LAPPIN, A.K.*; NISHIKAWA, K.C.; PIEROTTI, D.J.: A proposed mechanism for high power output during feeding in toads

Among anurans, toads exhibit the greatest tongue protraction velocities (>270 cm/s) and accelerations (>310 m/s) during feeding. This is accomplished via transmission of inertial forces from the rapidly opening mouth to the tongue. The mouth is opened by the depressor mandibule (DM) muscles, which are estimated to produce 1.165 W/g (3X power of frog semimembranosus during jumping). Sonomicrometry shows that the in vivo shortening velocity of the DM is 8.5 ML/s, near the upper extreme known for other vertebrate muscles studied to date. However, the isometric contractile properties of the DM do not reveal any design characteristics that explain their high power output. In addition, V_max was extrapolated to be <4 ML/s from a series of isotonic contractions, within the range known for other muscles and well below the in vivo shortening velocity. Other tests, however, reveal properties of the DM that begin to account for their remarkable power output. First, the maximum unloaded shortening velocity (V_o), as measured with a slack test, is 21.4 ML/s. This is the highest V_o measured to date in an ectotherm. Second, in a dynamic stiffness test, a 6% increase in muscle length during an isometric tetanus generates a 65% increase in force. Both the slack test and dynamic stiffness test indicate that the DM has the capacity to function as a very stiff spring. We propose that energy is stored in the active DM during pre-loading by the antagonistic adductor mandibulae. The high stiffness of the DM permits efficient recovery of elastic strain energy from the pre-loaded muscle thus resulting in high power output during rapid mouth opening.