Seal and Sea lion Whiskers Detect Slips of Vortices Similar as Rats Sense Textures

Abstract

Pinnipeds like seals and sea lions use their whiskers to hunt their prey in dark and turbid situations. There is currently no theoretical model or hypothesis to explain the interaction between whiskers and hydrodynamic fish trails. The current study, however, provides a theoretical and experimental insight into the mechanism behind the detection of the Strouhal frequency from a Von-Karman vortex street, similar to that of the inverted hydrodynamic fish trail. Herein the flow around a 3D printed sea lion head, with integrated whiskers of comparable geometry and material properties to a real seal lion, is investigated when exposed to vortex streets generated by cylindrical bluff bodies. The whiskers respond to the vortices with a jerky motion, analogous to the stick-slip response of rat whiskers; this motion is found to be the time derivative of the Gaussian function. Compared to the displacement response, the time-derivative of the whisker response decodes the Strouhal frequency of the Von-Karman wake, which improves the sensing efficiency in noisy environments. The study hypothesizes that the time derivative of the whisker bending moment is the best physical variable that can be used as the input to the pinnipeds neural system.

A Deep-Learning Model for Underwater Position Sensing of a Wake’s Source Using Artificial Seal Whiskers

“…for different locations of the cylinder along the x -axis and for a reference case of the free flow with no disturbance. In the reference case, with no cylinder placed upstream, the tip location varies slightly from its nominal position due to the self-induced vibration of the artificial whisker in the presence of the flow and the surface wave of the open channel [ 28 ]. After inserting the cylinder, the vortex street in its wake interacts with the whisker and causes a noticeable large-scale variation of both the mean tip deflection and the amplitude of the deflection’s fluctuations around the mean.…”

“…All of the fibers have the same diameter of 0.75 mm that is constant over their lengths. We capture the motion of 12 artificial whiskers that have the same length and were previously shown in [ 28 ] to have a similar response to real whiskers.…”

“…By fluid-whisker interaction, the seal is able to detect the direction in which it needs to swim to track the swimming body [ 24 ]. The wake of a cylinder ( Re ∈ [4000, 6000]), which mimics the trail generated by a prey (fish), was reported to induce time-variant deflection responses of artificial seal-like whiskers [ 28 ]. With the whiskers distributed over both sides of the frontal area of the pinniped’s head, the generated wake affects each whisker differently, depending on its adjacent local flow structure.…”

“…We believe that, by comparing the vibrations of different whiskers, information about the location of the wake’s source can be deduced. The whiskers’ vibrations in [ 28 ] were measured by tracking the whisker tips’ motion while using a camera. Other embedded techniques, such as the fiber-Bragg-grating [ 29 , 30 ] and strain gauges, can also be used to record the bending responses of such whisker beams.…”

“…These results inspired Alvarado et al to propose a whisker-type sensor design that could be tuned in order to amplify certain hydrodynamic signals and suppress others (e.g., noise) [ 38 ]. Recently, experimental studies were carried out in our group for underwater sensing using arrays of whiskers, where it was proven that the deflection signals of artificial whiskers can decode the specific shedding frequency of a Kármán vortex street [ 28 ]. The studies also highlighted the importance of the vortex-induced pressure-gradients as a major contribution to the jerky response of the whiskers.…”