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Sound to the Bone: How Baleen Whales Hear

Jan 30, 2015 12:24 PM EST
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New research has revealed that baleen whales, a group that makes up some of the largest animals on Earth, hear their surroundings and one another in a rather unique way. Their bones, it appears, can resonate with low frequencies, capturing sound and directing it to their ears, enabling supersensitive hearing that can stretch for miles.

That's at least according to a study recently published in the journal PLOS One, which investigated the incredible mechanics of this unique hearing system.

Researcher Ted Crawford explained in a recent statement that he was initially interested in better understanding whale hearing after learning just how far the whale songs of some massive species can reach. Ongoing whale studies, particularly of humpback whale song, has determined that some songs can be carried through the ocean for up to 3,000 kilometers (~2000 miles).

That due to the incredibly low frequencies of these songs, and while a couple thousand miles may be a bit too far to make out anything clear, a little closer could be well within "hollerin' distance," as Crawford puts it.

To better understand just how these hollers are heard in the first place, the San Diego State University biologist teamed up with University of California, San Diego engineer Petr Krysl to create a 3-D computation model of your average baleen whale skull.

According to the study, the pair accomplished this by getting their hands on a young fin whale's head, after the unfortunate creature beached itself and died on a shore in Orange County, Calif. By placing the head in an X-ray CT scanner - originally designed for rocket motors - the researchers were able to design their model, complete with appropriate density and acoustic properties.

"At that point, computationally," Cranford said, "it's just a simple physics problem."

The biologist explained that after the scan, he and Krysl essentially broke the skull down into its separate components. He compared this process to breaking up a LEGO brick model, where computed simulations of sound waves passing through the head helped them see how various frequencies affected which 'bricks' in different ways.

The key here was identifying the series of parts that could carry the sound to a whale's ears.

Boarding the Bone Train

According to the researchers, there were two possible ways in which sound could reach a whale's tympanoperiotic complex (TPC) - the inner ear structure that helps them process noise.

In one scenario, the pressure waves from sound could travel through the head's soft tissue. Unfortunately, sound would not be able to travel much further than the whales body, meaning that the massive swimming creatures would have to be uncomfortably close to one another to communicate. If you've ever watched Seinfeld, you know well enough how that's likely unacceptable, even for whales.

That led the pair to "bone conduction," a system that would require sound to easily travel through the skull.

We humans experience this phenomenon all the time in the pool. While our ears cannot capture much sound underwater, some frequencies still carry through after vibrating along our skulls. Unfortunately, due to the rigidity of our bones, the noises become warped and muffled.

Cranford and Krysl found that despite whale bones being similarly rigid, their skulls are specially designed to pick up and amplify these frequencies, carrying a clear, if-not-improved sound to the ears. (Scroll to read on...)

(Photo : Ted Cranford and Petr Krysl) A simulation of a fin whale skull deforming due to the vibrations of a 250 Hz sound wave. The motion depicted in this animation has been magnified to make it easier to see.

"Bone conduction is likely the predominant mechanism for hearing in fin whales and other baleen whales," Cranford explained. "This is, in my opinion, a grand discovery."

Sounds Like Trouble

So why is this important? The researchers stress that understanding how whales hear can help conservationists determine which man-made noises could specifically affect whale communities and how.

Other studies have revealed that many man-made noises, such as sonar signaling and the rumble of engines, can distrupt ocean life. Back in July it was even revealed that disruptive unnatural frequencies reaching the ocean floor could be leading to a mass decline in sea hare populations.

It was also thought that these noises - often at low frequencies - could be disrupting communication among whales that are already having a hard time. As things stand, blue whales, minke whales, right whales, humpback whales, and fin whales are all classified as threatened or endangered, according to the IUCN Red List.

And while baleen whale decline has certainly been associated with excessive whaling and pollution in the past, factors like disrupted communication could be making recovery hard-won for this species today.

For more great nature science stories and general news, please visit our sister site, Headlines and Global News (HNGN).

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