Many marine mammals live in a world shaped by sound - producing clicks to map their underwater environment out of echoes. Researchers in Hawaii have now discovered just how finely tuned this "echolocation" can be. The scientists found that toothed whales can focus their beam of sound - pinpointing a target with a narrow stream of clicks to study it in detail.
Laura Kloepper, a PhD student at the University Hawaii, led the study. She worked with a trained false killer whale - a member of the dolphin family - named Kina, which has been in a bay enclosure at the research institute since 1993. Kina came to the University of Hawaii's Hawaii Institute of Marine Biology when the US Navy closed its Hawaii laboratory.
"The Navy was reducing its animal inventory at that time," Ms Kloepper said. "And rather than risk reintroduction to the wild, the whale was given to [my supervisor] Dr Paul Nachtigall's newly established Marine Mammal Research Program at the University. "She's a dream to work with; we think she's probably the best echolocator in the world."
"In previous studies, she's managed to distinguish between two objects that differed in width by less than the thickness of a human hair." Scientists had suspected that this remarkable accuracy was partly due to whales' ability to adjust the focus of their echolocating sound beam. But Ms Kloepper and her team have carried out the first tests to actually measure the beam when the animal changed focus as an "echolocating task" became more difficult.
The researchers trained Kina to recognise a cylinder of specific dimensions, and to come to the surface and touch a response ball with her "nose" when she recognised it. Every time she identified her cylinder and touched the ball, she received a fish reward.
The researchers then presented Kina with a discrimination task using three different cylinders: the one she was trained to recognise; another with walls almost 1cm (0.39in) thicker and one with walls just 0.2mm (0.008in) thicker than her target cylinder. All three objects were the same length.
On a signal from her trainer, Kina swam into an underwater hoop ready for the test. "When she swims into the hoop," said Ms Kloepper, "a gate in front of her lifts so she can echolocate the object in front of her." During these trials, the researchers used an array of underwater microphones to measure the beam of sound that Kina produced.
"By recording from several positions, we're able to image the shape (and size) of her beam," Ms Kloepper said. These images revealed that Kina altered the size of her beam according to how difficult it was to identify the cylinder; she produced a larger beam when the cylinder in front of her was more difficult to distinguish from her target.
The scientists think that when Kina produced this large beam of sound, her melon acted as a responsive acoustic lens - focusing the wide beam down and directing all of the sound at the object of interest. "This way there's more acoustic energy that she's going to get back from the object she's investigating," explained Ms Kloepper. "It makes sense because echolocating is how [these animals] make their living, and during deep dives, they have very little light. So this means they can follow and track fish just by using sound," said Ms Kloepper.
The same team's subsequent studies have shown that harbour porpoises probably have this same focusing ability. Paul Nachtigall, who also took part in the study, explained that as well as adjusting their echolocating beam, Kina was able to alter the sensitivity of her hearing - making it super-sensitive when she was hunting, but "plugging her ears" to block out potentially damaging loud noise. Dr Nachtigall said that Kina was a "professional researcher".
Underwater Hunters
Toothed whales and dolphins, collectively known as odontocetes, use echolocation to hunt and navigate.
Echolocating clicks pass through a fatty structure at the front of their skull called the melon. It is this structure that forms a visible bulge on the animal's head and researchers say it acts as an adjustable acoustic lens, focusing the sound into a beam and altering the size of that beam.
Other non-echolocating marine mammals have different tricks for finding their way underwater. Seals have super-sensitive whiskers, which can detect the fattest fish by sensing the trail they leave behind.
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