RSS Feeds

Email a Friend Printer Friendly

Whiskers show seals the way

July 06, 2001

By using their whiskers (vibrissae) seals can find their way around in turbid waters: they are capable of detecting tiny movements caused by bodies moving under water over distances of up to 40 metres. This astonishing ability of these marine mammals has now been proved in behavioural experiments by biologists of the University of the Ruhr (Dr. Guido Dehnhardt, General Zoology and Neurobiology) in conjunction with researchers from the University of Bonn (Prof. Horst Bleckmann, Department of Comparative Neurobiology). The periodical SCIENCE reported on their findings in its 5th July issue.

Like other marine mammals, seals often live in habitats where visibility is reduced because the water is dark or turbid. Whereas toothed whales can apparently make up for this reduction in visibility by active use of echoes as a direction-finding technique, despite intensive efforts scientists had as yet never been able to demonstrate the existence of this ability in seals; they assumed that in addition to limited use of their eyes, seals and sea-lions mainly used their passive faculties of hearing in finding their way and detecting their prey in the oceans.

However, they do not simply rely on their eyes and ears: they use their system of vibrissae to locate moving objects under water. Like fish with their lateral line system, seals use their whiskers to detect movements in the water to the magnitude of less than a thousandth of a millimetre. Aquatic movements like these, such as those due to the swimming movements of a fish, show up as stable hydrodynamic trails; their shape is that of contra-rotating eddies arranged like rope ladders. „The flow pattern of a goldfish can be demonstrated in the lab using relatively simple methods even five minutes later," Professor Bleckmann explains.

The researchers were able to provide proof of this ability in behavioural experiments involving two seals. An automatic mini submarine, which was set in motion in their
vicinity, created hydrodynamic trails, both straight and curved ones. „We first placed the seals in a ring above water. We put stocking masks and headphones on the seals so that they could neither see nor hear the submarine," is how Dr. Dehnhardt explains the course of the experiment. After the submarine had been set on an unpredictable course for several seconds, it switched itself off and drifted silently in the water. When the headphones were removed, the seals set off. Still blindfolded by the masks, they left the ring and slowly dived, looking for traces of the submarine. As soon as they detected it, an obvious change of direction in their swimming behaviour showed that the seals were on its trail and were now following it. „Analysis of video footage confirms that the seals kept a fix on the direction taken by the submarine and followed even minor changes precisely," Dr. Dehnhardt says.

However, if the submarine's engine was still running when the headphones were taken off, the seals followed the sound and swam straight for its source, Horst Bleckmann, Professor of Zoology and head of the University of Bonn's Department of Neurobiology, adds. „Their sense of direction is in fact multi-modal, i.e. dependent on their ears, nose and vibrissae."

Dehnhardt and Bleckmann are convinced that this discovery will give new stimulus to research: „Our experiments with the detection of hydrodynamic trails demonstrate a completely new direction-finding system for the aquatic habitat. The utilisation of a hitherto unknown sensory quality in the direction-finding equipment of marine mammals thereby opens the door to a very exciting field of research."

Bonn, Universitaet