Bottlenose dolphins direct sonar clicks off-axis of targets to maximize Fisher Information about target bearing

Abstract

Aiming the sonar beam directly at a potential target maximizes the detection information available to an echolocating animal. In contrast, estimating the angular location of a target relies on changes in the received echo spectrum due to the frequency dependence of the transmit beam pattern. The Fisher Information quantifies the available information on angular location in terms of the sensitivity of the received echo spectrum as a function of bearing. We calculated the Fisher Information for a dolphin's echolocation signal and determined the maximum angular location information occurs when the sonar beam is such that the target falls slightly off-axis. To compare the predicted beam aim to the actual beam aim of an echolocating dolphin, we recorded the echolocation signals of a bottlenose dolphin with a 16-element hydrophone array while the animal performed a target detection task. The dolphin consistently pointed its sonar beam 7 degrees away from the target, which is the beam aim that maximizes the Fisher Information for the dolphin signal. [Work supported by NSF and ONR.]