Auditory Perception of Submerged Objects by Porpoises

Abstract

Two captive porpoises (Tursiops truncatus) were given a variety of tests for their ability to perceive and avoid objects in water by reflected sound signals. The tests were made in a specially constructed anechoic pool in which visual perception was eliminated by the turbidity of the water. The results showed that trains of underwater sound pulses were emitted periodically by the animals in this situation whether they were swimming or at rest. The signals increased when reflecting targets were lowered into the water. Sound pulses could also be triggered by surface splashes, but were not sustained after a splash if no target was present. These sonar noises partially resembled man-made sonar, but differed from it in several important characteristics. The porpoises would locate and approach food fish by this method but would avoid nonedible objects. They could also swim accurately through a field of obstructions without collisions[long dash]even in darkness. The ability to avoid obstacles by sound did not appear to be affected by artificial jamming noises.