The waters of the North Sea are among the murkiest on the planet, so dark and silty that a seal sometimes can’t see its own whiskers. Even so, the harbor seals there can hunt and catch fish. Marine biologists have known for several years that a seal relies on its whiskers to follow the wake a fish leaves behind. But according to a new study, whiskers supply detailed information that the seal may use to decide which fish are most worthwhile to hunt.
To find out just what a seal’s whiskers can tell it, biologist Wolf Hanke of the University of Rostock in Germany and colleagues enlisted the aid of a harbor seal named Henry, a veteran research participant who has been at the University of Rostock’s Marine Science Center since 2008. “When he starts working, he can’t be stopped,” Hanke says. The researchers presented Henry with different types of wakes using a test box of still water about 2 meters wide that sat at the bottom of a pool about 1 meter deep. Above the box hung a motorized arm with a series of paddles of different shapes and widths. While each paddle moved through the water, Henry waited outside the test box wearing headphones to block out the motor noise. He also wore a blindfold to ensure that he’d be using only his whiskers.
Once the paddle had come to rest, Henry left his station and dunked his body halfway into the test box. With the water swirling around his whiskers, Henry made his decision, then left the box and pressed one of two buttons with his nose, indicating whether the wake had been made by one of three paddle sizes he’d already been trained to recognize or by an unfamiliar one.
Henry chose correctly over 90% of the time, as long as the width of the familiar and unfamiliar paddles differed by about at least 4 centimeters, the team reports today in The Journal of Experimental Biology. Just by dipping his whiskers in the wake of a paddle that had stopped moving before he entered the tank, Henry could discern that it was a different size from one of three he had already encountered. His accuracy decreased as the difference in width dwindled to 3 centimeters or less. Randomly changing the speed of the paddles also decreased the Henry's performance. And he made no attempt to press any button when his whiskers were covered, showing that he relied on them as a source of information.
Henry had some success discerning different shapes. When the paddles were all the same width, he could distinguish trails made by a flat, rectangular paddle from those made by cylindrical, triangular, or undulating paddles. When the team altered the width of the paddles randomly, he had more trouble, except that he could always tell the wake of the flat paddle from that of the triangular one.
To pinpoint what Henry was analyzing, Hanke and colleagues used a technique called particle image velocimetry, in which a horizontal plane of water filled with reflective particles was illuminated with a laser, filmed, and digitally analyzed. The information “read” by Henry’s whiskers included the diameter of the vortex made by the paddle, the width of the wake, its total velocity, and sudden changes in its velocity. The triangular paddle left identical twin vortices, probably making it easier to identify.
“We knew a seal’s whiskers were sensitive, but these findings show how much a seal can discern about its prey,” Hanke says. Because a fish’s size, weight, and swimming style all affect the sort of wake it creates, this information could tell the seal how much meat has just passed by and how fast it’s swimming. “If a harbor seal can judge the size and shape of nearby fish, it can use this information to decide whether the fish is worth chasing,” Hanke says. The authors add that harbor seals are usually generalists when it comes to food, eating more than one type of prey, so the idea that they can choose among possibilities is novel.
Hanke adds that for this initial study, paddles were used as easily controllable and clearly distinguishable objects. Follow-up studies will mimic different types of fish more closely.
“It’s an elegant demonstration of an animal’s ability to use nonvisual senses,” says marine biologist Jacqueline Webb of the University of Rhode Island, Kingston. She also commends the researcher’s use of technology to recreate a detailed digital picture of what the seal was picking up on. “To my knowledge, it’s the first study showing that seals have this level of discriminative ability.”