D. R. Ketten
Dept. of Otolaryngol., Harvard Medical School, MEEI, 243 Charles St., Boston, MA 02114
Over 75 species of dolphins and whales are spread throughout every aquatic
habitat. Although echolocation abilities of some dolphins are well documented,
little is known about hearing in most whales. Dolphin signals range as high as
200 kHz, while baleen whales routinely produce 10- to 20-Hz signals. Whales
have, therefore, two important auditory considerations: (1) the broadest signal
range of any mammal group; and (2) the only mammalian ears adapted to
underwater hearing. In this study, three-dimensional morphometric models of
middle and inner ears from noncaptive toothed and baleen whales were used to
estimate their hearing ranges. The analyses show echolocating species have
basilar membrane stiffness coefficients 3x that of bats. Baleen whales have
stiffness coefficients lower than elephants that hear infrasonics. Ganglion
cell densities in whales are 2x bat and 3x human values. These data suggest
echolocating dolphins may use multiple, parallel processed signal detection
mechanisms. Cellular hypertrophy of inner ear support structures (stria
vascularis, spiral ligament, basement membrane, etc.) in all whales may mean
whales have inner ear mechanisms that decrease the potential for acoustic
trauma. [Work supported by ONR Grant No. N00014-92-J-4000.]