Flow sensing by pinniped whiskers

Abstract

Beside their haptic function, vibrissae of harbour seals (Phocidae) and California sea lions (Otariidae) both represent highly sensitive hydrodynamic receptor systems, although their vibrissal hair shafts differ considerably in structure. To quantify the sensory performance of both hair types, isolated single whiskers were used to measure vortex shedding frequencies produced in the wake of a cylinder immersed in a rotational flow tank. These measurements revealed that both whisker types were able to detect the vortex shedding frequency but differed considerably with respect to the signal-to-noise ratio (SNR). While the signal detected by sea lion whiskers was substantially corrupted by noise, harbour seal whiskers showed a higher SNR with largely reduced noise. However, further analysis revealed that in sea lion whiskers, each noise signal contained a dominant frequency suggested to function as a characteristic carrier signal. While in harbour seal whiskers the unique surface structure explains its high sensitivity, this more or less steady fundamental frequency might represent the mechanism underlying hydrodynamic reception in the fast swimming sea lion by being modulated in response to hydrodynamic stimuli impinging on the hair.

Somatosensation, Echolocation, and Underwater Sniffing: Adaptations Allow Mammals Without Traditional Olfactory Capabilities to Forage for Food Underwater

“…For both types of whiskers the researchers cal- culated a signal-to-noise ratio (SNR), or ratio of the power between 'wanted' signals and background noise. Results showed that while both whisker types responded accurately to vortices in the water tank, as supported by the aforementioned studies of these species, harbor seal whiskers significantly reduced background noise compared to California sea lion whiskers (Miersch et al, 2011). Walruses are the other members of the pinnipeds, but the role of their whiskers is largely unexplored.…”

“…Miersch et al (2011) isolated single whiskers of both animals and tested the whiskers' response to underwater wakes. For both types of whiskers the researchers cal- culated a signal-to-noise ratio (SNR), or ratio of the power between 'wanted' signals and background noise.…”

Active touch sensing

“…The next article on vibrissal touch by Miersch et al [45] switches the focus to the pinnipeds-seals and sea lions-which, if mechanoreceptor density is a good indicator, have the most sensitive vibrissae of all mammals. In addition to using their whiskers to investigate objects directly, several pinniped species have been shown to use their whiskers to detect the hydrodynamic trails left by swimming fish.…”

Passive wake detection using seal whisker-inspired sensing

“…Since then, a few other parties have carried out studies on the vibration properties of seal whiskers on both real and model specimens [91,143,95]. The important thing to note is that none of the experiments that provided quantitative results were conducted under conditions that allowed the whisker to vibrate freely in response to the flow.…”

“…In the few studies conducted with interfering cylinders of unequal diameter [115,82,40,41,39], it was found that the upstream cylinder also affects the response of the downstream cylinder, reducing its amplitude relative to the single cylinder case, while the frequency of oscillation of the downstream cylinder was found to be either equal to, or twice the value of the frequency of the upstream cylinder. Only one published study [91] has begun to show the harbor seal whiskers' response under WIV conditions.…”