May 12, 2011

Seal Whiskers More Sensitive Than Previously Thought

A study published in the Journal of Experimental Biology shows that seals can detect the size and shape of objects by sensing differences in the trail of disturbance they make in the water and use this ability to identify the best fish, BBC News is reporting.

Dr. Wolf Hanke and scientists from the Marine Science Center at the University of Rostock, Germany, first showed how sensitive seals' whiskers were last year when they reported that a trained seal named Henry was able to sense an artificial fish up to 100 meters away using only his whiskers.

Their next portion of the study was to answer whether seals used their whiskers to discern size and shape. In an open-air pool in the Cologne Zoo, researchers set up a box with a series of rotating paddles inside. These paddles created trails similar to those made by swimming fish.

With an eye mask and headphones on the seal to restrict his other senses, Henry swam through the box to one of two targets on the other side and received a fish reward.

Scientists found that the seal could tell the difference between the trails left in the water between a control paddle and one that varied in thickness or shape. In trails made by the control paddle, Henry selected a target to the right, and for anything thicker, thinner or of a different shape, he touched the target above the exit gate.

"Seals can tell the size and shapes of objects that have been moved through the water by reading the water movements that the objects leave behind, the so-called hydrodynamic trail, using their whiskers," said Hanke.

The team then tested which aspects of the wake the seal picked up on. "We randomized the speeds of the paddles so that the maximum flow velocity wasn't a distinguishing cue for the widest paddles, but the structure of the wake had to be recognized by the seal and he could do that too, but with slightly less accuracy."

"Hydrodynamic wakes are of major importance to harbor seals because vision is often very limited under water, and hearing is often rendered useless because the seals do possess acute hearing, but swimming fish are often quite silent."

The research suggests that sensitive whiskers are of huge benefit to the species, allowing them to hunt fish with the highest calorific reward.

Hanke continues, "The significance of these abilities to the seal is that it seems to be able to discriminate fish of different size and shape, which can help to save time and energy when hunting underwater."

The researchers believe whiskers could allow foraging seals to optimize their hunting behavior to suit the size and shape of their prey.


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