Dolphins studied at Kewalo Basin have shown they can use their sound system not only to communicate and pick up signals, but to see the shape of objects, researchers report. Dolphins’ sonar found
to rival eyesightHawaii experiments show they
can sense shapes acousticallyBy Helen Altonn
haltonn@starbulletin.com"What we've demonstrated here for the first time is that dolphins can 'see' through sound," said Louis Herman, president of the Dolphin Institute and director of its Kewalo Basin Marine Mammal Laboratory.
Adam Pack, vice president of the institute and assistant director of the laboratory, said the animals can identify complex objects "holistically" with their sonar.
The study, conducted by Pack and Herman with graduate students Matthias Hoffmann-Kuhnt and Brian Branstetter, was recently published in the scientific journal Behavioural Processes.
The goal was to determine if a dolphin can spontaneously recognize an object only through sonar that it saw previously through vision. Also, if it can recognize an object through vision alone that it inspected earlier through sonar.
Dolphin sonar involves pulselike signals generated in the dolphin's nasal passages that are transmitted through its head and reflected off of objects in the water. The echoes are received through the dolphin's lower jaw and transmitted to its middle and inner ears.
Herman and Pack designed a box that they suspended underwater with the front made of a thin sheet of black Plexiglas. It was impenetrable by light but allowed transmission of sound. The dolphin could inspect an object inside the box through sonar but not vision.
The scientists held three other objects in the air that the dolphin Elele could see only with vision. Then they gave her a task of matching four choices to assess her sonar abilities.
Elele swam to the box in the water and inspected its contents through sonar. The other three objects, presented to her in the air, had features similar to the one in the box but differed in overall shape. Her task was to point with her rostrum to the object matching the object in the box. If none matched, she was to press a paddle to indicate "none of the above."
During most of the trials, Elele chose a match based on the global shape, rather than individual features, the scientists said.
"Her excellent performance was equivalent in both directions, whether she was matching from echolocation to vision or from vision to echolocation," they said.
Pack said the study "gives us more information about what appears to be a very elegant perceptual system for the dolphin. It tells people who study dolphins in the wild about the sophistication of their echolocation sense. ... If you think about it, it makes a lot of sense about how dolphins are able to function so well acoustically when vision is limited."
He noted, for instance, that spinner dolphins off the Waianae Coast feed largely at night where their vision is severely limited.
"What we are learning is the echolocation (sonar) system provides an excellent way not only for avoiding predators, but locating prey and navigating in a visually limiting environment."
Herman found in studies in 1975 that dolphins have excellent vision in the air and underwater but will rely on visual sense in clear water, Pack said. "What we're learning is how dolphins make their way in the natural world."