Harbour seals have the ability to detect the size of fish using only their whiskers, research has shown.
Hunting in the North Sea, harbour 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.
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. Knowing that a fish's size and shape can dramatically affect its wake structure, researchers then decided to find out how well seals can distinguish between the wakes of objects with different shapes and sizes.
Teaming up with Henry the harbour seal at the Marine Science Centre, Germany, scientists began testing Henry's ability to distinguish between the wakes of differently sized paddles.
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.
The researchers blindfolded Henry and covered his ears, then they swept a paddle through a large box in Henry's enclosure and allowed him to enter it three seconds later. 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.
—Dr. Wolf Hanke