http://www.nature.com/nsu/010712/010712-2.html

Seals' whiskers may feel the wake of far off dinner.
6 July 2001

ERICA KLARREICH

Seals' whiskers are certainly cute. But new research suggests they may also
be a crucial hunting tool.

Seals' whiskers enable them to home in on the wake of prey as much as 180
metres away, new research suggests. Dolphins' sonar can locate fish only up
to about 110 metres away.

"People thought whiskers could only detect things about a millimetre or a
centimetre away," says Guido Dehnhardt, a zoologist who conducted the study
with his colleagues at the University of Bonn in Germany.

The way in which seals track their prey had been something of a mystery.

"The consensus has been that seals don't use a sonar system, as dolphins
do," Dehnhardt explains. "People claimed that they must use vision or
passive hearing."

Yet seals are known to hunt successfully in water so murky that their eyes
are of little use. In England, there are even thriving grey seals that are
totally blind.

The animals' whiskers are fine-tuned to pick up tiny water disturbances.
The 80 follicles contain about 25,000 receptors that pick up vibrations.

Having shown previously that the whiskers can detect water movements of
just a millionth of a metre, Dehnhardt's team suspected that seals might
hunt with only their whiskers for guidance.

The team trained two harbour seals (Phoca vitulina) to follow a submarine
the size of a large trout, which produced a trail in the water similar to a
fish's wake. They then blindfolded each animal and launched the submarine,
holding the seal's head above the water until the submarine was turned off,
so that the seal could not rely on sight, smell or sound.

In most of the trials, the seals successfully followed the submarine's
trail, even when it zigzagged unpredictably. When the researchers pulled
stockings over the seals' heads, blocking the activity of their whiskers,
the hunters' success rate plummeted to zero1.

"These trails really are information for seals," Dehnhardt says. "It's a
totally new underwater orientation system."

The use of whiskers to follow a water trail is akin to the way a dog tracks
prey by following its scent, says Michael Fedak of the Sea Mammal Research
Unit at the University of St Andrews in Fife, Scotland.

"This research hints about the perceptual world of these creatures, which
is hard for us to imagine," Fedak says. "It's a nice indication that their
capabilities may be well beyond what we think."

The next step for the research team is to take their experiments to the
open ocean, to see how the seals behave in their natural environment.

Many other marine mammals, such as manatees and sperm whales, may rely on
their whiskers, Dehnhardt says. Dolphins lack whiskers.
 
 

References

1.Dehnhardt, G. et al. Hydrodynamic trail-following in harbor seals (Phoca
vitulina). Science, 293, 102 - 104, (2001).