Underwater dance


POSTED: Monday, November 10, 2008

In a stunning display of teamwork and coordination, Hawaii spinner dolphins herd their food through elaborate choreography and share equally in the feast, Hawaii and Oregon researchers have discovered with high-tech acoustics.

Studying groups of dolphins off Leeward Oahu, they found the animals engage in a synchronized “;dance”; to enclose small, deep-ocean prey such as lanternfish, shrimp and juvenile squid.

The dolphins swim by pairs into a high density school of fish, feed for about 15 seconds, then back out and let the next pairs in line take a turn, the scientists have discovered.

“;We actually had no idea how coordinated those groups were—not just groups, but how well they were working together,”; said Kelly Benoit-Bird, a marine ecologist at Oregon State University and lead author of the study, in a telephone interview.

The findings, by Benoit-Bird and Whitlow Au, chief scientist of the University of Hawaii's Marine Mammal Research Program, was published recently in the Journal of the Acoustical Society of America. In a companion article, they describe how they listened to the spinner dolphins feeding at night with underwater hydrophones, which pick up sound waves.

Other dolphins can be observed during the daytime, Au said. “;What they typically do is swim around in a circle and drive the prey toward the surface.”;

But spinner dolphins can't be observed directly because they feed at night about 20 to 30 feet below the surface, he said.

Sonar readings from multi-beam echo-sounders gave the researchers an “;eye-opening”; acoustical picture of the dolphins' feeding ritual.

As Benoit-Bird describes it: About 20 dolphins swim side by side until finding food. Within about 15 feet from the fish, they form 10 pairs, pull into a circle and sequentially swim up and down vertically, “;in essence, doing 'the wave' like fans at a sporting event.”;

With the fish in chaos, two dolphin pairs move into the prey from opposite sides of the circle, then return to the circle and two more pairs would feed, she said.

Au explained that while the dolphins swim round the prey all together on the outside like a cylinder, each pair is at a slightly different depth so they tend to entrap the prey.

Their coordinated behavior is amazing because they can hardly see the fish unless they are bioluminescent, Au said, “;and that we don't know.”; The dolphins work hard, diving and preying for long periods throughout the night, he said.

“;It's like running and eating at the same time. It's a very arduous task.”;

The researchers calculated that spinner dolphins, which grow to about 6 or 7 feet, require about 3,200 calories a day and need to eat at least 650 fish each night, plus another 200 to 300 fish for the energy used in hunting food.

They can only eat one fish at a time and their prey is small, only about 2.5 inches long, Benoit-Bird said. “;It's like living off of one kernel of popcorn at one time.”;

After about five minutes, and still in a circle, the dolphins would rise to the surface, take one breath, then dive down and begin eating again, she said. No dolphin ever broke from the circle, she said.

That, she said, begs the question, “;How do they communicate with each other and how do they pass on that knowledge to their young?”;

The underwater hydrophones revealed the dolphins making a series of “;clicks”; with the highest rates occurring just before their foraging, she said.

Au said clicks are used for echolocation and it isn't clear how else they're used.

“;We're not sure exactly how they coordinate their behavior,”; he said. “;It sure indicates cognitive function.”;

Dolphins can use frequency-modulated whistles that were thought used to cue coordinated behavior. No whistles were measured, Au said, “;but that doesn't mean they're not whistling.”;

Benoit-Bird earned a doctorate degree from the University of Hawaii in 2003, working with Au at the Institute of Marine Biology, and returns here for about a month each year to continue their joint research.