KNOWLES, Bridgett I*; MCKENNA, Megan F; CRANFORD, Ted; BERTA, Annalisa; San Diego State University; San Diego State University; San Diego State University; San Diego State University: Comparative Anatomy of the Hearing Apparatus in selected Cetaceans using CT Scans

To investigate possible mechanisms of mysticete hearing and to expand our understanding of odontocete hearing, we compared the in situ cephalic anatomy from CT scans of one mysticete, a neonate fin whale, (Balaenoptera physalus), and three odontocetes, a neonate beaked whale, (Ziphius cavirostris), and two adults, one bottlenose dolphin (Tursiops truncates), and one northern right whale dophin, (Lissodelphis borealis). The primary goal was to quantify relative volumes and surface areas of tissue structures deemed functionally important to the hearing apparatus. Previous work with odontocetes has shown that sounds pass through the mandibular fat bodies to the bony ear complex. Acoustic pathways in mysticetes are not well known. We measured the volume and surface areas of the head, ears, and mandibular fat bodies for each specimen. These measurements, extracted from varying densities between tissues, reveal that: The contact surface area between the fat body and the tympanic bulla in Ziphius was greater than that of the other odontocetes. The ratio of fat body volume to the volume of the entire head was very large in Tursiops compared to the other two odontocetes. And, the fin whale ears were much larger with respect to the animal’s head compared to the other species. This is the first study to quantify the area of contact between the tympanic bulla and the branches of the mandibular fat bodies; interfaces assumed to be critical in odontocete hearing. In all likelihood, these geometric relationships are functionally (acoustically) significant. This anatomic geometry supports the idea of acoustic frequency filtering and channeling functions in odontocetes and low frequency hearing in mysticetes.