Experimental evidence for ultra-high frequencies in dolphin sonar signals

The presence of high frequency components in dolphin echolocation signals and their possible significance in dolphin sonar is reported here, based on echolocation experiments conducted with several bottlenosed dolphins. Measurements on the animals' sonar capability under varied conditions, including imposed difficulties for target detection are discussed. Blindfolded dolphin test subjects were utilized in echo-ranging experiments at a fixed station as well as in free-swimming modes. The experiments involved detection of small targets at ranges measured in meters, in a presence or absence paradigm, where the animals were rewarded for correct decisions as to the target's presence or absence. Measurements on the time domain and power spectra of the biosonar transmissions, as well as angular response in terms of beampatterns were made. Results show that the animals changed their sonar parameters toward more intense, higher resolution capability when the difficulty of target detection was increased. The increase of detection difficulty was effected through the use of absorbing panels and the insertion of band limited noise in the dolphin sonar scenario. It was found that the animals increased their sonar transmit levels, their pulse repetition frequency, and high frequency content of their transmitted signals, which were contained in increasingly narrow sonar beams. The resulting parameter adjustments are analyzed in terms of the increased sonar difficulties imposed. The results are interpreted in terms of potential significance in man-made sonar architecture.