Wake-induced 'slaloming' response explains exquisite sensitivity of seal whisker-like sensors

Blindfolded harbour seals arc able to use their uniqucly shaped whiskers to track vortex wakes left by moving animals and identify objects that passed by 30 s earlier, an impressive feat, as the flow features have velocities as low as 1 mm s(-1). The seals sense while swimming, hence their whiskers are sensitive enough to detect small-scale changes in the flow, While rejecting self-generated flow noise. Here we identify and illustrate a novel flow mechanism, causing a large-amplitude 'slaloming' whisker response, which allows artificial whiskers with the identical unique undulatory try as those of the harbour seal to detect the features of minute flow fluctuations when placed within wakes. Whereas in open water the whisker responds with very low-amplitude vibration, once within a wake, it oscillates with large amplitude and, importantly, its response frequency coincides with the Strouhal frequency of the upstream cylinder, making the detection of an upstream wake and an estimation of the size and shape of the wake generating body possible. This mechanism has sonic antics with the flow mechanisms observed in actively controlled propulsive foils upstream wakes and trout swimming behind bluff cylinders in a stream, but there are also differences caused by the unique whisker morphology, Which enables respond passively and within a much wider parametric range.