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Feet used for powerful swimming: true frogs


XN Rana sp 01 / Guido Gerdin.. / LicenseGFDL - Gnu Free Document License

The webbed back feet of true frogs are used to swim by pushing back against the water creating vortex rings.

"Frogs propel themselves by kicking water backwards using a synchronised extension of their hind limbs and webbed feet. To understand this propulsion process, we quantified the water movements and displacements resulting from swimming in the green frog Rana esculenta, applying digital particle image velocimetry (DPIV) to the frog’s wake.

"The wake showed two vortex rings left behind by the two feet. The rings appeared to be elliptic in planform, urging for correction of the observed ring radii. The rings’ long and short axes (average ratio 1.75:1) were about the same size as the length and width of the propelling frog foot and the ellipsoid mass of water accelerated with it. Average thrust forces were derived from the vortex rings, assuming all propulsive energy to be compiled in the rings. The calculated average forces (Fav=0.10±0.04·N) were in close agreement with our parallel study applying a momentum–impulse approach to water displacements during the leg extension phase.

"We did not find any support for previously assumed propulsion enhancement mechanisms. The feet do not clap together at the end of the power stroke and no ‘wedgeaction’ jetting is observed. Each foot accelerates its own water mantle, ending up in a separate vortex ring without interference by the other leg." (Stamhuis and Nauwelaerts 2005:1445)
About the inspiring organism
Med_xn_rana_sp_01 edible frog
Rana esculenta Linnaeus, 1758

Habitat(s): Wetlands
Natural History Information: Also called Pelophylax esculentus.
Learn more at
Some organism data provided by: ITIS: The Integrated Taxonomic Information System
Organism/taxonomy data provided by:
Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist

Threat Categories LONG_LC IUCN Red List Status: Least Concern

Bioinspired products and application ideas

Application Ideas: Flexible appendages for ships, planes, or boats that can be used to increase or decrease drag as needed; flexible stabilizing attachments for marine infrastructure; flexible fabrics for safety equipment.

Industrial Sector(s) interested in this strategy: Transportation, shipping, marine siting, textiles

Ocean Ecosystems
Eize J. Stamhuis
University of Groningen
Foy S; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
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Stamhuis, E. J.; Nauwelaerts, S. 2005. Propulsive force calculations in swimming frogs II. Application of a vortex ring model to DPIV data. Journal of Experimental Biology. 208: 1445-1451.
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Nauwelaerts, S.; Stamhuis, E.J.; Aerts, P. 2005. Propulsive force calculations in swimming frogs I. A momentum–impulse approach. Journal of Experimental Biology. 208: 1435-1443.
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