Article: Cranford, TW., Krysl, P. 2015. “Fin whale sound reception mechanisms: Skull vibration enables low-frequency hearing”. PLoS ONE. 10(1): e0116222 doi:10.1371/journal.pone.0116222
With the myriad of human actions that are causing impacts to the world’s ocean (climate change, ocean acidification, pollution, over-harvesting of resources, just to name a few), noisiness tends not to be one that is readily on people’s minds. After all, who thinks of the ocean as a noisy place? (It is, by the way, quite noisy. Just ask any scuba diver.)
While the movement of water and the various marine organisms all contribute to the noisiness of the ocean, human-made sounds add on to those noises and can greatly impact a system. Activities like boating, ocean drilling, and even fishing can create quite a bit of noise (and sometimes, scientists are not entirely innocent either). Add on to the fact that humans are near constantly in or on the ocean doing something at any one time, this can add up to a lot of extra noise (see this nifty piece by National Geographic for more information on marine noise pollution). While all this noise certainly would be irritating to live with, why should it be such a big concern to us?
To quickly flash back to high school physics, sounds travel as waves through whatever medium they are produced in (air for us land lubbers, sea water for marine organisms). As water is a denser medium, sound waves travel faster and farther in the ocean than in air, making seemingly mild noises loud and far-reaching. Additionally, lots of animals (marine mammals in particular) use sound as an essential tool for foraging. An example is using echolocation to locate food, like bats do, and to communicate. In fact, many species of whales take advantage of the fact that sound travels so well in water, and use it to communicate of vast distances.
Unfortunately for these whales, many human-generated noises fall in the same frequency as their whale-noises. Scientists hypothesize that this extra noise can greatly interfere with their communication. Sadly, the scientific community is not very knowledgeable about whale hearing to confidently test these hypotheses, keeping us in the dark on how noise pollution might impact marine mammals.
Previous studies regarding whale hearing has been limited to anatomical studies (how the shapes of the structures and organs influence what sounds they might be able to hear), playback studies (how whale behavior changes in response to other whale recordings), or just plain old inference (whales can probably hear the sounds they produce, right?). However, a recent study published in the journal PLoS ONE has applied a new method to better explain how whales hear – computer modeling.
In late 2003, a newborn male fin whale (Balaenoptera physalus) washed onto shore in San Diego, CA. Necropsies revealed that the calf died of natural problems, likely resulting from natural medical concerns. While the whale calf was being processes by the San Diego State University Museum of Natural History, Drs. Ted Cranford and Peter Krysl found themselves a fantastic opportunity to study whale hearing. During the necropsy process, the entire head of the whale was removed and was placed in a CT scanner, which created a detailed 3-dimensional model of the whale’s head (Fig 1 and 2), including the soft tissues and bones. They used these images to create a finite element model, which reduces the different structures and tissues of the head to simplified boxes that interconnect with each other. This simplification allowed the researchers to focus on how these elements were interconnected with each other and how they might work together to transmit sound to the whale’s ear.
Once they determined how sound was picked up by the whale, they were able to test many different wavelengths of sound to determine the sensitivity range of the fin whale (and possibly other species of whales as well).
Using this finite element approach, Drs. Cranford and Krysl were able to model how sounds waves in the environment can be picked up through the whale’s head and directed to the ears. They found that the pressure of sound waves hitting the head of a whales get transmitted through the soft tissue and are directed towards the ears – more specifically, to the tympanoperiotic complex, a series of rigid bone structures that vibrate when picking up sounds). This is called the pressure mechanism. Such a finding is not entirely surprising as many anatomical studies have suggested that soft tissues in the head act this way. In fact, many studies in toothed whales (like dolphins and orcas) have shown their heads certainly do act as large antennas for sound waves. However, an unexpected discovery made from the models was that baleen whales use a second mechanism to direct sound waves to their ears – bone conduction.
Drs. Cranford and Krysl found that the jaw bones in the fin whale pick up an entirely different type of sound and direct the sound to the ears through the many bony connections in the skull leading to the ear. This has never been found before in whales. Comparison of the different anatomies of toothed and baleen whales show that this bone conduction mechanism may be unique to baleen whales.
The two researchers then tested the sensitivities of these two hearing mechanisms by inputting a wide range of sounds to their model and measuring how much sound is transferred to the ears. The researchers produced a modeled, or theoretical, audiogram, outlining the sensitivity range of hearing in fin whales (Fig 3). Unsurprisingly, the two methods of hearing were quite different in their sensitivities, but worked together to enhance the whale’s ability to hear. The pressure mechanism is able to pick up on higher frequency sounds, such as the clicks we commonly associate with dolphins. Unfortunately, the pressure mechanism only works with high frequency/short wavelengths of sounds and long wavelengths (longer than the whale’s body) are not picked up by this mechanism. This is where baleen whales employ bone conduction. The audiogram shows that bone conduction is nearly 4 times more sensitive to long wavelengths of sound, such as the echoing songs we hear on white noise albums. The pressure and bone conduction mechanisms work in tandem to increase the overall range of sounds a whale can hear.
This entirely new discovery marks a huge step in our understanding of whale hearing. While this model is specific to the fin whale that washed up on shore, Drs. Cranford and Krysl predict that the model should be fairly generalizable and be applicable to many different species. Anatomical studies on the tympanoperiotic complex show a varying degree of stiffness and connectivity which may alter the relative importance of the pressure and bone conduction mechanism. With their model, they anticipate being able to resolve these species specific features with relative ease.
Additionally, these models are now able to give scientists, and possibly more importantly, policy makers, the information they need to make noise pollution regulation a reality. For example, specific activities that generate noises that may overlap with whales’ hearing or interrupt their communication could potentially be mandated to alter their frequency through changes in technology or design to protect the whales. Rather than relying on speculation, inferential studies, or anecdotes, we now have quantitative data on the sensitivity of whale hearing. Rather than assuming which noises may interfere with communication or hearing, we may be able to predict which specific activities pose the greatest risks to whales, and how best to address those activities.