The Role of Asymmetry of the Left and Right External Ear of Bottlenose Dolphin (Tursiops truncatus) in the Spatial Localization of Sound

It is generally accepted that Odontoceti lost their external ears (pinnae) in the process of adapting to aquatic habitats. However, their hearing localizes sound with an accuracy of 1° in the frontal and median planes and is directional. These facts indicate the presence of morphological structures functionally performing the role of evolutionarily new external ears adapted to the aquatic environment. The data available to date suggest that this role is played by the left and right row of mental foramens (MFs) and the morphological structures of the rostrum and skull of the dolphin. In this study, for the first time for Odontoceti, the paths of sound travel along MFs and mandibular canals of the lower jaw of bottlenose dolphin (Tursiops truncatus) are measured, and the relative time delays of sound between the MF and the degree of their acoustic shielding by the rostrum and skull depending on the localization of sound in space are calculated. It was established that the left and right outer ear form unique temporal and spectral signs of the spatial localization of sound with a maximal accuracy realized rostrally. Localization mechanisms are based on asymmetry, including rostral–caudal and left–right mutually complementary asymmetry of MF architecture, dorsal–ventral asymmetry in the size of the rostrum, as well as rostral–ventral asymmetry in the position of the left and right row of MFs on the rostrum and rostral–caudal asymmetry in the sizes of the rostrum and skull. Thus, unlike the outer ears of terrestrial animals and human beings limited by the auricles, the outer ears of the dolphin are integrated into the streamlined shape of the rostrum and head of the dolphin, which reduces the resistance to its movement from the water side and, most importantly, does not worsen the signal-to-noise ratio of the flow around it in its hearing with increasing speed. Based on the morphology similarity of Odontoceti, it is natural to assume that their MFs and the morphological structures of the rostrum and skull play the role of external ears and form signs of spatial localization of sounds.