44 Summary Marine mammals are acoustically diverse with wide variations not only in ear anatomy, but also in frequency range and amplitude sensitivity. In general their hearing is as acute as that of land mammals, and they have wider ranges. Although marine mammals exhibit habitat and size related hearing trends that parallel those of land mammals in that larger species tend to have lower frequency ranges than smaller species, the majority of species have some ultrasonic capability and there are multiple specialized, auditory adaptations in odontocetes that provide large species exceptional high frequency hearing capabilities. Both mysticetes and odontocetes appear to have soft tissue channels for sound conduction to the ear. Sirenians may have analogous adaptations. It remains unclear whether pinnipeds use soft tissue channels in addition to the air-filled external canal for sound reception. Comparisons of the hearing characteristics of otarids and phocids suggest that there are at least two types of pinniped ears, with phocids being better adapted for underwater hearing. Sea otter ears are the most similar to those of land mammals of all marine mammal ears that have been investigated, but they do have some aquaticrelated features, and it is not known how well they hear underwater. No data are available on polar bear hearing.   All marine mammals have middle ears that are heavily modified structurally from those in terrestrial mammals in ways that reduce the probability of barotrauma. The end product is an acoustically sensitive ear that is simultaneously adapted to sustain moderately rapid and extreme pressure changes, and which appears capable of accommodating acoustic power relationships several magnitudes greater than in air. It is possible that these special adaptations may coincidentally provide acoustically protective mechanisms that lessen the risk of injury from high intensity noise, but no behavioral or psychometric studies are yet available that directly address this issue.   One irony of sensory system research is that the more tools we invent to explore animals and their senses the greater the hints we receive that our reach is still too short. How extensive is our research arm currently? We know marine mammals use frequencies we cannot hear but we can technologically detect and transduce their frequency range into something we can analyze. Tools that help us probe and visualize how marine mammal sounds are produced and processed, like fast biomedical imaging, are helpful but still comparatively limited. The anatomical sophistication and the extensive cortical space allotted to temporal divisions of the brain in virtually all cetaceans, including baleen whales, implies a more important role for auditory processing than we have previously expected. Our greatest short-coming is that we cannot yet measure or observe reliably and frequently in the truly relevant environment for marine mammals: at depth in a free-ranging animal but technology that will make these studies routine are rapidly becoming available - and ironically will certainly have to employ acoustics to obtain definitive answers.