Hearing in Mysticetes vs. Odontocetes
Evolution of Cetaceans and their Communication
Hearing in Mysticetes vs. Odontocetes
Ear structures have not been studied in as much detail in mysticetes as in odontocetes, and animal protection laws make studying them harder. Therefore, Geisler and Luo studied fossils of the extinct Herpetocetus sp. as an example of mysticetes. From comparing their studies to previous studies in other cetaceans, they concluded the following:
- Laminar gap is wider in Herpetocetus sp. than in odontocetes and it widens quicker from the base to the apex. That is important because laminar gap approximates how wide the basilar membrane is, which determines frequency sensitivity. A narrower sensitivity corresponds to better sensitivities to frequencies. Therefore, the anatomical difference in the laminar gap suggests a low frequency hearing range for Herpetocetus sp. as for modern species, rather than a high one as in odontocetes.
- The secondary lamina in Herpetocetus sp. is weaker and thinner than in odontocetes, which suggests low sensitivities to high frequency sounds in mysticetes.
- The distribution of relative fiber densities in the cochlear foramina suggests that the cochlea in Herpetocetus sp. is specialized for the detection of low-frequency sounds. That is, cochlear nerve densities are generally highest in the parts of the cochlea most sensitive to high frequency sounds as in echolocation.
- Herpetocetus sp. and other mysticetes have a spherical vestibule as opposed to a curved and tubular one found in odontocetes. The spherical shape is similar to that found in terrestrial mammals, and thus it suggests low frequency sensitivities as found in terrestrial mammals, such as humans. (10)
Figure 5. Skull of an extinct mysticete. Permission from Wikipedia under the GNU Free Documentation License.
These findings suggest a specialization for low-frequency hearing in mysticetes rather than a loss of high-frequency hearing function suggested by Milinkovitch et al. (1993) based on mitochondrial ribosomal DNA sequence evidence.(10,22)
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