Tunnel-tube and Fourier methods for measuring three-dimensional medium reaction force in burrowing animals

Subterranean digging behaviors provide opportunities for protection, access to prey, and predator avoidance for a diverse array of vertebrates, yet studies of the biomechanics of burrowing have been limited by the technical challenges of measuring kinetics and kinematics of animals moving within a medium. We describe a new system for measuring 3D reaction forces during burrowing, called a 'tunnel-tube', which is composed of two, separately instrumented plastic tubes: an 'entry tube' with no medium, in series with a 'digging tube' filled with medium. Mean reaction forces are measured for a digging bout and Fourier analysis is used to quantify the amplitude of oscillatory digging force as a function of frequency. In sample data from pocket gophers digging in artificial and natural media, the mean ground reaction force is constant, whereas Fourier analysis resolves a reduced amplitude of oscillatory force in the artificial medium with lower compaction strength.