for National Geographic News
Harbor seals, and possibly all pinnipeds, rely on their whiskers much
more than previously realized to help them track prey in murky water,
according to a group of German scientists.
Some marine animals, like dolphins and toothed whales, use echolocation, a kind of internal sonar system that allows them to determine the location of their prey by measuring how long it takes an echo to bounce back from it. Many fish species have what is called a lateral line system that helps them detect the movement of prey in dark water.
Seals have neither, yet they must operate in the same conditions, searching for food in an environment where sight and smell are effectively neutralized. Harbor seals are carnivores and feed on fish, shrimp, squid, and other animals. How they find their prey has long puzzled scientists.
A team of zoologists based in Germany reports in the July 6 issue of the journal Science that it's all in the seals movement-sensitive whiskers.
Following the Hydrodynamic Trail
Experiments conducted by lead author Guido Dehnhardt, a zoologist from the University of Bochum, Germany, and his colleagues in 1998 showed that seals use their whiskers to detect the hydrodynamic trail fish leave in their wake. However, the general consensus until now has been that the movement of the swimming seal would inhibit hydrodynamic trail detection to only very short distances.
The current study has shown that the whiskers play a much bigger role in seal navigation.
"Previously it was thought that hydrodynamic reception works only in the millimeter to centimeter range," says Dehnhardt, who specializes in sensory ecology. "For seals this would mean that they could have used the whiskers in a sit-and-wait strategy, not more. Now we have shown for the first time that hydrodynamic information can be used for long-distance object location.
"This is a small revolution in the field of research on hydrodynamic receptor systems."
Henry and the Submarine
The researchers trained two harbor seals, Henry and Nick, to search for a trail left by a miniature submarine. Even blindfolded, the seals found the trail and tracked it to the submarine around 80 percent of the time. When their whiskers were covered, however, they never found the miniature sub.
"As fish trails are much more stable and last for a longer time [than the trail left by the sub] we calculated that it should be possible for a seal to locate a herring by hydrodynamic trail-following that is about 180 meters (600 feet) away," says Dehnhardt. "This assumption is extrapolated from what we know about trails of small gold fish and has to be tested experimentally now."
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