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Toothed whales (odontocetes) make use of high-frequency sounds to echolocate, differing significantly from their sister group baleen whales (mysticetes), which make use of low-frequency sound for long-distance communication. This divergence in auditory ability has led to considerable speculation as to how hearing functioned in the ancestral archaeocetes, and when the specializations of modern species arose. Numerous studies have attempted to infer auditory capabilities from morphological correlates valid in modern species. Here, we build upon these previous methods with a focus on cochlear structures that have well-understood links to function. We combine this with information on the sound conduction apparatus to chart the evolutionary trajectory of cetacean hearing. Our results suggest an initial move toward low-frequency specialization in early Eocene cetaceans, which coincides with the appearance of new sound conduction pathways. This paved the way for the later movement toward higher-frequency hearing in protocetids; however, the ultra-high- and low-frequency hearing specializations of both modern cetacean clades evolved after their divergence. We use these data to test the hypotheses that evolutionary brain size increases in cetaceans were related to the origin of high-frequency echolocation. We show that no shift in relative brain size coincides with any changes toward high-frequency perception. However, this does not rule out a role for other changes in hearing ability such as some simple forms of echolocation, similar to that suggested for hippopotamuses or bowhead whales, which may have been present in even the earliest cetaceans.
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Intraspecific variation of cochlear morphology in bowhead and beluga whales
Abstract The bony labyrinth of the petrosal bone, a distinctive feature of mammal skulls, is often identified in micro‐computed tomography imaging to infer species' physiological and ecological traits. When done as part of a comparative study, one individual specimen is normally considered representative of a species, and intraspecific variation is considered low. Yet tests of intraspecific variability have been performed on few species and on limited morphological traits. Studies of intraspecific variability are not only valuable to help us assess the need for multiple specimens in comparative work, but relative levels of variability can also be used to reveal insights into a trait's functional significance. In this study, we report measurements of intraspecific variation on two cetaceans with vastly different auditory specializations, a low‐frequency specialized mysticete and an echolocating odontocete. We examine the internal structures of the cochlea in beluga and bowhead whales and relate this to their hearing abilities. Overall levels of intraspecific variability are higher in the bowhead than the beluga, reflecting the more specialized auditory system of the latter. However, the levels of variation differ through the length of the cochlea (base to apex) and these appear to reflect known frequency specializations of the species, with the bowhead having lower variation in some measurements at the low‐frequency apical end than the beluga.
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- Award ID(s):
- 2142526
- PAR ID:
- 10640306
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- The Anatomical Record
- ISSN:
- 1932-8486
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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