Date Published: June 10, 2012
Publisher: Wiley Subscription Services, Inc., A Wiley Company
Author(s): Maya Yamato, Darlene R Ketten, Julie Arruda, Scott Cramer, Kathleen Moore.
Cetaceans possess highly derived auditory systems adapted for underwater hearing. Odontoceti (toothed whales) are thought to receive sound through specialized fat bodies that contact the tympanoperiotic complex, the bones housing the middle and inner ears. However, sound reception pathways remain unknown in Mysticeti (baleen whales), which have very different cranial anatomies compared to odontocetes. Here, we report a potential fatty sound reception pathway in the minke whale (Balaenoptera acutorostrata), a mysticete of the balaenopterid family. The cephalic anatomy of seven minke whales was investigated using computerized tomography and magnetic resonance imaging, verified through dissections. Findings include a large, well-formed fat body lateral, dorsal, and posterior to the mandibular ramus and lateral to the tympanoperiotic complex. This fat body inserts into the tympanoperiotic complex at the lateral aperture between the tympanic and periotic bones and is in contact with the ossicles. There is also a second, smaller body of fat found within the tympanic bone, which contacts the ossicles as well. This is the first analysis of these fatty tissues’ association with the auditory structures in a mysticete, providing anatomical evidence that fatty sound reception pathways may not be a unique feature of odontocete cetaceans. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.
In all minke whales examined, there was a distinct, depigmented (white) line on the epidermis projecting posteriorly from the aperture of the external auditory meatus. This marker is rarely, if ever, mentioned in the literature but would be helpful in locating the minuscule external auditory meatus. The auditory canal appeared to be continuous from its external opening to the glove finger, though winding and narrow.
Sound reception in terrestrial mammals involves an air-filled outer ear. In odontocetes, which receive sound under water, the air-filled ear canal has been replaced by multiple lobes of fatty tissues leading to the tympanoperiotic complex (Norris, 1968). Two of the fat lobes are oriented anteriorly from the ears, including the inner fats filling the enlarged mandibular hiatus and the outer fats covering the lateral and ventral portions of the mandible (Ketten, 1994). These two anterior lobes are separated by the mandible, which has a thinned region termed the “pan bone” (Norris, 1968). Although Norris (1968) states that this “thin bone is transparent to the sounds used by porpoises,” the precise role of the pan bone in odontocete sound reception is still unclear (Ketten, 2000; Cranford et al., 2008a). In addition to the inner fat body and the outer fat body, a third fat lobe is located lateral to the tympanoperiotic complex and is thought to be a better sound reception pathway for lower frequency sounds (Bullock et al., 1968; Renaud and Popper, 1975; Popov and Supin, 1990; Ketten, 1994, 1997; Popov et al., 2008). All fatty lobes have well-defined connections with the tympanoperiotic complex.