Paper, The elastic secrets of the chameleon tongue

The ballistic projection of the chameleon tongue is an extreme example of quick energy release in the animal kingdom. It relies on a complicated physiological structure and an elaborate balance between tissue elasticity, collagen fibre anisotropy, active muscular contraction, stress release and geometry. A general biophysical model for the dynamics of the chameleon tongue based on large deformation elasticity is proposed. The model involves three distinct coupled subsystems: the energetics of the intralingual sheaths, the mechanics of the activating accelerator muscle and the dynamics of tongue extension. Together, these three systems elucidate the key physical principles of prey-catching among chameleonides.

Among animals, chameleons have strikingly distinctive features: they have zygodactylous feet, prehensile tails, colour-changing ability, panoramic eyes and ballistic projection of their tongue for prey-catching. What distinguishes prey-catching in chameleons is not only the extension of the tongue—up to 2.5 body lengths—but also the extreme acceleration and short duration of the entire ballistic projection (figure 1). Anderson [1] estimates the total duration of tongue projection, depending on species, to last between 10 and 55 ms. Maximum accelerations between 500 and 2590 m s−2 are reported, requiring peak power density between 3000 and 14 040 W kg−1 [1–4]. The accepted theory is that the tongue projection is triggered entirely by intrinsic muscular activities [5]. It is further understood that the peak power recorded in tongue extension cannot be solely due to muscle activation, but is the result of a release of the energy stored in the extension of the tongue’s collagenous tissue [3]. By contrast, the relatively slow process of tongue retraction results from direct muscular contraction as demonstrated experimentally [6]. The chameleon’s ballistic mechanism is a clear example in biology of elastic forces generating rapid motion.

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