Correlations Between Cranial and Inner Ear Morphologies of Odontocetes
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Glass, Abigail M
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Abstract
The odontocete skull shows features that indicate specialized behaviors implemented during the animal's life such as echolocation and feeding preference. Echolocation is a highly specialized adaptation for feeding and navigation that involves the production of high frequency sound and the reception of its echo. Previous studies show that measurements of bony labyrinths of the inner ear show interspecific differences in hearing frequency ranges of odontocetes. We can use CT scanning technology to take detailed measurements of these features. Features of the inner ear that correlate with high frequency hearing include a low basal ratio, greater inter-turn distance, and lower number of turns of the cochlea. Cranial features such as asymmetry and telescoping may be involved in high frequency sound production. Odontocetes use echolocation to assist in a variety of methods to capture prey such as suction and raptorial feeding. These feeding strategies may be determined by cranial morphological features such as longirostry (long rostrum) and brevirostry (short rostrum). Finding adaptive and functional relationships between these features is an important part of understanding the evolution of odontocetes and how they behave today. To determine if there are any evolutionary correlations between cranial features for feeding, sound production, and sound reception, I collected existing measurement data of inner ear and cranial features ("characters") from a total of 65 extinct and extant odontocete species. These data incorporate all the major extinct and extant odontocete families. I mapped the characters onto phylogenetic trees. Phylogenetic generalized least squares statistical analysis was used on the phylogenetic trees to assess correlations between characters. The results provide evidence that cranial asymmetry is correlated with echolocation. There is no evidence that cranial telescoping is correlated with echolocation. The findings in this study also indicate that brevirostrine odontocetes emit higher frequencies than longirostrine odontocetes. Lastly, this study finds a positive correlation between most inner ear features, except for comparisons involving number of cochlear turns and basal ratio. This reflects how size dimensions of the cochlea are directly proportional to one another. This work highlights and clarifies relationships among cranial features for echolocation and feeding within odontocetes.
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Odontocetes, Echolocation, Rostral proportional index, Cranial asymmetry, Cranial telescoping