Researchers in San Diego (aka “Whale’s Vagina”) have digitally recreated the entire head of a fin whale, revealing how these baleen giants are able to hear: bone vibrations. Their skulls have acoustic properties capable of capturing the energy of low frequencies. The finding, published in PLOS ONE this week, helps solve a long-standing mystery about one of the world’s largest and highly inaccessible animals.
Baleen whales can emit extremely low frequency vocalizations that travel far distances under water. But many of these are in the same frequency range as man-made noises from commercial shipping, energy exploration, and military exercises—which may limit how they communicate with each other about food or mates. But there’s a lot we don’t know about how baleen whales actually hear. Much of what we do know is inferred from anatomic studies of their ears and sound playback experiments.
Using a different approach, Ted Cranford from San Diego State and Petr Krysl from University of California, San Diego, built a 3D computer model of a baleen whale head to simulate how sound travels through it. They based the skin, tongue, muscles, and everything in between on X-ray CT scans of a young fin whale (Balaenoptera physalus) who beached on Sunset Beach in Orange County. (They used a device originally designed for rocket motors.) Using these scans, they applied “finite element modeling” (pictured below) to break the data up into millions of tiny bits to track their various connections to each other. It's a bit like dividing the whale's head into a series of LEGO bricks, Cranford explains in a SDSU release.
There are two ways for sound to reach the interlocked ear bones attached to a whale’s skull: pressure waves through soft tissue or vibrations along the skull (called bone conduction). Pressure waves are ineffective if the sound waves are longer than the whale’s body; with bone conduction, on the other hand, longer wavelengths are amplified as the skull vibrates.
When the duo simulated sound waves passing through the computerized skull, they were able to see how each miniscule component of the head responds. And bone conduction, they found, was about four times more sensitive to low frequency sounds than the pressure mechanism. In fact, for the lowest frequencies used by fin whales (in the 10 to 130 hertz range), bone conduction could be up to 10 times more sensitive.
We experience something similar when we’re submerged in a swimming pool. "Our ears are useless, but we still hear something because our head shakes under the pushing and pulling of the sound waves carried by the water,” Krysl explains in a UCSD release.
Cranford adds: “Anatomic structure is no accident. It is functional, and often beautifully designed in unanticipated ways." Here’s a very cool video showing the deformations and motion of the skull for a 250 Hz wave:
Images: SDSU (top), Ted W. Cranford and Petr Krysl (middle)