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UBC Theses and Dissertations

A comparative morphometric analysis of the sensory hair cells of the cochleas of echolocating mammals Girdlestone, Cassandra Dawn


Morphometric analysis of the inner ear of mammals can provide information for cochlear frequency mapping, which can determine the encoding frequency of lesions in the cochlea resulting from noise-induced hearing loss. A frequency map is a species-specific designation of the locations in the cochlea at which different sound frequencies are encoded. If the frequency map is known and there is a lesion in the cochlea due to noise exposure, the related frequencies of the anthropogenic source may be determined. Morphometric variation occurs in cells of the organ of Corti from the apex to the base of the cochlea. The base of the cochlea encodes for high frequency sounds, while low frequencies are detected in the apex. These changes in cell shape and spacing are linked to the frequencies detected at different locations, which has previously been shown in the guinea pig (Cavia porcellus). Here, we show that morphometric analysis also seems to be a viable alternative to physiological techniques when predicting the frequency as a function of location in other mammals, including those that echolocate. Parnell’s mustached bat (Pteronotus parnellii) already has a well-documented frequency map to compare morphometric measurements to. Using both traditional and geometric morphometrics to analyze scanning electron micrographs, our research shows a relationship between cochlear morphometrics in six mustached bats and their frequency map. Traditional morphometrics were also collected in a Wistar rat (Rattus norvegicus). These results from both species were further compared to traditional morphometrics measured in beluga whales (Delphinapterus leucas). Five out of eight morphometric parameters analyzed showed a strong similarity in their trends along the cochlea, including the distance between the rows of hair cells, width of outer hair cells, and gap width between hair cells. Using a multiple linear regression model revealed that five parameters are responsible for 83.5% of the variation in these morphometric data. Based on this information we created the first cochlear frequency map for the beluga whale. Determining the biologically relevant measurements related to frequency detection can give us a greater understanding of how hearing works and how it is affected by anthropogenic noise.

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