Is The Hearing of Whales and Dolphins Fully Developed at Birth?: An investigation of the odontocete inner ear

Abstract

Sound is an essential component of toothed whale and dolphin (odontocete) biology. The advanced hearing-dependent activities of precocial calves lead us to believe that hearing is fully functional by birth. Nevertheless, few studies have investigated the ontogeny of cetacean ears. This thesis investigates the odontocete ear region (bones of the tympanoperiotic complex, TPC), with emphasis on the organ of hearing (cochlea) in the inner ear. I used radiography to examine ossification in six odontocete foetuses, and micro-CT scans to reconstruct 3D cochlear models for four life stages (foetal, neonate, juvenile and adult) from four ecologically divergent species from New Zealand: Hector’s dolphin (Cephalorhynchus hectori), bottlenose dolphin (Tursiops truncatus), long-finned pilot whale (Globicephala melas), and Gray’s beaked whale (Mesoplodon grayi). Individuals ranging from newborns to adults showed no significant intraspecific differences in the size and shape of their cochlear canals (p= 0.998) and TPCs, but there were significant interspecific differences (p<0.001). Consistent with their phylogeny, M. grayi cochleae significantly differed from the three delphinid study species (p<0.05), while the cochlea of the three delphinids did not significantly differ from each other (p>0.05). No ecological parallels were apparent within cochlear structures. Radiographs of foetuses ranging from Stages 7-12 (of Štěrba et al., 2000 classification) did not show evidence of TPC ossification. The more advanced Stage 11 G. melas foetuses, however, showed minor mineralisation of the TPC, and a cochlear canal could be observed in the largest foetus (69cm). Cochlear and TPC size and shape results suggest rapid prenatal development of hearing structures and are consistent with the hypothesis that calves have fully functional hearing abilities at birth.