Under consideration for publication in J. Fluid Mech. 1 Wake-induced ‘slaloming’ response explains exquisite sensitivity of seal whisker-like sensors Heather R. Beem 1,2 †, Michael S. Triantafyllou 1 1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA 2 Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA (Received ?; revised ?; accepted ?. - To be entered by editorial office) Blindfolded harbor seals are able to use their uniquely shaped whiskers to track vortex wakes left by moving animals and identify objects that passed by 30 seconds earlier, an impressive feat as the flow features have velocities as low as 1 mm/s. 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 geometry as those of the harbor 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 some similarities with the flow mechanisms observed in actively controlled propulsive foils within upstream wakes and trout swimming behind bluff cylinders in a stream, but there are also differences caused by the unique whisker morphology, which enables it to respond passively and within a much wider parametric range. Key words: 1. Introduction Behavioral experiments with harbor seals have demonstrated their outstanding abil- ity to detect and track hydrodynamic signatures left by swimming animals and moving objects (Dehnhardt et al. 2001; Schulte-Pelkum et al. 2007; Wieskotten et al. 2010a ,b ): figure 1. Despite having their auditory and visual sensory cues blocked, the seals suc- cessfully followed the paths of bodies, which had swum ahead of them by 30 seconds or longer. Detection thresholds were measured on the vibrissae (whiskers) of a stationary seal responding to a dipole stimuli in water (Dehnhardt et al. 1998) and reported to be sensitive enough to detect the minute movements left in these hydrodynamic trails. Various animals utilize similar hair-like structures as direct touch sensors (Barth 2004; Solomon & Hartmann 2006; Grant et al. 2013). For aquatic animals that swim forward at † Email address for correspondence: beem@mit.edu