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Who: Guido Dehnhardt, Björn Mauck, Wolf Hanke, and Horst Bleckmann, of the Universität Bonn. Dehnhardt and Mauck are also affiliated with Ruhr-Universität Bochum in Germany.
What: A study entitled "Hydrodynamic Trail-Following in Harbor Seals," reporting new findings about the sensory abilities of seals.
Where: The Zoo in Cologne, Germany.
When: Findings are in the 6 July 2001 issue of the international journal, Science.
Purpose: The researchers wanted to figure out how seals are able to successfully navigate dark, murky waters to forage for food. Their study shows seals use their highly sensitive whiskers for long distance prey location.
Background: Marine animals must forage for food in dark, murky waters. How do they find their way around in the dark? Some animals, like dolphins, have a special sensory ability called echolocation, where they determine the location of their prey by measuring how long an echo takes to return from it. But pinnipeds, or seals, don't have that ability. Some scientists believed a seal's whiskers were sensitive to the minute water movements made by a swimming fish, but only over short distances. This new study suggests the whiskers play a much bigger role in seal navigation.
Methodology: The team used two seals--"Henry" and "Nick"--in different types of pools. One was filled with naturally murky water, the other with fresh, clear water. They used a custom-made miniature submarine to simulate the hydrodynamic trail a fish leaves in its wake. They measured that water trail and counted the number of times the seals were able to successfully find and follow it.
Experiment: In the first experiment Henry was trained to locate the submarine in the murky water without any sensory restrictions. Then researchers put a blindfold and headphones on the seal and positioned the seal so its head was above water. The submarine was started under water, and stopped a few seconds later. When researchers removed the headphones, that was the signal to begin the search. The researchers conducted the second experiment in clear water with Nick.
Findings: A frame-by-frame video analysis shows Henry clung to the murky water trail nearly 80 percent of the time, overall. Moreover, the seal:
- Located the submarine in 256 out of 326 total attempts;
- Found the submarine 24 out of 30 times when it wasn't allowed to begin searching immediately;
- Found the submarine 26 out of 30 times when researchers altered its course mimicking a fish; and
- Failed 30 out of 30 attempts when his whiskers were covered with a stocking mask.
In the fresh water pool Nick had similar results, even with a mask over his whiskers. In his case, researchers randomly tested whether the seal could find the trail after receiving a short, acoustic cue. The seal:
- Located the submarine in 37 out of 45 trials (82 percent); and
- Located the submarine in 10 out of 10 trials, beginning his search before the submarine motor was stopped.
Future Plans: Dehnhardt and his team say in order to figure out the greatest distance at which a seal can detect a fish, researchers need to know more about authentic conditions in the wild. In the experiments they were able to test the seal's abilities under optimal conditions. Now they plan to continue experiments with trained animals in the ocean.
Special Notes: Electronic images of Henry and Nick are available. Funding for the research was provided by Deutsche Forschungsgemeinschaft. The seals used in the experiments were treated in accordance with all regulations, and remain healthy and active.