes from having tongues that are both long (the pangolin's tongue is anchored at its hips) and slender.

It's a simple matter of geometry: the volume of a tongue is equal to the area of its cross section times its length. Consider two hypothetical tongues, both eight inches long. One of them is chunky, with a radius of two inches. The other is thin, with a radius of only one inch. Now squeeze both tongues so that each radius loses half an inch. The area of the slender tongue's cross section shrinks drastically, by 75 percent; that of the chunky tongue shrinks by less than half. And since the volume does not change, the tongues compensate for the smaller diameters by stretching out, the chunky tongue to a little over fourteen inches and the slender to an impressive thirty-two inches.

But no mammal can compete with the ability of some reptiles and amphibians to extend their tongues far and fast. Some species of salamander can, in milliseconds, shoot their tongues out more than half the length of their bodies. The tongues of these animals are attached in two places: the back end is anchored to the pelvis, and the front is connected to a long piece of cartilage shaped like a two-tined fork. Surrounded by rings of muscle, the tines of cartilage taper to a point as they extend back into the body. Stephen Deban, a biologist at the University of Bremen, Germany, has discovered that when these protractor muscles squeeze down on the cartilage, it shoots entirely out of the salamander's body. The principle is the same as pinching a watermelon seed to make it sail across the room: force applied to the tapered end of an object will cause the object to move away from the source of the pressure. As the cartilage shoots forward, it pulls the tongue along with it. Inertia (the tendency of an object in motion to stay in motion) does the rest. Once the salamander has hit its target, it reels its tongue back in by contracting the retractor muscles attached to its pelvis.

Chameleons can hurl their tongues even farther than salamanders, shooting them twice the length of their bodies in less than a second. They do this with a sort of reverse watermelon-seed strategy. In their case, the "seed" is a shaft of bone reaching up from the throat and lodging in the core of the tongue. This bone is tapered, like the salamander's cartilage, but its tip points toward the mouth, not toward the tail. When the rings of tongue muscle contract around the bone, they create a force that wants to push the bone back. But because the bone is anchored, it can't move. Instead, the force generated by the squeezing muscles pushes the muscles themselves out of the chameleon's mouth. (It's like a pushup. When you push against the floor, you go up; the floor doesn't go down.) Like the salamander, the chameleon has a lot of loose muscle fibers in its tongue; when the tongue flies forward, these fibers unfold and stretch out. As the tongue leaves the chameleon's mouth, it unfurls at a rate of about twenty feet a second.

Rings of muscle aren't mandatory for sticking out a tongue, however, as demonstrated by many species of frogs and toads, including the much-studied marine toad (Bufo magnus). At rest, the marine toad's tongue lies flopped upside down in its mouth, the tip pointing backward. To catch a bug, it jerks its jaw down with a tremendous force thirty times that of gravity--that pulls the tongue to the edge of its mouth, where the force of the jerk flips the tip of its tongue over and forward. This sudden rotation gives the tongue even more energy, just as the flick of a wrist can crack a whip. The tongue whizzes out of the toad's mouth at about nine feet a second, and the toad, like the chameleon and the salamander, lets inertia take over.

Kiisa Nishikawa, a Northern Arizona University expert on frog tongues, has been studying a variant of this technique in the African pig-nosed frog. This frog opens its jaw with only enough energy to gently flip the tongue out of its mouth. If a marine toad did this, its tongue would fall limp to the ground. But Nishikawa, working with biologists William Kier, of the University of North Carolina, and Kathleen Smith, of Duke University, has discovered that the pig-nosed frog has a sheet of muscle sandwiched inside its tongue. Once the tongue flips forward, the frog contracts the sheet, squeezing the tongue to half its usual height and, relatively slowly, doubling its length. (You might think that contracting the sheet of muscle could make the tongue wider as well, but because the frog's tongue is encased in a tough sheath of connective tissue, length is the only dimension that can change.)