Credit: Image courtesy of Andre Seale, University of Hawaii.

Like bats — and submarines — dolphins use echolocation to investigate their environment. Unlike bats and submarines, though, how dolphins use this specialized sensory system is not well understood. A study of dolphins in the wild now reveals that they use a unique gain control system.

One problem faced by any echolocating organism is loss of the signal over distance. An echo from a distant prey animal will be much fainter than an echo from the same animal when it is very close. Bats overcome this problem using a system of auditory gain control — a specialized muscle in their middle ear contracts when they emit an echolocation call, which reduces the sensitivity of the auditory system and prevents the animal from being deafened by the call. Then, as the time from the call increases, the muscle relaxes and the ear gradually becomes more sensitive. The system is calibrated so that the amplitude of the echo signal remains constant regardless of how close the target is, within a certain range. Sonar systems engineered by humans use a similar principle.

Au and Benoit-Bird find that echolocating dolphins also use gain control to maintain a consistent signal — but rather than adjusting their receivers, they vary the strength of their calls with the distance to the target. The net effect is the same — the calls are precisely calibrated so that the returning signal has the same strength regardless of the range to the target.

So dolphins and bats, faced with the same problem in their unusual sensory system, have evolved two very different mechanisms that produce the same outcome. Bats use a neural mechanism that couples muscle contraction with the echolocation call; however, it seems that the dolphin's gain control is a natural result of their system of sound production. As they get closer to a target and the echoes are returned more rapidly, dolphins produce echolocation clicks more quickly; and because of the way the clicks are produced (which relies on pressurization of the nasal system) the amplitude of the clicks will naturally fall as the repetition rate increases. What at first seems like a physiological limitation has been turned into an efficient gain control system.