Biological Mechanical Analysis of the Odontocete Ear

Laura Jahchan, Aidan Licoppe, Alessio Palladino, Shu Yuan Zhang

Abstract

The report that follows intends to discuss the biological mechanical and material properties of the ear while using the odontocete whale as a point of reference. The functions that the auditory must accomplish are detailed as well as the architectural mechanical properties of the odontocete ear which allow for such functions to be accomplished. The information is then put together to draw a clear image on the operation of the organ as a whole as well as to draw comparisons with terrestrial mammals. Finally, specific mechanical hearing problems are explored detailing the solutions that exist to these conditions.

Introduction

The world of today is characterized by its rich biodiverse nature. Interestingly, some of the world’s most biodiverse habitats are not visible to humans during their everyday lives but are instead found below sea level. As terrestrial animals made their way to the open water in a very distant past, evolution naturally followed. Our oceans are now home to aquatic creatures of all shapes and sizes, but our research seeks to dive into the King of the Deep’s physiology, and, more precisely, how whale ears function from a biomechanical perspective.

From head to toe, whales went through gradual, extensive transformations and their ears were in no way left out from this process. In fact, one can compare the way humans make use of their vision to the way whales use their auditory senses, that is how much they have evolved (Frost, 2015). With auditory signals ranging from songs to echolocation, and to underwater clicks, it is only logical to note that auditory system making up whale physiology is just as convoluted as the underwater sounds they attempt to hear (Frost, 2015).

However, whales cannot all be put into a single category and their complex auditory systems differ between these groups. Distinctions can be made with regards to what is found in their mouths, thus splitting the species into two groups: baleen whales and toothed whales — odontocetes — (Kennedy, “Differences Between Baleen and Toothed Whales”). The following report will have a particular focus on the ears of toothed whales.

Functions of the Ear

Although the way in which the odontocete ear is architecturally constructed may not be identical to the way a more familiar auditory system such as the human one is, it is still very much possible to relate them in terms of the general functions of the organ itself. In fact, the whale ear seeks to fulfill the three following functions common to the mammalian auditory system (Ketten, 1997):

1. The capturing of sound;
2. The transmission of sound;
3. The transduction of sound.

The way odontocete whales capture sound may not be related to their ears in any fashion. However, it feels inevitable to bring up the most intriguing hypotheses that were found during our research. For the time being, these theories seem to be researched ideas rather than concrete facts as the odontocete auditory system does not appear to be completely understood as far as sound capturing and transmission are concerned. First, Ketten (1997) provided data supporting Kenneth S. Norris’ hypothesis that fat channels in the toothed whale mandible appear to be responsible for capturing vibrations and bringing them to the inner parts of the ear. Second, a similar idea regarding how the mechanical properties of the whale rostrum also provide evidence that it is capable of capturing and transmitting these vibrations (Zioupos et al., 1997).

These captured vibrations can then be transmitted to the middle ear as is the case with most mammals. Sound transmission then continues through the vibrations of the middle ear ossicles and velocity amplification occurs due to a lever mechanism in the middle ear (Mooney et al., 2012).

Finally, transduction of sound takes place as vibrations reach the cochlear liquids of the inner ear. The third function is completed as the hair cells transform the vibrations into neural signals through electro-mechanical transduction (Pujol et al., 2016).

Mechanical Architecture of the Ear

Now that the functions of the toothed whale ear are understood, it is possible to take a closer look at how the ear’s architecture aligns with the previously mentioned biological mechanical functions. Restricting our analysis to the auditory organ itself, it is possible to divide the ear into three sections: the outer ear, the middle ear and the inner ear (Ketten, 1997). However, as previously mentioned, sound in toothed whales appears more likely to be captured through mandibular fat channels (Ketten, 1997) or the rostrum (Zioupos et al., 1997) as opposed to the outer ear itself. Thus, while reviewing the auditory system as a whole, it would be rather appropriate to identify a fourth section to odontocete whale ear anatomy, the aberrant regions, comprising the mandible and the rostrum.

