Giant Galápagos tortoises walk without inverted pendulum mechanical-energy exchange

Animals must perform mechanical work during walking, but most conserve substantial mechanical energy via an inverted-pendulum-like mechanism of energy recovery in which fluctuations of kinetic energy (KE) and gravitational potential energy (GPE) are of similar magnitude and 180° out of phase. The greatest energy recovery typically occurs at intermediate speeds. Tortoises are known for their slow speeds, which we anticipated would lead to small fluctuations in KE. To have an effective exchange of mechanical energy using the inverted-pendulum mechanism, tortoises would need to walk with only small changes in GPE corresponding to vertical center-of-mass (COM) fluctuations of <0.5 mm. Thus, we hypothesized that giant Galápagos tortoises would not conserve substantial mechanical energy using the inverted-pendulum mechanism. We studied five adult giant Galápagos tortoises Geochelone elephantopus (mean mass=142 kg; range= 103-196 kg). Walking speed was extremely slow (0.16±0.052 m s^-1; mean ± 1 S.D.). The fluctuations in kinetic energy (8.1±3.98 J stride^-1) were only one-third as large as the fluctuations in gravitational potential energy (22.7±8.04 J stride^-1). In addition, these energies fluctuated nearly randomly and were only sporadically out of phase. Because of the dissimilar amplitudes and inconsistent phase relationships of these energies, tortoises conserved little mechanical energy during steady walking, recovering only 29.8±3.77% of the mechanical energy (range=13-52%). Thus, giant Galápagos tortoises do not utilize effectively an inverted-pendulum mechanism of energy conservation. Nonetheless, the mass-specific external mechanical work required per distance (0.41±0.092 J kg^-1 m^-1) was not different from most other legged animals. Other turtle species use less than half as much metabolic energy to walk as other terrestrial animals of similar mass. It is not yet known if Galápagos tortoises are economical walkers. Nevertheless, contrary to biomechanical convention, poor inverted-pendulum mechanics during walking do not necessarily correspond to high mechanical work and may not result in a high metabolic cost.