Dolphins change sonar volume, UH researchers find

By Jan TenBruggencate
Advertiser Science Writer

University of Hawai'i researchers have found that dolphins, unlike bats and mechanical sonar, change the volume of the sonar signals they emit as they near a target.

Bats and human-built echolocation devices keep the volume at the same level, but change the hearing or receiving sensitivity.

It's a whole different way of dealing with the fact that as echolocation users get closer to a target, the echo comes back faster and progressively louder. Without an adjustment, the echo would be deafening.

Whether you reduce the volume of the output as dolphins do, or increase the sensitivity of the receiver as bats do, it has the same effect, said Whitlow Au, chief scientist at the UH Marine Mammal Research Program at the Hawai'i Institute of Marine Biology, and Kelly Benoit-Bird, a post-doctoral fellow at the institute. They reported their findings last week in the journal Nature.

The dolphin technique seems to work in two phases, which allow it to gain information both about the distance and the size of a target, Benoit-Bird said.

First, the dolphin determines the distance to a target by how long it takes for its echolocation signal to reach the target and return. Then, by automatically adjusting its volume to the distance, it can learn the size of the object.

"It's a very quick way to say, 'How big is this?' " Benoit-Bird said.

Once the volume is adjusted for distance, the bigger the return, the bigger the target. A dolphin's echolocation "click" bouncing off the side of a salmon would make a smaller sound than the click rebounding off the side of a whale.

Au said the team conducted echolocation studies with fellow researchers on white beaked dolphins in Iceland, Atlantic spotted dolphins in the Bahamas, and killer whales off Vancouver, Canada.

In most cases, they simply dropped an array of receivers into the water, and curious dolphins began approaching them, echolocating as they came.

The scientists weren't looking for the unique echolocation volume-control mechanism when they started.

"This phenomenon we never knew happened. It took us a while to say, 'Hey, this is what's happening!' " Au said.

Besides figuring out how big their targets are, the dolphins also may adjust their volume simply because it's annoying to get a really loud response from a nearby object, Au said.

Since the initial research, the team has found that dusky dolphins in New Zealand and spinner dolphins in Hawai'i use the same system.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.