For a long time, marine researchers have been mystified by how baleen whales hear. Now a new research has shed light on how these mammals hear underwater, highlighting the role of skull bones in the process.
While the largest animals on the planet, baleen whales are mostly endangered, except for the gray whale. Baleens include right whales, minke whales, blue whales and fin whales. Ted Cranford from the San Diego State University and Petr Krysl from the University of California, San Diego used the skull of a young fin whale that was beached on Orange County's Sunset Beach to determine how baleen whales hear.
Baleen whales emit vocalizations at extremely low frequencies that can travel great distances underwater. Most of what scientists know though about the whales' hearing process is brought about by inferences about vocalization frequency ranges and an anatomic understanding of their ears complemented by some experiments on sound playback.
Cranford and Krysl took a different approach; building a highly complex 3-D computer model of the head of a baleen whale and then simulating how sound likely travel through it. To do this, the head of the young fin whale was scanned and then finite element modeling was applied. This resulted in a 3-D model in which every part of the skull would react in response to vibrations in sound waves.
Researchers knew that there were two ways that sound would travel to the tympanoperiotic complex, an interlocking puzzle of bones in the ear that is attached rigidly to the skull. One way was through sound pressure traveling through soft tissues and the other was through bone conduction where sound travels through vibrations on the skull.
Applying various wavelengths to the 3-D model of the skull revealed that bone conduction resulted into four times more sensitivity to low-frequency sounds. More importantly, at the lowest frequencies fin whales are capable of, bone conduction led to up to 10 times more sensitivity to sound.
"What our contribution does is give us a window into how the world's largest animals hear, by an odd mechanism no less. This research has driven home one beautiful principle: Anatomic structure is no accident. It is functional, and often beautifully designed in unanticipated ways," said Cranford.
The results of their research are published in the journal PLOS ONE. The skull of the young fin whale used in the study now calls the SDSU Museum of Biodiversity home.
