Whiskers point out the best meals
Harbour seals can detect the fattest fish using just their whiskers, according to research.
Tests with a trained seal have revealed that the animals can sense underwater objects, even with their hearing and sight restricted.
The seal detected objects' sizes and shapes by sensing differences in the trail of disturbance they made in the water.
Scientists suggest that seals use this ability to identify the best prey.
Harbour seals have 40 to 50 whiskers on each side of their snout
There are around 1,500 nerves at the base of each whisker, ten times that found in rats or cats
The whiskers simultaneously pick up on any displacements in the water, providing seals with quick information about their surroundings
Dr Wolf Hanke and scientists from the Marine Science Centre at the University of Rostock, Germany, first showed how sensitive seals' whiskers were last year.
They reported that a trained seal, Henry, was able to sense an artificial fish up to 100m (328ft) away using just his whiskers.
The researchers then focused their investigation on whether seals used their whiskers to discern size and shape.
Their findings have been published in the Journal of Experimental Biology.
In an open-air pool in Cologne zoo, the team set up a box with a series of rotating paddles inside. These paddles created trails similar to those made by swimming fish.
Wearing a mask and headphones to restrict his other senses, Henry swam through the box to hit one of two targets on the other side and get a fish reward.
The seal seems to be able to discriminate fish of different size and shape, which can help to save time and energy when hunting underwater
Dr Wolf Hanke
Comparing a control paddle and one that varied in thickness or shape, scientists found that the seal could tell the difference between the trails left in the water.
For 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 Dr Hanke.
"Hydrodynamic wakes are of major importance to harbour 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 seal wore a mask and headphones to restrict its senses but could still find food
Although they only tested one trained animal, the research team say their evidence proves that the ability is present in all harbour seals.
They suggest that sensitive whiskers are of huge benefit to the species, allowing them to hunt fish with the highest calorific reward.
"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," said Dr Hanke.
The researchers believe whiskers could allow foraging seals to optimise their hunting behaviour to suit the size and shape of their prey.
Harbour seals (Phoca vitulina) are also known as common seals and are abundant in the waters of the north Atlantic and north Pacific Oceans.
The species are considered highly adaptable for their ability to live in both the turbid North Sea and clear Pacific Ocean.
Almost 5% of the world's population live off the UK's coast.