Outer Ear

As viewed from the outside, the exterior of the toothed whale outer ear does not look the same as the common mammalian ear. In fact, pinnae – used for channeling sound into the ear canal – are not present (Ketten, 1997). Through this evolution of the exterior ear, whales could develop into faster swimming animals without having pinnae sticking out of their sides (Ketten and Madin, 2005). The rest of the outer ear is characterized by a small external meatus connecting to a narrow external auditory canal not linked to the tympanic membrane and filled with cellular debris and thick ear wax (Ketten, 1997). Through the architectural build of the odontocete outer ear, it appears as though the region is not built with the intentions of favorably capturing sound and channeling it to the middle ear, forcing researchers to direct their attention to alternate sound caption structures which we have defined as the aberrant regions of the ear.

Aberrant Regions

The first of the two structures making up the aberrant regions is the mandible (Fig. 1). This region in odontocetes has evolved to favor the transmission of sound upon its reception (Norris, “Peripheral Sound Processing in Odontocetes”). A thin and flexible zygomatic arc at the back of the mandible is structurally different from the stout bony strut of terrestrial mammals to reduce sound interference (Norris, “Peripheral Sound Processing in Odontocetes”). Moreover, the previously mentioned acoustic fats lie just above the pan bone which Norris (“Peripheral Sound Processing in Odontocetes”) describes as the thinnest part of the mandibular region. It is critical for the pan bone to be thin as sounds hitting the outer region of the mouth must pass through it to reach the fatty tissues (Norris, “Peripheral Sound Processing in Odontocetes”). Transmission through these tissues is considered optimal when sound arrives at an angle between 15° and 30° as the path towards the middle ear becomes direct (Norris, “Peripheral Sound Processing in Odontocetes”). The properties of the mandibular acoustic fats are remarkably interesting and have been subject of a lot of research but should instead be studied from a chemical perspective.

The second structure found in the aberrant regions that contribute to the toothed whale’s auditory system is the rostrum (Fig. 1). The rostrum represents the beak-like part in cetaceans, crustaceans, and certain fish (Kennedy, “Rostrum, As Used in Marine Life”).

During research conducted on the rostrum of the Mesoplodon densirostris, the rostrum was labeled as “the most extreme bone yet described” (Zioupos et al., 1997). In terms of stiffness, hardness, and mineral content, it succeeded in ranking superior in each one of these categories (Zioupos et al., 1997). By contextualizing the mechanical properties of the rostrum, it is possible to relate these characteristics to the hypothesized function that the structure must accomplish. The extreme stiffness and density of the rostrum aligns perfectly with the ideas that sound velocity is maximal, and that energy attenuation is minimal when such mechanical properties of the bone are met. (Ashman et al., 1984). Further research is required to verify to what extent the rostrum is involved in odontocete acoustic functions. However, what may be even more encouraging is the fact that the tympanic bulla, which encloses parts of the middle and inner ear, has very similar mechanical properties to that of the rostrum further pointing to a possible acoustic-related nature of the structure (Zioupos et al., 1997).

Location of the Middle and Inner Ear

Before discussing the specific architectural design of the middle and inner ear respectively, a focus should be placed on the placement of these parts of the organ within odontocetes as a whole. In fact, the architecture of their ears is designed in such a way that the middle and inner ear are found outside the case of the brane (Ketten, 1997). Not only are these sections separated from the skull, but each ear is also isolated from the other (Lee, 2015). In fact, the ears are suspended by ligaments and isolated by mucosal cushions, aligning the organ with the pan bone (Ketten, 1997). This form of architecture is directly related to the idea that whales must be able to efficiently locate sound from underwater positions (Ketten, 1997). Without such a peculiar architecture, the entire skull of the animal would vibrate in response to sound being transmitted to the middle and inner ear and it would not be possible for the whale to accurately identify where sounds are coming from (Lee, 2015).

