Harbour porpoises roam the cold, murky waters of the Northern hemisphere,where it can be hard to spot tasty schools of fish. To locate its dinner, a porpoise sends out high-frequency clicks and carefully listens out for the echoes bouncing off any prey lingering in the hunter's transmission beam. By locking their sonar onto inanimate objects like nearby rocks, porpoises also gain an intimate knowledge of their underwater surroundings, which is especially useful in dark, murky waters. But do porpoises also use echolocation to work out where they are? Although it's assumed that porpoises use sonar for spatial awareness, nobody has shown conclusively that the marine creatures use echolocation as a navigational tool. While investigating the swimming and echolocation behaviour of two harbour porpoises, Ursula Verfuß, Lee Miller and Hans-Ulrich Schnitzler discovered that porpoises really do rely on sonar to navigate, even though the animals they studied were swimming through familiar territory(p. 3385).
`Researchers interested in echolocation behaviour usually give dolphins or porpoises a target object to find, like a fish hanging in the water',Verfuß explains. As a result, we have plenty of evidence that these toothed whales use echolocation to track down their prey. But to provide clear evidence that porpoises also use echolocation for spatial orientation,Verfuß needed to show that porpoises use echolocation when there are no target fish in the water.
Teaming up with Miller and Schnitzler, Verfuß travelled to Kerteminde in Denmark to find out whether porpoises use echolocation while swimming across a familiar pool. The team studied two captive harbour porpoises in an outdoor pool supplied with a flow of seawater from the Great Belt and Kerteminde Fjord. They decided to see if porpoises use sonar while performing a simple task – swimming in a familiar pool containing relatively clear water, which meant that the animals should be able to see where they are going. The team soon trained the porpoises to swim from one side of the pool to the other and used two surveillance cameras suspended above the water to film the porpoises as the animals swam across the empty pool. At the same time, they recorded the porpoises' high-frequency clicks using hydrophones placed underwater at each end of the pool.
Would the porpoises still emit clicks, despite the fact that there were no fish to lock their sonar beam onto? Since the animals were swimming in a familiar pool containing clear water, the team suspected that the porpoises would mainly rely on their eyesight to make their way across the pool. But to their surprise, they recorded a continuous stream of clicks from the porpoises as the animals swam across the pool. What's more, they noticed that the time interval between clicks decreased as the porpoises approached the end of the pool. Verfuß explains that a decreasing click interval as a porpoise approaches an object reveals that the animal has locked its sonar onto that specific object. In other words, as they made their way across the pool, the porpoises appeared to be locking onto specific landmarks at the other end of the pool. The team concludes that, even when harbour porpoises swim along a familiar and visible route, these small whales use echolocation to make sure that they know where they're headed.