Some Aspects on an Asymmetry of Nasal Bones in Toothed Whales

The arrangement of cranial bones in cetaceans in both Odontoceti and Mysticeti has evolved to enable better adaptation to aquatic habitats, and as a result, cetacean skulls differ from those of terrestrial mammals. Their unique skull morphology has been studied to the extent that it is now known Odontoceti have unusual asymmet­ ric cranial bone structures, whereas Mysticeti have sym­ metric structures similar to other mammals (Miller 1923). Ness (1967) measured the asymmetry of odontocete skulls of 314 specimens across seven families, 26 genera, and 41 species. He revealed the presence of a leftward deviation of midline structures and reported that the point most deviant from the midline axis usually lies on the tangent between both nasal bones. Cranial asymmetry includes both the deviation of the midline suture to the left and enlargement or distortion of the bony elements (Heyning 1989). This phenomenon is known as direc­ tional asymmetry, which is unique to Odontoceti among mammals. Cranial asymmetry has been pointed out to play a role in the sophisticated acoustic system characteristic of Odontoceti. The source of their biosonar signals is thought to be a structure associated with the upper nasal passage (Yurick and Gaskin 1988; Cranford et al. 1996; Cranford 1999), and the cranial bones related to that airway around the nasal bones are likely to have the most apparent asym­ metry, leading to an expectation of a correlation between cranial asymmetry and biosonar generation. Aroyan et al. (1992) suggested that these skull bones act as an acousti­ cal mirror to reflect and amplify the sounds the whales generate. However, no evidence exists to explain why an asymmetric structure is needed for nasal phonation or the way in which each cranial bone serves as an acoustic component. Why do different odontocete species have different cranial asymmetry? The skull bones in some species, such as the porpoise, secondarily became more symmetrical to shift the produced sound outside of their predators’ hear­ ing range after ancient Odontoceti had obtained an asym­ metric skull, which was originally explained by Morisaka and Connor (2007) and further endorsed by Murakami et al. (2014). Past studies of odontocete skulls dealing with specified delphinid species, such as phocoenids, have been performed (Perrin 1975; Yurick and Gaskin 1988). Despite such studies trying to account for the phenome­ non, there is no reasonable explanation for asymmetric variation in the nasal bones because only a few studies have measured and compared the shapes of the skull bones, especially the nasal bones, in various odontocete species. Ness (1967) concluded that the magnitude of asymmetry decreased as follows: Kogia > monodontids > delphinoids (except Orcinus) > ziphiids > Orcinus. How­ ever he did not adequately describe how the asymmet­ ric region varied between species. As Heyning (1989) pointed out, in order to analyze the nasal bone asymme­ try, the size and/or shape asymmetry should be quantified in some ways which are independent from the rostral length. In addition, when we consider Cetacea as a whole, mysticete skulls are assumed to be symmetrical but have not been investigated fully or in detail, partly because of the difficulty in gathering a sufficient number of speci­ mens. In this study, we emphasize the importance of nasal bones in cranial asymmetry by examining nasal bone asymmetry in seven Odontoceti families through mea­ suring the position and shape of the nasal bones. We calculated the ratio of asymmetry by using bilateral structures to eliminate the influence of the proportional differences by rostral length among odontocete families. The same regions were then compared with those in the common minke whale Balaenoptera acutorostrata to confirm that the cranial bones of the mysticetes are sym­ metrical. We also compare the asymmetric anatomy of odontocete nasal bones not only among odontocetes but also to mysticetes.