Movement trajectories reflect active information acquisition by an echolocating porpoise in a target discrimination task

Voluntary movements of echolocating animals with respect to targets in natural foraging or laboratory target discrimination tasks have long been interpreted as a closed-loop sensorimotor feedback driven by information in previously received echoes. However, what can we infer about sensorimotor integration and auditory information-gathering from animal movement trajectories? In this work, we use unsupervised clustering to analyze the movement trajectories of a free-swimming, echolocating harbor porpoise trained to select a sphere against prolate spheroids of varying aspect ratios presented at different angles, and show that the animal's discrimination performance and overall trajectory can be explained by task difficulty based on the similarity of target echo spectra received during its initial approach. The porpoise continued to evaluate its target selection via incoming echoes throughout the trials, and reversed its decision at very close ranges in a subset of trials. In more challenging scenarios, the animal engaged in prolonged, focused ensonification of a single target, sometimes via buzzes, which we interpret as an evidence accumulation process toward decision making. Our findings highlight movement, in addition to acoustic emissions, as a key behavioral readout in the active information acquisition process embodied in echolocation.