Comparing terrestrial locomotion strategies of amphibious fishes using robophysical models

Many amphibious fishes can move on land. Their sustained terrestrial locomotion falls into three strategies by their propulsive mechanisms: using fins (e.g., mudskipper), using body lateral bending (e.g., ropefish), and combining fin use and body lateral bending (e.g., bichir). Biological studies focused on quantifying kinematics and muscular control. Recent fish robot simulations revealed that the mudskipper's gait (fin use only) is better for its shorter body whereas the bichir's gait (body bending and fin use together) is better for its longer body. To further advance our understanding of advantages and tradeoffs of these three strategies over a broader range of actuation employed by fishes, we created a robophysical model of amphibious fishes that can execute all three strategies and tested it on solid ground with systematically varied body and/or fin actuation. For a given stride frequency, the mudskipper gait became faster as fins swept backward more and were tucked in closer to the body, which increased stride length and raised the body more to reduce its ground contact (presumably reducing drag), respectively. The ropefish gait struggled to move regardless of body waveform but turned in place, due to limited body-ground friction anisotropy to break symmetry. The bichir gait was the fastest of the three strategies, with speed increasing with body bending amplitude. Its speed was also sensitive to the phase offset between the fin and body motions, sometimes even becoming negative (moving backwards).