The role of hyoid muscles in biotremor production in Chamaeleo calyptratus.

The production of biotremors has been described in veiled chameleons (Chamaeleo calyptratus), but the mechanism by which they are produced is unknown. We gathered muscle activation data via electromyography (EMG), with simultaneous recordings of biotremors using an accelerometer, to test for the role of hyoid muscles in biotremor production. We recorded a mean biotremor frequency of 150.87 Hz for females and 136.01 Hz for males. The durations of activity and the latencies to onset and offset for the M. sternohyoideus profundus (SP), M. sternohyoideus superficialis (SS), Mm. mandibulohyoideus (MH) and M. levator scapulae (LS) were all significantly correlated with biotremor durations and biotremor onset and offset, respectively. Linear mixed-effect regression model comparisons of biotremor duration indicated that models containing either the MH and/or the SP and LS account for the most variation in biotremor duration. Twitch times for the SP (100 ms) and the SS (132 ms) at field active body temperature, however, were individually too slow to produce the biotremors at the observed frequency without alteration after production by other anatomical structures. These results implicate the SP, SS, MH and LS in the production of biotremors, but the exact mechanism of production requires further study.