How whales and dolphins focus sound beams on prey

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.

Their findings are published in the Journal of Experimental Biology.

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 told BBC Nature. “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.

Read full story: BBC by Victoria Gill

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