Learning from Freja the harbor porpoise to appreciate movements as an important behavioral readout to understand echolocation-based target discrimination

Abstract

Toothed whales routinely discriminate and select prey via echolocation. Over a decade, the harbor porpoise Freja in Fjord and Bælt, Kerteminde, Denmark participated in a series of target discrimination experiments involving metal objects. Different from experiments in which the subjects were constrained to be stationary, Freja’s free and voluntary movements offered insights into the tightly coupled nature of acoustic sampling and movements in echolocation-based target discrimination. Compared with observations from wild animals with unknown prey, the controlled experimental task and stimuli made it possible to interpret her behavior based on predictable target echo features. Here, we test the hypothesis that an echolocating animal plans its movement based on prior echo returns to gather better target discrimination information. We trained Freja to approach and select a sphere against a spheroid in a two-alternative forced-choice discrimination task, and show that her performance and movement trajectory varied depending on the aspect ratio and presentation angle of the spheroid. Consistent with our hypothesis, we further show that Freja’s movement patterns were likely driven by echo information received earlier during the approach that is correlated with the discrimination difficulty. These results highlight movements as an important behavioral readout of the dynamic, closed-loop sensorimotor feedback in echolocation. [Work supported by ONR.]