Feet Used for Powerful Swimming

a green and brown spotted bullfrog peeks their yellow eyes above water, webbed feet hang underwater behind them

Biological Strategy

AskNature Team

Image: Victoria Pickering / Flickr / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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

Image: Guido Gerding / Wikimedia Commons /

OLYMPUS DIGITAL CAMERA

Image: Guido Gerding / Wikimedia Commons /

OLYMPUS DIGITAL CAMERA

Image: Ib Rasmussen / Wikimedia Commons /

Rana esculenta, photographed in a garden at Stillinge Strand, Denmark.

Image: Ib Rasmussen / Wikimedia Commons /

Rana esculenta, photographed in a garden at Stillinge Strand, Denmark.

Image: Monika Betley / Wikimedia Commons /

Rana esculenta

Image: Manfred Heyde / Wikimedia Commons /

Rana esculenta

Image: Anna S. / Wikimedia Commons /

DCF 1.0

Image: Drachentoeter / Wikimedia Commons /

Teichfrosch mit aufgeblasenen Backen

Image: Sebaho / Wikimedia Commons /

“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)