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Journal of Experimental Biology 168,1-21 (1992)
Published by Company of Biologists 1992

The Mechanism of Tongue Projection in Chameleons: I. Electromyographic Tests of Functional Hypotheses

PETER C. WAINWRIGHT ¹ and ALBERT F. BENNETT ²

¹ Department of Ecology and Evolutionary Biology, University of California Irvine, CA 92717, USA; Department of Biological Science, B-157, Florida State University, Tallahassee, FL 32306-3050, USA.
² Department of Ecology and Evolutionary Biology, University of California Irvine, CA 92717, USA

In this paper we document the activity of key muscles of the tongue, hyobranchial apparatus and head during prey capture in the lizard Chamaeleo jacksonii Boulenger and use these data to test current hypotheses of chameleon tongue function. Electromyographic recordings were made during 27 feedings from nine individuals and synchronized with high-speed video recordings (200 fields s^-1), permitting an assessment of the activity of muscles relative to the onset of tongue projection, contact between tongue and prey, and tongue retraction. Four major results were obtained. (1) The hyoglossi muscles exhibit a single burst of activity that begins between 10 ms before and 20 ms after the onset of tongue projection and continues throughout the period of tongue retraction. (2) The accelerator muscle exhibits a biphasic activity pattern, with the first burst lasting about 185 ms and ending an average of 10.6 ms prior to the onset of projection. (3) The accelerator muscle shows regional variation in morphology that corresponds with variation in motor pattern. The anterior region of the muscle, unlike the posterior portion, exhibits only a single burst of activity that begins 2.5 ms after the onset of tongue projection and is thus not involved in launching the tongue. (4) The geniohyoidei, sternohyoidei, sternothyroidei, depressor mandibulae, adductor mandibulae and pterygoideus all exhibit activity patterns consistent with previously reported kinematic patterns and their proposed roles. The major implications of these results for models of the chameleon feeding mechanism are (1) that the hyoglossi do not act to hold the tongue on the entoglossal process during a loading period prior to tongue projection, and (2) that the presence of 185 ms of intense activity in the accelerator muscle prior to tongue projection suggests the presence of a preloading mechanism, the nature of which is the subject of the companion paper.

Key words: Chamaeleo jacksonii, chameleon, contractile kinetics, electromyography, feeding, lizard, skeletal muscle, tongue

Accepted on March 23, 1992

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