Animals

Mystery of Baleen Whale's Hearing May Be Solved

Skulls of at least some baleen whales have acoustic properties that capture the energy of low frequencies and direct it to their ear bones.

In what a researcher calls a "grand discovery," the question of how baleen whales hear may have been answered, solving a long-standing mystery.

Baleen whales, the largest animals on Earth at about 65 to 80 feet long, can emit vocalizations at very low frequencies, at wavelengths sometimes longer than the whales themselves. But how they hear, has remained to scientists a bit of a puzzle.

Rather than use more traditional whale-hearing analysis -- relying on anatomic study and sound-playback experiments with whales in controlled environments -- two researchers from San Diego decided to use computational horsepower and 3D software to tackle the problem.

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San Diego State University biologist Ted W. Cranford and University of California, San Diego engineer Petr Krysl created a three-dimensional computer model of a baleen whale's head, one that would include the skin, skull, eyes, ears, tongue, brain, muscles, and jaws.

For their test subject, the pair obtained the head of a fin whale that beached in 2003 and then ran it through an X-ray CT scanner.

Once they had the head scan, Cranford and Krysl ran simulations of how sound travels through the whale's brain. To get the detail they needed, they used a technique called finite element modeling, in which the data representing the head parts and skull were separated out into tiny elements by the millions, the relationships between the elements tracked.

Sound can reach a baleen whale's ear bones on its skull in two ways: the sound's pressure waves can go through the animal's soft tissue; or the sounds can vibrate along the skull itself, in a process called "bone induction."

The problem with the soft-tissue, pressure, route, the researchers said, is that it's ineffective when sound waves are longer than the whale's body. But with the bone induction process, those longer waves become amplified as they vibrate in the creature's skull.

The scientists' computer modeling showed that the bone induction process was about four times more sensitive to low-frequency sounds than the soft-tissue, pressure mechanism.

What's more, their modeling predicted that bone induction is 10 times more sensitive to the lowest frequencies used by fin whales (10 Hz-130 Hz).

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"What our contribution does is give us a window into how the world's largest animals hear, by an odd mechanism no less," Cranford said in a release. "This research has driven home one beautiful principle: Anatomic structure is no accident. It is functional, and often beautifully designed in unanticipated ways."

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

Next up, Cranford and Krysl hope to apply their computational methods to other species of baleen whale. Their research has just been published in the journal PLOS ONE.

The fin whale skull used for this study now resides in SDSU's Museum of Biodiversity.