Many animals use several different gaits — patterns of locomotion that change abruptly at a critical speed. People walk to go slowly and run to go fast. Horses and other quadrupedal mammals walk, trot and gallop. In flight, birds and bats have slow and fast gaits. Many fish swim slowly with their fins and fast with their whole bodies. On land, frogs crawl slowly and hop faster.

Now we learn from Sandra Nauwelaerts and Peter Aerts that frogs also change gait when they swim (J. Zool. 258, 183–188; 2002). Frogs swim by kicking water backwards with their webbed feet. Usually they kick with both hind legs simultaneously (in-phase swimming), but Nauwelaerts and Aerts find that in slow swimming the hind legs move alternately (out-of-phase swimming). The subject of their attention, the species Rana esculenta, is shown here in static pose.

Measurements of oxygen consumption show that humans and horses change gait at speeds at which the slower gait becomes less energy-efficient than the faster one. The same may be true for flying birds. Fish, however, seem to change gait because their fin muscles cannot provide enough power for high speeds. Do swimming frogs save energy by changing gait, or do they have to change gait to develop enough power to swim fast?

From measurements on video sequences, Nauwelaerts and Aerts calculate that out-of-phase swimming uses less power than in-phase swimming at the same speed. During in-phase swimming, the animal accelerates by kicking with both feet, then decelerates as it brings its feet forward for the next kick. During out-of-phase swimming, the frog's speed fluctuates much less because each leg is brought forward while the other is kicking. Hydrodynamic drag is approximately proportional to the square of the speed, so speed fluctuations increase the mean drag.

This argument suggests that out-of-phase swimming should be the more economical gait at all speeds. However, it may be less effective at generating thrust, because the feet interact when they are used together. So frogs may be obliged to use the less economical gait to generate enough thrust to swim fast.