MONROY, JA; NISHIKAWA , KC; Northern Arizona University: Modulation of muscle pre-activation as a function of prey distance in freely behaving toads

In toads, the mouth opening muscles, the m. depressor mandibulae, power ballistic tongue projection. During rapid mouth opening, momentum is transferred from the lower jaw to the tongue to launch the tongue from the mouth. Because tongue projection is dependent upon mouth opening, the velocity of the tongue and the distance the tongue extends from the mouth are controlled by modulation of m. depressor mandibulae activity. Recent studies have demonstrated that these muscles are pre-activated as much as 200 ms prior to rapid mouth opening. Research suggests that during muscle pre-activation, elastic strain energy is stored and then recovered at the onset of mouth opening. Energy storage and recovery provides a means of increasing power output by increasing shortening velocity for a given load. Based on these findings, we hypothesized that mouth opening velocity increases with pre-activation duration and that freely behaving toads modulate the duration to control the velocity and reach of the tongue. As prey distance increases, the tongue must extend farther from the mouth at faster velocities. This suggests that one behavioral strategy of toads is to modulate pre-activation to increase tongue accuracy for a given prey distance. Using a RedLake high-speed digital imaging system, we measured the feeding kinematics of Bufo alvarius at different distances from the prey. Electromyography of the m. depressor mandibulae was recorded and the duration and rectified integrated area of the pre-activation period were analyzed. Our results demonstrate strong correlations between mouth opening velocity, tongue velocity, tongue reach, prey distance, and pre-activation duration. Thus, toads may modulate pre-activation of the mouth opening muscles in response to prey distance to maximize tongue velocity and reach.