(a,b) Pressure
contours and flow streamlines at two instants during a propulsive
swimming stroke.
Left half of body is indicated by black shape.
(November 3, 2015) A
central and long-standing tenet in the conceptualization of animal swimming is
the idea that propulsive thrust is generated by pushing the surrounding water
rearward. Inherent in this perspective is the assumption that locomotion
involves the generation of locally elevated pressures in the fluid to achieve
the expected downstream push of the surrounding water mass. Here we show that
rather than pushing against the surrounding fluid, efficient swimming animals
primarily pull themselves through the water via suction. This distinction is
manifested in dominant low-pressure regions generated in the fluid surrounding
the animal body, which are observed by using particle image velocimetry and a
pressure calculation algorithm applied to freely swimming lampreys and
jellyfish. These results suggest a rethinking of the evolutionary adaptations
observed in swimming animals as well as the mechanistic basis for bio-inspired
and biomimetic engineered vehicles.