Baleen whales hear with their bones, study finds

A biologist and an engineer have published a study in the journal PLOS ONE that suggests the skulls of baleen whales have evolved the ability to feel sound in their bones.

Baleen whales are a filter-feeding suborder of cetacea that includes fin whales and blue whales – the two largest living animal species in the world. The blue whale can grow to be almost 100 feet long and weigh over 400,000 pounds, making it the heaviest known animal to have ever lived. By comparison, the largest known dinosaur, Argentinosaurus, weighed only about half of that. Needless to say, their size makes these animals incredibly difficult to study.

So in 2003, when rescue efforts failed to save a young fin whale that beached in Orange County, Calif., San Diego State University biologist Ted W. Cranford and University of California, San Diego engineer Petr Krysl saw their chance: they took the whale's head. 

Then, Cranford and Krysl modeled the head with a high-powered CT scanner. With a computational process called finite element modeling, they were able to break up that complex structure into tiny individual components. This way, they could see in minute detail how these anatomical features – skin, bone, and muscle – connected and interacted with each other. And with their computer-generated whale skull, Cranford and Krysl were able to simulate how sound would travel through the whale’s head.

“It is yet another confirmation that hypotheses can follow from engineering models of natural objects, like animals,” Krysl says.

Whales hear sound by way of a bony, middle-ear structure called the tympano-periotic complex (TPC). Pressure waves from a sound can travel through a whale’s soft tissue into the TPC, but only if the waves are no longer than the whale’s body. This would pose a problem for blue and fin whales, who communicate in low frequency, long wavelength vocalizations. But thanks to a process called bone conduction, the sounds are amplified as they vibrate along the skull.

“In toothed whales, the ears sort of hang off the skulls,” Cranford says, “so they're isolated from this bone conduction mechanism. But baleen whales have their ears very rigidly attached to the skull. So right away we had an inkling something might be different.”

Marine biologists had postulated that baleen whales used some degree of bone conduction, but it had never been shown empirically. Cranford and Krysl found that not only do fin whales use bone conduction, but it is probably their predominant mechanism for hearing. The skulls of fin whales are keenly adapted to this unique method – at the lowest frequencies used by fin whales, bone conduction can be 10 times more sensitive than the pressure mechanism.

“The way function develops by way of evolution in natural systems is eye-opening and humbling for any engineer,” Krysl says.

Oceanic noise from shipping and military exercises has long been thought to harm whales. But without an understanding of whale hearing sensitivity, it has proven difficult to legislate regulations. Cranford says that, while his findings didn’t account for the effects of man-made noise on whales, they could be valuable to regulatory agencies or future research to that end.

“Anybody who was concerned with what effects it might have [on the whales] would be concerned with sensitivity,” Cranford says. “We don’t really know what effect the noise levels in the ocean have on the animal’s biology, but now we can start to think about those questions.”

Cranford and Krysl believe that similar systems could be employed in other baleen whales, as well as some fish. But could there be any outside applications for the fin whale’s bizarre hearing method?

“Too early to say if our group will come up with some application,” Krysl says. “There's always something an engineer can learn from nature, however. Of that, I'm certain.”