Whale ears: Structural analyses and implications for acoustic trauma

Abstract

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 3× that of bats. Baleen whales have stiffness coefficients lower than elephants that hear infrasonics. Ganglion cell densities in whales are 2× bat and 3× 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.]