Rotation-independent discrimination of object shape by echolocating bottlenose dolphins (<i>Tursiops truncatus</i>)

Previous research has examined the perception of aspect-dependent objects by echolocating bats and dolphins; however, the importance of perceived spatial features as opposed to acoustic cues in discriminating and recognizing targets is not fully understood. In this study, two bottlenose dolphins (Tursiops truncatus) learned a two-alternative forced choice task requiring discrimination of a two-cylinder “dipole” target with 12-cm center-to-center spacing (S + ) from one of three S− targets: a “monopole” target, or one of two dipoles with 6- or 18-cm spacing. Targets rotated in the horizontal plane on a trial-to-trial basis so that echo timing and azimuth changed depending on the dipoles’ orientations relative to the dolphin. Target cylinders were identical in composition and size so that acoustic cues other than the echo timing and azimuthal separation were not available, and random positional roving reduced the possibility of any single acoustic feature indicating the S+. After achieving consistently high performance with a limited set of target rotations, testing was repeated using novel rotation angles. Results—including first trial data with novel rotations—support dolphins forming a distal, spatial representation of target shape instead of relying solely on proximal, auditory cues, a capability likely necessary in cluttered aquatic environments. [Work funded by ONR.]