Abstract:

The components of the dolphin's biosonar are well studied, but their combination and dynamic adjustment during echolocation in open waters are poorly understood. A lack of adequate measurement and research techniques has delayed progress, but new technology and methods now allow for the continuous recording of the animal's interpulse-interval, head-attitude, azimuth, and pulse waveform during search and detection in very shallow open water. Such measurements were made in controlled tests with bottom objects in random positions on random sand backgrounds. The data provided a 3-D description of the variation in biosonar search behavior as a function of object position and practice effects. Basic findings included the adaptation of the search pattern to the spatial distribution of previously detected objects, correlations between the animal's orientation, pulse rate, and object location, as well as an inverse relation between the number of pulses on object and the object's distance. Steady pulse rates on nearby objects were also observed with only half of the ``over-two-way'' interval previously reported. Coincident with these dynamic adjustments, the pulse spectra displayed considerable variation with detected objects, changing distance and grazing angles. The spectral data also indicate a dual-frequency source.