Motor pattern control for increasing crushing force in the striped burrfish (Chilomycterus schoepfi)

The relationship between muscular force modulation and the underlying nervous system control signals has been difficult to quantify for in vivo animal systems. Our goal was to understand how animals alter muscle activation patterns to increase bite forces and to evaluate how accurate these patterns are in predicting crushing forces. We examined the relationship between commonly used measures of cranial muscle activity and force production during feeding events of the striped burrfish (Chilomycterus schoepfi), a mollusc crushing specialist. We quantified the force required to crush a common gastropod prey item (Littorina irrorata) of burrfish using a materials testing device. Burrfish were fed these calibrated prey items while we recorded electromyograms (EMGs) from the main jaw closing muscles (adductor mandibulae A1beta, A2alpha, and A2beta). We quantified EMG activity by measuring the burst duration, rectified integrated area, and then calculated the intensity of activity from these two variables. Least squares regressions relating force to crush (Fcrush) and all EMG variables were calculated for each fish. Multiple regression analyses were used to determine how much of the variation in Fcrush could be explained by muscle activation patterns. We found that 20 cm burrfish are capable of generating extremely high crushing forces (380 N peak force) primarily by increasing the duration of muscle activity. EMG variables explained 71% of the total variation in force production. After accounting for the inherent variation in Fcrush of snails, EMGs do a very good job of predicting bite forces for these fish.