Studying how Cetaceans use Sound to Explore their Environment

Many biologists implicitly assume that mechanisms for echolocation and communication are separate and compartmentalized. For example, the high-frequency vocal and auditory specializations of dolphins are typically only discussed in terms of echolocation and the low-frequency sounds of baleen whales are usually presented as signals for long-range communication. However, signals that evolved for one purpose may develop other functions. Some porpoises appear to use rhythmic patterns of “echolocation” clicks as communicative signals. When a whale makes a low-frequency sound for communication, the sound may echo from the seafloor and possibly provide the whale with important information about its environment. Research on the evolution of echolocation in marine mammals suffers from a dearth of studies of ecological function and from a lack of broad comparative reviews. If studies of marine mammal sonar included more analysis of the problems for which sonar may have evolved, we might discover fascinating new kinds of biosonar. For example, low-frequency sound is better suited than high frequency for long-range sonar in the sea, and many targets of great importance to marine mammals, such as large bathymetric features and fish with resonant swim bladders are also well suited to low-frequency sonar. Some marine mammals have the skills required to engage in bistatic sonar, in which one animal may listen to how the sounds of another individual are modified by the environment. Targets such as concentrations of fish with resonant swim bladders may absorb more energy than they scatter, leading to significant advantages for bistatic sonar in a forward propagation mode. These examples blend features typically associated with the domains of sonar and communication. I suggest that auditory and vocal skills evolved to function in one of these domains may preadapt animals for developing abilities in the other domain. Vocal learning, in particular, is required for many forms of sonar, and it also enables the evolution of very different communication systems than are possible when vocal output is unaffected by auditory input.