Middle Ear

As it is common to the mammalian middle ear, the ossicles are found in this section of the organ where velocity amplification and transmission of sound occur (Mooney et al., 2012). By analyzing the properties of the middle ear ossicles, the elastic transmission of sound in the middle ear can be explained. Materials with the greatest elastic modulus may transmit sound through a body at the greatest of speeds (Allan, 2018). In the case of odontocetes, it was found that elastic modulus in their middle is quite high (Tubelli et al., “Elastic Modulus of Cetacean Auditory Ossicles”). Moreover, this stiffness is mainly related to the synostosis formed between malleus and the tympanic bone as well as the stapes’s annular ligament (Fleischer, 1978; Miller et al., 2006). It so happens that the middle ear structure of certain toothed whales has such a high elastic modulus that it approaches the value Zioupos et al. (1997) associated with the Mesoplodon densirostris rostrum, further hinting towards the potential auditory functions of the rostrum.

Certain properties of odontocete ossicles appear to be related to the fact that these whales can hear sounds at extremely high frequencies. Toothed whale ossicles are very rigid and calcified (Ketten, “Structure And Function In Whale Ears”). In fact, high ossicle mineralization, density, and stiffness are related to higher frequency hearing in toothed whales (Fleischer, 1978; Miller et al., 2006). Furthermore, the ossicles were not found to have isotropic material properties as elastic moduli varied in different regions of the bones (Tubelli et al., “Elastic Modulus of Cetacean Auditory Ossicles”). Thus, the ossicles of the odontocete middle ear should not be considered a bulk modulus as their movement does not resemble that of rigid bodies when they are subject to remarkably high audible frequencies (Tubelli et al., “Elastic Modulus of Cetacean Auditory Ossicles”).

Furthermore, middle ear architecture is designed in such a way that the difference in acoustic impedance between the middle and inner ear is accounted for by the lever mechanism between the malleus, incus, and stapes of the middle ear (refer to Mould, 2020 in Appendix). Sound travels from the air of the middle ear to the liquids in the cochlea that have a much higher acoustic impedance which would result in a minimal amount of acoustic energy being transmitted (refer to Mould, 2020 in Appendix). The mechanical properties of the lever, with the incus as the pivot point, allow for less acoustic energy to be reflected upon its passage to the inner ear (refer to Mould, 2020 in Appendix) (Fig. 2). Sound is further amplified as ossicles can concentrate energy as the tympanic membrane is much larger than the footplate of the stapes (Howard, 2019).

Inner Ear

The odontocete inner ear is home to the cochlea. It has been speculated that the spiral shape of the cochlea is related to the idea of conserving space. Nevertheless, newer research has shown that the architecture of the spiral cochlea may also be related to better vertical sound localization (Huang et al., 2012). At the base of the cochlea is the basilar membrane. At the apex, the basilar membrane is wide and flexible whereas, at its base, it is narrow and stiff, leading to a decrease in elasticity away from the base (Howard, 2019) (Fig. 3). Thus, at different distances from the base, specific frequencies will resonate in a process known as tonotopy (Howard, 2019).

The basilar membrane then lies within the organ of Corti, designed according to a sandwich-like structure with hair cells lying between the basilar and tectorial membranes (Howard, 2019). Upon movement of the basilar membrane, the hair cells will brush against the tectorial membrane and vibrations are transduced into electrical energy (Howard, 2019).

Operation of the Ear

With the knowledge of the mechanical function and architecture of the toothed whale ear, a closer inspection upon the overall operation of how this majestic creature captures the waves of sound follows. From the cephalic anatomy of toothed whales, it has long been recognized that the sound reception and transduction apparatus is an essential component of a biosonar system used by the odontocete, which is composed of the sound generation and transmission apparatus, as well as the central nervous system (Cranford et al., 2008). It is the culmination of these sophisticated processes that enables the toothed whales to perceive the environment through sound. The current essay, however, will only investigate how the odontocete manage to hear the vibrations of water in this extraordinary environment completely foreign to human ears.

To discover how sound propagates through the cephalic structure of a toothed whale, Ted W Cranford and colleagues (2008) utilized a finite element modeling (FEM) space constructed using a simulated whale head based on CT (Computed Tomography) data sets and physical measurements of sound propagation characteristics of tissue samples. They observed two sound reception pathways that could both be employed by the odontocete.

The conventional sound pathway first suggested by Norris in 1968 is often known by the colloquial term “jaw hearing”. After a detailed study of the mandibular structure, he proposed that sound enters the odontocete head through a fatty pad called the “acoustic window” which lies between the skin and the thin posterior portion of each mandible (Norris, “The evolution of acoustic mechanisms in odontocete cetaceans”; Norris and Harvey, 1974). As discussed in the architecture section of this paper, the sound passes from the acoustic window through the thin lamina of the pan bone and is conducted along the internal mandibular fat body to the tympanoperiotic complexes.

In addition to the conventional notion of jaw hearing, their studies strongly suggest that sound can also travel through the oral cavity and the gular region (Fig. 4). When the simulated acoustic pressure wave, travelling in the horizontal direction, encounters the cone of soft tissues surrounding the head, it is refracted around and enters the internal mandibular fat bodies through the opening created by the absence of the medial bony wall of the mandible and propagates to the tail of the bony ear complex. It has also been discovered that the gular pathway is functional in the bottlenose dolphin, which has its most sensitive region for sound entering the middle ear slightly forward of the acoustic window proposed by Norris (Møhl et al., 1999). This unique result can be attributed to the absence of the medial bony lamina of the posterior portion of the mandible, implying broad taxonomic distribution, especially among all other tooth whales which share the same condition (Cranford et al., 2008).

The functional elements of the inner ear and the middle ear — the cochlear labyrinth and ossicular chain in particular — have essentially the same structure as found in terrestrial mammals, and, therefore, similar functionalities (Tubelli et al., “A model and experimental approach to the middle ear transfer function”). As the vibrations are propagated to the middle ear, the waves of sound are captured by the tympanic membrane and transferred onto the ossicles in the middle ear, which is further conducted to the cochlea. There have been arguments stating that the middle ear is effectively dysfunctional, and that the transmission to cochlea is done mainly through bone conduction (Cranford and Krysl, 2015). Andrew A. Tubelli and his colleagues (“A model and experimental approach to the middle ear transfer function”) constructed a series of models using anatomical and physical properties of ear tissues to estimate the output at each functional division of the ear in the humpback whale, which has an essentially identical structure of inner and middle ear to the toothed whales. An additional experiment on a cadaveric ear was also conducted to acquire the most extensive data upon the middle ear transfer function (METF) to determine acoustic energy transmission to the cochlea. The results demonstrate a nearly perfect efficiency of the velocity transfer from the tympanic membrane tip to its insertion on the malleus at 1 kHz (Fig. 5). Although the magnitude differences between the two outputs are more pronounced in the experiments than for the models at that frequency, it still clearly illustrates the significance and the efficiency of the inner ear for the perception of sounds in whales.

The waves of sound will ultimately pass through the ossicles and reach the cochlea within the inner ear (Fig. 6). At the oval window — where the inner ear and the middle ear intersects — the vibrations are transferred into the fluid in helicotrema. These frequencies are then captured by the hair cells lining the organ of Corti, sending their signals back to the central nervous system where the information is interpreted and processed, helping the toothed whales to perceive the environment and guide them through the boundless seas.

Comparative Study

As odontocetes are aquatic mammals, the structure and function of their ears evidently differ from that of their terrestrial counterparts. In fact, if no such distinctions existed, humans would be able to hear underwater in similar fashion to how sound is perceived above it. Yet, when terrestrial mammals such as humans dive into water, sound perception is altered: noises appear to be muffled and the location of incoming soundwaves is basically unknown. Thus, comparing the hearing mechanics of odontocetes and humans can provide evidence as to how the auditory system of these aquatic mammals has evolved.

Perhaps the most obvious distinction between the ears of the two mammals lies within the architecture of the outer ear. As previously stated, pinnae are inexistant in odontocete outer ears. However, the human outer ear is made up of fleshy pinnae that stick out of the body. Because the acoustic properties of air and flesh are different, human pinnae can help funnel sound into the ear (Lee, 2015). Underwater, the fleshy pinnae of the human ear prove to be ineffective as its acoustic properties resemble that of water (Lee, 2015). Thus, the different architecture of the human outer ear demonstrates how this part of the organ can accomplish a different function compared to the odontocete outer ear through the way the structure is built.

Another distinct approach to ear architecture that hinders humans from adequately hearing underwater revolves around the location of the inner and middle ear inside the head. Because these interior parts of the human ear can be found within the skull, the vibrations channelled to the middle and inner ear cause the entire human skull to vibrate underwater leading to both ears being simultaneously affected (Lee, 2015). In air, such a problem does not occur. Making a parallel with odontocete ear architecture, it is evident why they do not encounter these difficulties. Recall that toothed whale ears are not found within their skulls, thus, isolating each ear from each other, preventing total vibration and allowing for better localization of sound. The architecture of the odontocete skull, middle and inner ears has evolved to account for this underwater vibration observed in the case of humans.

As for the operation of the middle and inner ear itself, the process of transmission and transduction of sound is very much similar between both mammals. Sound is amplified in the middle ear through a lever mechanism to account for acoustic impedance and hair cells of the cochlea will send signals to the central nervous system.

Thus, it is evident that the auditory system of odontocetes and humans resemble each other when analyzing the auditory organ as a whole. However, it is also clear that, over time, these aquatic mammals have adapted to their environment with their auditory systems consequently evolving. Their systems have become more complex, allowing them to hear a greater range of frequencies as their use of sound in the deep, dark open waters are much more important to the animals’ overall survival as opposed to humans who rely particularly on their vision above all (Frost, 2015).

Hearing Conditions and Solutions

With the intent of discussing various hearing conditions that stem from the mechanical properties of the ear as well as their related solutions, a specific interest will be placed on the human ear. Since the process of odontocete hearing is not understood to perfection, let alone the hearing conditions that may arise, humans — whose mammalian internal ear structure is slightly similar to that of odontocetes and whose hearing conditions are actively treated — provide a better subject for analysis.

The ear is a very intricate and delicate system, in which small abnormalities to the system can lead to serious issues in an individual’s ability to perceive sound. There is a plethora of conditions that can affect a person’s ear health and by consequence their ability to perceive sound.

Firstly, one of the most common issues that can arise with the ears, involve the most visible and least delicate portion of the ear: the outer ear. Problems with the outer ear may not impact hearing though they can cause an effect when there is a blockage in the ear canal (Kesser, 2020).  These blockages can result from cysts, abscesses, infections, earwax, or foreign objects. These conditions are pretty benign towards overall ear health and can be resolved with minimal intervention — antibiotics, removal of wax, draining the cysts, etc. — except for extreme situations. A more extreme blockage to the ear canal would be noncancerous and cancerous ear tumors. Though noncancerous tumors do not pose an immense threat towards overall health, from a mechanical standpoint, they can severely hinder hearing if they are blocking the pathway to the ear canal significantly enough. These tumors are treated through surgical removal which allows for better sound transmission towards the inner ear (Kesser, 2020).

As with any body part, with a significant amount of stress, the components of the ear, especially the more delicate components of the middle and inner ear can degrade over time, resulting in hearing loss. This occurs with all people with varying degrees of hearing loss, which can depend on genetics, overall stress, infections and diseases, or ototoxic drugs (“Basics of Sound, the Ear, and Hearing” | Hearing Loss: Determining Eligibility for Social Security Benefits | National Research Council (US) Committee on Disability Determination for Individuals with Hearing Impairments, 2004). There are three types of hearing loss that can occur: conductive, sensorineural, or mixed hearing loss which consists of a mix of both. Conductive hearing loss arises from problems in the external or middle ear, resulting in the inability to overcome the loss in sound transmission from the medium of the outer ear to the inner ear fluid. These problems include a perforation in the tympanic membrane, loss of ossicular continuity, or increased stiffness in the tympanic membrane, which can mostly be resolved surgically. Sensorineural hearing loss results from issues with the neural transduction of sound typically from the loss of hair cells and neurotrophic factors which can then also lead to nerve cell loss. The most common forms of adult hearing loss are presbycusis and noise exposure, both resulting from damage to the hair cells at the base of the cochlea, causing high-frequency sensorineural deficit (“Basics of Sound, the Ear, and Hearing” | Hearing Loss: Determining Eligibility for Social Security Benefits | National Research Council (US) Committee on Disability Determination for Individuals with Hearing Impairments, 2004).

Hearing loss causes an individual’s ability to discriminate between frequencies – usually 0.5 % change in tonal frequency – and sound level – typically 1 dB – to diminish depending on its severity which can pose many challenges towards daily life (“Basics of Sound, the Ear, and Hearing” | Hearing Loss: Determining Eligibility for Social Security Benefits | National Research Council (US) Committee on Disability Determination for Individuals with Hearing Impairments, 2004). People with hearing loss experience the same upper limit of audibility as those with normal hearing, meaning change in loudness increases more drastically as a function of sound level. Furthermore, people who experience hearing loss have wider critical bands, meaning signals can be masked by sounds with a larger variation of frequency in the spectral region of their loss. These people will require a more intense sound to detect one masked by other sounds of a similar frequency (“Basics of Sound, the Ear, and Hearing” | Hearing Loss: Determining Eligibility for Social Security Benefits | National Research Council (US) Committee on Disability Determination for Individuals with Hearing Impairments, 2004).

The least intrusive biomedical device that can be used as a solution for hearing loss is the hearing aid, which is used to partially overcome the various challenges posed by hearing loss. There have been various forms of hearing aids developed over the years, following a trend of decreasing size for better accessibility and comfort. All hearing aids function in a similar manner in which the signal passes through certain blocks typically represented through a block diagram (Fig. 7).

The first block encountered is the microphone, which serves to convert the sound to electricity. Directional microphones have two entry ports which are more sensitive to frontal sound than sound originating from other directions. This is useful as it can improve how intelligible speech can be perceived in the presence of other noise. The following block is the pre-amplifier, which serves to render the signal received by the microphone more powerful. There are various measures set in place in tone control to decrease the signal if it is amplified to a level that is too high. Following the pre-amplifier is the final amplifier which can represent the sound in an analog or digital manner. Finally, this signal is then sent to the receiver which takes the code from the amplifier and turns it into an acoustic output to be sent into the ear. Hearing aids can be adjusted towards the individuals certain hearing conditions to amplify different frequency ranges at different levels (Dillon, 2012).

Though there are many other issues that can occur with the ear, there are two more common diseases or conditions that can be solved recently through both surgery and prostheses in the ear. Firstly, cholesteatoma (Fig. 8) is a disease in which a noncancerous skin cyst grows in the middle ear and mastoid. It is not certain on how this disease forms, though evidence links it to the improper function of the Eustachian tube as well as patients typically have experienced previous problems with middle ear fluid. Though the condition itself is not painful, cholesteatoma is usually accompanied by an infection which can be treated with antibiotics.

