Spring-Loaded

Every beat of a dolphin's tail stores elastic energy that helps propel the animal forward.

Story by Adam Summers  ~  Illustrations by Sally J. Bensusen

The two Pacific white-sided dolphins riding the bow wave of our boat are a paean to power and a testament to biomechanical efficiency. Though the boat motors along at eighteen knots, the dolphins keep pace without visible effort, their tail flukes barely beating in the clear blue water. When they slide off the wave, moving away from the boat, they increase their pace to an easy lope more akin to the stride of a long-distance runner than that of a sprinter. Besides symbolizing grace, speed, and exuberance, dolphins are a wonderful aquatic example of an important principle of locomotion: the temporary storage of energy in a spring.

The subdermal sheath attaches some of the dolphin's muscles to its skeleton and provides some of the "spring" in the animal's movements.

To experience the benefits of spring-loaded locomotion, just hop on one of the icons of 1950s popular culture, a pogo stick. The downward part of each bounce compresses a spring at the bottom of the stick; energy stored there is then converted into the upward thrust of the next bounce. To bounce higher (or farther), all you have to do is push down a little harder on the spring; a small investment of energy produces a significant increase in jumping power. Kangaroos capitalize on this principle when they hop.

As swimming muscles lift the dolphin's tail, blubber on the top side is compressed while blubber and the fibers of the subdermal sheath on the bottom are stretched, storing energy for the downstroke.

As the muscles relax, the previously compressed blubber springs back and may help push the tail down. The now compressed blubber and sheath on the tail's underside may also contribute to the downward pull on the tail.

A clue that dolphins (and probably many other cetaceans) use biological springs to store energy lies in data collected by Terrie Williams, of the University of California, Santa Cruz. She trained bottlenose dolphins to swim in place against a stationary target designed to register the force with which they were pushing. The faster they beat their tails, the harder they pushed (and the faster they would have swum if they had been unrestrained). Responding to signals, the dolphins willingly maintained several different tail-beat frequencies, even though all that churning got them nowhere. Meanwhile, Williams measured how much oxygen the animals consumed while generating the force, since oxygen consumption is an indication of energy used. She found that even when the dolphins nearly doubled the force they were producing, their metabolic rate-as well as the frequency with which they popped above the surface to breathe-did not change.

The biomechanics of marine mammals, and particularly how they store elastic energy, is the subject of research in the lab of Ann Pabst and William McLellan at the University of North Carolina at Wilmington. Much of their work focuses on the animals' hind end, which in whales and dolphins tapers to a narrow tailstock that is higher than it is wide. The tailstock ends in the flukes; locomotion is powered by the up-and-down motion of these broad, flattened, propulsive blades. Muscles contract to pull the tail up, but at some point the tail must stop, reverse direction, and go down again. This reversal could be accomplished actively, through the contraction of opposing muscles, or passively, by the action of springs.

Pabst and McLellan's studies have led them to speculate that blubber and a connective tissue called the subdermal sheath are the springs in the dolphin's pogo stick. A stiff, helically wound membrane, the subdermal sheath surrounds the entire tailstock and attaches some of the underlying muscles to the skeleton. When the tail is on its way up, the sheath on the bottom side of the tail is stretched, storing some energy for the downstroke. On the way down, energy gets stored on the tail's top side. Perhaps even more important is the role of the blubber, a thick layer of fat that lies beneath the skin and, unlike the fat of your next-door neighbor's beer belly, is reinforced with a highly organized, three-dimensional weave of collagen fibers. This fibrous material gives dolphin blubber a stiffness and resilience not found in other forms of fat. Specialized triangular wedges of blubber run along both the top and the bottom of the tailstock. When a swimming dolphin flips its tail up, the top wedge compresses and the bottom wedge stretches. Once the tail can go no higher, the wedge of blubber on the top pushes it (and the wedge on the bottom pulls it) back down.

The combination of these two biological springs-the blubber and the sheath-may be what helps propel the dolphin at speeds greater than twenty miles per hour, making them the envy of swimmers and engineers alike. Perhaps our Olympians are on the wrong track, losing body fat to go faster. The trick might be to acquire better-designed fat, not less of it.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu).