Harbour seals can detect the fattest fish using just their whiskers, according to new research.
Hunting in the North Sea, harbour seals, often known as Common seals, often encounter murky water that impedes their vision; but it doesn't affect their ability to chase prey. Extending their vibration-sensitive whiskers, the mammals are almost as efficient at pursuing their quarry as they would be if guided by sight.
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.
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.
In an open-air pool 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.
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 research team 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 also believe their whiskers could allow foraging seals to optimise their hunting behaviour to suit the size and shape of their prey.
Harbour seals have 40 to 50 whiskers on each side of their snout, with around 1,500 nerves at the base of each whisker - ten times that found in rats or cats. These whiskers simultaneously pick up on any displacements in the water, providing seals with quick information about their surroundings.
Scotland's internationally-important populations of harbour seals have declined dramatically in recent years. Although a number of different factors are thought to be responsible, the unnecessary shooting of seals by fish farmers and fishermen is an important issue. OneKind is not just concerned about populations and numbers, but we believe the life and death of each individual animal matters. OneKind's successful campaign for better legislation to protect seals in Scottish waters has helped conserve seal numbers as well as better protecting the welfare of each individual sentient seal.