The second condition is otosclerosis (Fig. 9), where a callus of bone accumulates on the stapes bone, limiting its ability to vibrate resulting in hearing loss. This condition is purely genetic and is associated with progressive hearing loss typically beginning in one ear followed by the other about 60 % of the time (Holt, 2003).

For a cholesteatoma, the type of surgery performed as well as if a prosthesis is required depends on how severely the condition has progressed. The main focus of the surgery is to remove the disease — to prevent secondary conditions and infections — as well as to restore or maintain hearing depending on its size and location. One possible form of surgery which can restore hearing is an ossiculoplasty, in which the ossicles are repaired or reconstructed. Over 80 % of patients require an ossiculoplasty due to a cholesteatoma or chronic suppurative otitis media. The materials used as a prosthesis may vary depending on the patient (Mudhol et al., 2013). Autografts (tissue grafts from the individual’s body), most commonly from the incus body reshaped to fit between the manubrium of the malleus and stapes capitulum, are typically the gold standard for ossicle reconstruction. Autografts may not always be possible due to the patient’s condition, so a closer and less common alternative is a homograft – grafts form another individual – from a cadaver which is less commonly used. Finally, synthetic prostheses fulfilling criteria of biocompatibility have been the most recent development giving the most advantageous hearings results. The materials used for these prostheses include metals such as titanium and gold, plastics such as plastipore and polyethylenes, and biomaterials such as ceramics and hydroxyapatite. Alloplastic materials are losing some popularity due to higher extrusion rates and lack of long-term results, though hydroxyapatite is becoming more successful in terms of functional hearing (Mudhol et al., 2013).

Depending on the severity of hearing loss experienced by otosclerosis, the patient can decide to monitor its condition with a doctor over time if hearing loss is insignificant, employ a hearing aid, or receive a stapedectomy. A stapedectomy is a surgical procedure involving the partial or total removal of the stapes bone, replaced by a prosthesis to restore its function. This is a relatively non-invasive surgery performed through the ear canal. The exact process varies from surgeon to surgeon, as well as the prostheses (Fig. 10) used which can include a stainless-steel piston or a wire. However, in the majority of cases, most of the stapes bone is removed and replaced by a Robinson prosthesis. This prosthesis has been used since the mid 1960’s boasting excellent results in the improvement of the patient’s hearing and biocompatibility (Holt, 2003).

Conclusion

The above investigation into the odontocete whale’s auditory system reveals how these majestic creatures adapt to the unique environments of the boundless ocean and overcome the challenges they face with exceptional engineering ingenuity which we, the humans, are yet to fully understand. Having the external pinnae removed and the bones containing the middle and inner ear separated from the skull permits them to clearly capture and interpret the vibrations of sound in water without obstructing their ability to swim swiftly, further revealing how closely functions and structures are related to one another.

Despite extensive knowledge on the anatomy of whales, how the odontocete perceives the environment through sound still remains largely a mystery. In the hearing conditions section, auditory issues related to human beings were discussed due to the lack of available research regarding the auditory problems that the toothed whales might be experiencing, which is not a surprising situation considering that the recent arguments on how sound reaches the cochlea are yet to be fully settled as new discoveries are made (Cranford et al., 2008; Tubelli et al., “A model and experimental approach to the middle ear transfer function”). The difficulties in the study of whale hearing arise mainly from the inability to conduct direct experiments on the toothed whales as well as the difficulty in fully simulating the complex structures and the exact diverse environment where this auditory system is intended to be applied. Further research into their sophisticated hearing system will not only help us to better protect these beautiful animals from the harmful noise pollution produced by oil tankers and cargo ships, but also shed light on how we could capture 3D sound data using sonar in the deep dark ocean.

Appendix

“There’s a Lever in Your Ear and It Does Something Amazing.”, 9 April 2020, Steve Mould, YouTube, https://www.youtube.com/watch?v=geSDcollRos.

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