Passive thrust production in animal propulsion: exploitation of velocity gradients for energy conservation

Animals move through fluid environments that are rich in velocity gradients, yet how they exploit these flows to reduce locomotor costs remains elusive. We hypothesize that vertebrates utilize shear layers to passively produce thrust and establish a force equilibrium, thereby reducing locomotor cost. We discovered a shear-layer gait where fish preferentially position themselves within velocity gradients to maintain a steady posture and position at ~50% of their sustained maximum speed. Compared to the matching speed of uniform flow, fish moved into an adjacent velocity gradient reduced active propulsion effort by 98% and metabolic costs by 74%, reaching a resting state. The mechanism is a force equilibrium between viscous drag on the high-velocity side of the body and anteriorly directed suction forces on the low-velocity side, fundamentally a passive thrust production driven by the pressure gradients. Such discovery has direct applications in bioinspired propulsion design and broad implications for movement ecology.