The results of this study provide a description of North Atlantic right whale ear morphology and measurements of their cochlear and basilar membrane dimensions. The highly endangered status of the North Atlantic right whale has resulted in concerted efforts to perform complete necropsies on all dead right whales that can be recovered. This has resulted in the collection of baleen whale ears that are in good condition when from relatively fresh specimens. Better specimens could be collected from fresh kills in populations of baleen whales where whaling still occurs. This is not an option for any right whale population anywhere in the world.
This study indicates that collection of ear bones from right whales in any state of decomposition is worthwhile. This may have great application to collection of specimens from other species for which research approaches are similarly limited. Ears collected from highly decomposed specimens often retained the ossicular chain position in situ. Specimens with moderate decomposition retained the tough glove finger and lining of the tympanic chamber. All specimens allowed for measurement of size, length, and number of turns in the cochlea of the right whale. Decalcification and histological processing of two Code 3 (moderate decomposition) specimens resulted in very useful basilar membrane measurements and ganglion cell counts. Specimens in any condition from other right whale populations would be of use to determine whether there are significant differences in ear anatomy among the three proposed species of right whales.
The CT scanning of specimens proved effective for a variety of in situ measurements of right whale ears. Both 2D and 3D images were useful in describing the right whale cochlea. The 2D reconstructions were used to evaluate the condition of the middle ear and to detect any remnants of the VIIIth cranial nerve and in some cases the basilar membrane condition to assist with selection for further dissection and histological processing. Cochlear length could be determined both from measurement of radii from 2D cross-sections and from direct measurement of 3D reconstructions of the canal. The total number of turns was best determined from the 3D reconstructions.
The dimensions, position, and orientation of other ear structures are simultaneously available from CT scans of intact ears, including the vestibular system and ossicles (Fig. 2). Measures of these structures will allow for future estimates of middle ear transfer functions for this species. Scans of entire temporal complexes could be used for measurement of the size of the tympanic and the periotic bones. All of these observations can be made without disturbing the positions or destroying the specimen.
Gross Dissection and Histology
The gross dissections of ears provided data relating to middle ear structures in the bony ear complex of right whales. The spongy cancellous bone covering the densest dome of bone of the periotic may provide insight into the mode of sound transduction in right whales (see also Nummela et al., 2007, this issue). If bone conduction of sound is important for hearing in right whales, then the flanges may function to direct the sound to the dense bone surrounding the cochlea, with the spongy pad reducing incoming bone conduction of sound from other directions. Alternatively, the spongy pad of bone against the very dense bone surrounding the cochlea could function to isolate the cochlea from vibrations of the skull. The bony strut found supporting the ossicles and the presence of a well-developed stapedial muscle indicate that the ossicular chain in right whales may be functional. Even with the strut and the muscle present, the ossicular chain can be moved. Further mechanical studies need to be conducted to determine the functional role of the ossicles in baleen whale hearing.
Decalcification times for right whale ears was longer than would be predicted simply from the mass of the ears, primarily because of the exceptional density of the periotic, the bone surrounding the cochlea. Decalcification of an isolated, pared down, periotic bone took from 6–20 months, depending on the size of the specimen and the time in acid. The 5% trichloroacetic acid was used in an attempt to accelerate the rate of decalcification. The acid did substantially increase the rate of decalcification. For example, EG 18's left ear decalcified in 6 months solely in acid, but EG 18's right ear took 20 months in EDTA. However, acid alone as a decalcificant created multiple and significant artifacts, including the loss of soft tissue within the cochlea (Fig. 10). In baleen whales, the width and thickness of the basilar membrane toward the apical turn is the most important feature for accurate measurements to estimate low frequency hearing sensitivity. Unfortunately, membranes in the apex are the most fragile and the first lost in acid decalcification. None of the specimens decalcified in acid retained any basilar membrane beyond the first turn of the cochlea. A chelating agent such as EDTA is far superior. EDTA minimizes artifacts from decalcification and better preserves fragile membranes (Schuknecht, 1993). There is evidence that even specimens decalcified in EDTA can be overdecalcified. The extremely dense section of bone that projected laterally from the cochlear canal was the last area to decalcify, and it is likely that the cochlear canal itself was sufficiently decalcified to allow cutting significantly earlier. Future attempts at decalcification should focus on removing as much of the high density periotic as possible before and during decalcification as more is exposed to accelerate the process. Specimens from calves and adults made it possible to observe aging effects, such as demineralization of the periotic bone in older whales. Figure 10a–c shows ears from calves (<6 months in age) while Figure 10d is an ear from an adult, at least 23 years of age. The latter has lower bone density surrounding the cochlea. This change is not merely a result of differences in decalcification of the preserved specimen, but rather a real density difference that was evident from CT scans before the ear was dissected and decalcified.
The basilar membrane dimensions of the right whale are consistent with previously described measurements of baleen whale basilar membranes (Wartzok and Ketten, 1999). The base of the basilar membrane is thicker and narrower than the apical turn, which is extremely thin and wide. The apical turn of the right whale ear has membranes that may be thinner than can be accurately measured by traditional light microscopy and are perhaps best examined by transmission electron microscopy.
The distribution of ganglion cells in the best-preserved specimens indicates that there may be variable ganglion distribution and possibly hair cells in different regions in the cochlea of right whales. However, the postmortem decomposition of the specimens makes interpretation of the remaining ganglion cells difficult. Combining the counts of the best preserved sections of the basilar membranes from EG 4 and EG 9 still yielded a total ganglion cell count that was significantly lower than any reported ganglion cell count for any cetacean species due to large segments with total loss of cells. The direct counts are slightly greater than seen in human ears (30,000) but much less than half of what has been reported for other baleen whales (156,000) (Ketten, 2000). The density of cells in relatively well preserved areas was consistent with previous cetacean ganglion cell data (Ketten, 2000). The estimated ganglion cell densities/mm coupled with the average length of the right whale basilar membrane yielded a count comparable to those of other cetaceans. It is notable that the ganglion cell count of the right whale, presented here, and other baleen whales rival those of odontocetes and these counts are much higher than the average for any terrestrial mammal (Ketten, 2000).
The total hearing range for the right whale predicted from measurements presented here is 10 Hz–22 kHz with functional ranges probably being 15 Hz–18 kHz. These estimates were made using the model described in Ketten (1994). The model has been shown to accurately predict the frequency range of hearing in both odontocetes and bat species. Currently, there are no direct measures of baleen whale hearing; therefore, the results from this model cannot be compared with behavioral or physiological hearing curves for right whales. The robustness of this model makes it likely that the frequency range of hearing presented here is a close approximation to the hearing abilities of this species. The apical measurements of the basilar membrane indicate better low frequency hearing than in humans, while the capacity suggested by the basal end of the membrane is slightly higher in frequency but similar to human ears. As expected, this range corresponds well to the sounds produced by right whales (Parks and Tyack, 2005). Both this frequency range and the frequency range of right whale sounds overlap with the frequency range of many anthropogenic noise sources, suggesting that noise could potentially have a negative impact on hearing, localization, and communication by right whales.
This study represents a rare look at multiple ear specimens from a single baleen whale population. It provides a comprehensive description of multiple ear specimens collected from an endangered baleen whale species. It is difficult to collect baleen whale ear specimens as large whale strandings are relatively rare in comparison to those of small odontocetes. As expected, there was variation in the size, length, and number of turns of cochlea from different individuals, but consistent intraspecies spiral form and length. Further research is needed to ground-truth these model predictions with field tests of the functional upper frequency of hearing of right whales and the relative sensitivity of right whales.