The acoustic mechanics of stick–slip friction in the California spiny lobster (Panulirus interruptus)

Abstract

SUMMARY The dynamic interplay between static and sliding friction is fundamental to many animal movements. One interesting example of stick–slip friction is found in the sound-producing apparatus of many spiny lobster species(Palinuridae). The acoustic movements of the spiny lobster's plectrum over the file are generated by stick–slip friction between the two surfaces. We examined the microscopic anatomy, kinematics, acoustics and frictional properties of the California spiny lobster (Panulirus interruptus)toward the goal of quantitatively characterizing the frictional and acoustic mechanics of this system. Using synchronous high-speed video and sound recordings, we tested whether plectrum kinematics are correlated with acoustic signal features and found that plectrum velocity is positively correlated with acoustic amplitude. To characterize the frictional mechanics of the system, we measured frictional forces during sound production using excised plectrums and files. Similar to rubber materials sliding against hard surfaces, the static coefficient of friction in this system was on average 1.7. The change in the coefficient of friction across each stick–slip cycle varied substantially with an average change of 1.1. Although driven at a constant speed, the plectrum slipped at velocities that were positively correlated with the normal force between the two surfaces. Studies of friction in biological systems have focused primarily on adhesion and movement, while studies of stick–slip acoustics have remained under the purview of musical acoustics and engineering design. The present study offers an integrative analysis of an unusual bioacoustic mechanism and contrasts its physical parameters with other biological and engineered systems.