Springs in Swimming Animals

Animals can lower the metabolic cost of swimming by using appropriatelytuned, elastic springs. Jet-powered invertebrates use springsthat lie in functional parallel to their swimming muscles topower half the locomotor cycle. The parallel geometry constrainsthe spring to be non-linearly elastic; muscle power is divertedto load the spring only when swimming muscles are not capableof producing maximal hydrodynamic thrust. The springs of jellyfishand scallops are forced at or near their resonant frequency,producing large energy savings. Measuring the contribution ofelastic energy storage to jet-powered locomotion has been facilitatedby the relatively simple geometries of invertebrate locomotorsystems. In contrast, complex musculoskeletal systems and kinematicshave complicated the study of springs in swimming vertebrates.Skins, tendons and axial skeletons of some vertebrate swimmershave appropriate mechanical properties to act as springs. Todate, though, there exist just a handful of studies that haveinvestigated the mechanical behaviors of these locomotor structuresin swimming vertebrates, and these data have yet to be integratedwith measures of swimming power. Integrating mechanical, kinematic,hydrodynamic and metabolic data are required to understand morefully the role of elastic springs in vertebrate swimming energetics.