Passage IV

NATURAL SCIENCE: This passage is adapted from the article "In Nature, Animals That Stop and Start Win the Race" by Elizabeth Pennisi (©2000 by the American Association for the Advancement of Science).

In 1995, marine physiologist Terrie Williams was stumped. After studying the oxygen requirements of diving dolphins, she had carefully calculated that dives to 200 meters required 28% more oxygen than the animal could possibly inhale or have in reserve. Deep, prolonged dives might well be fatal. Yet in field experiments, somehow her subjects-trained bottlenose dolphins-easily plummeted to depths well below 200 meters and returned safely, with ample reserves of oxygen. After 5 years of arduous field experiments in the Pacific and Antarctic oceans, Williams and her colleagues finally discovered the diving dolphins' secret.

As she reports, rather than swimming—and consuming oxygen—all the way down, dolphins take a few strokes and then glide as long as possible, a trick bio-mechanicists call intermittent locomotion. By doing less work, the animals use less oxygen, and so can dive deeper and longer. This was quite a surprise, for dolphins and whales have been intensely studied for years and no one had any inkling that this diving behavior existed.

For decades researchers have emphasized steady-state locomotion, bringing organisms into the laboratory and watching them move at a steady pace. But Williams's finding is just one of a stream of recent results indicating that that focus was only a first step. The new work shows that animals from aquatic invertebrates to humans move like window shoppers, stopping and starting as they seek out food, mates, or shelter.

Probing the fitful nature of locomotion is helping researchers understand how various organisms' bodies and biochemistry are adapted for movement, and it may even have applications in human medicine.

For example, at a recent symposium organized by comparative physiologist Randi Weinstein, researchers compared notes and found that intermittent locomotion has surprising benefits for organisms-everything from allowing time to notice the surroundings to saving energy. In some respects human muscles appear to be more efficient when working intermittently than when working steadily, for example. This suggests better ways of helping people make the best use of their bodies, says Weinstein. "By inserting rest and pauses, and changing the [exercise] interval, it might be possible to decrease the physiological load on the body [of] people with compromised systems," such as those recovering from heart problems, she says.

There are good reasons why researchers analyzing locomotion previously concentrated on steady-state experiments. Such analyses suited theorists, as modeling the mechanics and energetics of steady movement is more tractable than the mathematics of changing gaits or repeated starts and stops. And designing effective experiments in natural settings can be a challenge, says Williams, who should know.

To solve the paradox of how marine mammals dive so deeply, her team first had to develop or track down technologies to monitor the animals as they swam. The researchers coaxed a prosthetics manufacturer to make a custom-molded plastic housing to fit over a dolphin's dorsal fin, mounted a pressurized case containing cameras on the plastic, and then stabilized the device with a thin strap around the dolphin's belly. On some dives the camera was mounted facing backward to view the fluke, and other times it faced forward to view the flippers. Other sensors tracked depth, temperature, speed and acceleration.

The videos revealed that a dolphin strokes with its tail at the beginning of a dive, but then spends as much as 2 minutes motionless. Intermittent swimming and gliding continue for both the descent and the ascent Weddell seals and blue whales have adopted this strategy too, Williams's team found. For dives of less than 50 meters, the seals stroked the whole way down, but to go deeper, they did a series of strokes and glides. Like the dolphins, the seals glided about 80% of the time during dives deeper than 300 meters. Moreover, "the deeper they dive, the more time they spend gliding," Williams points out. Once below 80 meters, the air in the lungs is compressed enough that the animals start to sink without effort, and gliding becomes quite effective. "The animals take advantage of the change in pressure and the resulting change in buoyancy," she says.

That was apparent in one dive by a blue whale. By the time the whale reached 90 meters, it was spending 80% to 90% of its time sinking. Strokes were so slow that the researchers had to speed up the film to see them, Williams says.