A new model helps explain where spinner dolphins get their twist.

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New Twist on an Old Spin

One of most entertaining sights along coasts are the acrobatic shows put on by spinner dolphins. They jettison themselves out of the water, pirouette through the air, then--BOOM!--belly flop back into the sea. Video data and a mathematical model have now revealed just how they carry out this impressive display--an antic that may keep freeloading fish off their backs.

More petite than their bottlenosed relatives, spinner dolphins cruise the tropical and subtropical Indian, Pacific, and Atlantic Oceans in groups of about 60. They start their displays with a slow twirl underwater. For years, however, researchers thought the subsequent rapid pirouette was the result of the dolphin flexing its tail in thin air. In retrospect, the idea is flawed, says Frank Fish, a functional morphologist at West Chester University in Pennsylvania. Dolphins aren't that flexible, he says, and it's unlikely that they could really use air to "push off" into a spin.

So Fish teamed up with West Chester University engineer Anthony Nicastro and Daniel Weihs, a hydrodyamicist at Technion, The Israel Institute of Technology in Haifa, to look at the forces involved. Based on basic principles gleaned from fish studies, the threesome created a mathematical model that incorporated the physical principles involved in initiating and maintaining a dolphin's spin. The model predicted that while underwater, the dolphins use their flippers to begin to slowly twirl around but don't go very fast because of water resistance. As the dolphin emerges, that resistance greatly decreases and, with one last push of its fluke, it accelerates into a pirouette. The model suggests that a dolphin heading full out--6 meters per second--into the air could finish an astonishing seven rotations, the researchers report in this month's Journal of Experimental Biology.

These aerial gymnastics aren't just fun and games. When Fish and his colleagues broke down the behavior into the various forces involved, they were able to calculate how hard the dolphins would hit the water. The impact was enough to tear the remora fish--which latches onto marine mammals for free transportation and easy access to food--from their bodies.

"The paper is the first careful biomechanical and hydrodynamic study of the behavior," says Malcolm Gordon, a marine biologist at the University of California, Los Angeles. "The traditional, physically naïve view of how the spinning is produced is clearly wrong."

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