How whales, dolphins, seals dive so deep

A nerve-racking effort to attach cameras to marine mammals has shown how four species dive the impossible dive.

Previous calculations just hadn’t explained how certain champion divers use so little oxygen, notes Terrie M. Williams of the University of California, Santa Cruz. Now, she and seven colleagues report that a 10-year project that fit video cameras onto free-swimming animals has revealed a crucial trick.

The blue whale, bottlenose dolphin, Weddell seal, and elephant seal all cut diving energy costs 10 to 50 percent by simply gliding downward, the team reports in the April 7 Science. The creatures’ buoyancy decreases because water pressure squeezes their bodies into a smaller volume and flattens air sacs in their lungs. As the animals sink, they save energy for the swim back up.

“This is the first time we’ve really been able to see what marine mammals do on deep dives,” Williams says.

Scuba diving scientists have managed to swim with animals for only about the first 30 meters of the drop. “We’re pathetic,” Williams says. Her team documented seal depths of nearly 400 m.

Previously, instruments recorded times and depths but not swimming technique. Fixed cameras gave only glimpses, although they did show that the animals’ chests caved in as pressure increased.

“We had to wait for the technology to come along,” Williams recalls. Finally, researchers at Texas A&M University in College Station and the National Geographic Society in Washington, D.C., created video backpacks: a small camera plus monitoring instruments. Then came the puzzle of how to attach the package to an animal.

Glue works on the hairy seal bodies, but “nothing sticks to a dolphin,” Williams sighs. Tapes slip off its skin, and even superglue fails. The dolphin team finally used suction cups like those on a non-slip shower mat.

The suction cup had to be dinner-plate-size for the blue whale. John Calambokidis of Cascadia Research in Olympia, Wash., drew on his long experience in politely approaching whales to place the pack. He eased a 22-foot inflatable boat up to an 80-foot whale off the California coast. Leaning over the bow, John Francis of the National Geographic Society maneuvered an aluminum pole to position the camera. Fortunately, “blue whales aren’t flighty,” he says.

The researchers didn’t just wave good-bye to cameras worth tens of thousands of dollars. They hauled a backpack-bearing elephant seal out to sea and predicted that it would hasten back to its basking place on the beach. Their dolphin returned on command, thanks to its Navy training. The whale suction cup had magnesium plugs that dissolved in 90 minutes, breaking the vacuum so the camera floated up for retrieval.

In Antarctica, the scientists hovered around a hole that they had drilled in a solid expanse of ice and hoped the three backpack-bearing Weddell seals wouldn’t find another place to surface. “We were breaking open a champagne bottle the first time the Weddell’s camera came back,” Williams remembers.

Body motions captured by the cameras let the researchers correlate the animals’ swim strokes with their progress. To judge oxygen expenditure, a team sampled exhalations of Weddell seals after dives and shallower explorations. Williams found that stroking demanded more oxygen than gliding downward.

Work on robotic fish had raised the odd possibility that gliding demands more energy than stroking, observes Frank Fish, who studies swim biomechanics at West Chester (Pa.) University.

The new work, however, makes this unlikely in marine mammals, he notes. He adds, “It’s interesting in that it adds to the database of how clever animals are.” Morphologist Ann Pabst of the University of North Carolina at Wilmington praised the new study for being “so broadly comparative.” She points out that the dolphin is only about one-tenth the length of a blue whale, but the gliding strategy still works. Also, she says, the similar underwater diving routines of the four species must have evolved in at least two separate lines of mammals.