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Comparative Study
. 2011 Nov 12;366(1581):3077-84.
doi: 10.1098/rstb.2011.0155.

Flow sensing by pinniped whiskers

L Miersch  1 , W Hanke, S Wieskotten, F D Hanke, J Oeffner, A Leder, M Brede, M Witte, G Dehnhardt
Affiliations
Comparative Study

Flow sensing by pinniped whiskers

L Miersch et al. Philos Trans R Soc Lond B Biol Sci. .

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.

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Figures

Figure 1.
Figure 1.
(a) Photographic presentation of the structure of vibrissal hair shafts of the harbour seal (P. vitulina) and the California sea lion (Z. californianus). The vibrissae are presented in two views perpendicular to each other: top two, seal; bottom two, sea lion. The seal vibrissa is flattened and possesses an undulated shape. The sea lion vibrissa is oval and smooth in shape without undulation. (b) The basic frequency (in Hz) of whisker hairs in flow as a function of flow speed (in cm s–1). (c) The amplitudes of the dominant basic frequencies (in V, logarithmic scale) of the whiskers in flow as a function of flow speed (in cm s–1). Most signals of the seal whiskers possess amplitudes below 3 mV. For (b) and (c), black dots represent the data of the seal whiskers and grey dots the data of the sea lion whiskers. Each point reflects the mean value of 20 single measurements.
Figure 2.
Figure 2.
Vortex shedding frequencies (fVS in Hz) of the Kármán vortex streets generated by a cylinder theoretically predicted for as well as measured by (a) seal whiskers (PvV1, PvV2, PvV3) and (b) sea lion whiskers (ZcV1, ZcV2, ZcV3). Black bars represent the frequencies determined from the vibrissal signal. Grey bars show the expected values calculated by means of flow velocity and cylinder diameter (16 mm). Above each value pair, the flow velocity (in cm s−1) at which the data were obtained and which was used for the calculation is indicated.
Figure 3.
Figure 3.
Signal-to-noise ratios (in dB) of the whiskers while detecting the vortex-shedding frequency fVS of the generated Kármán vortex street as a function of the flow speed (in cm s–1). Conventions as in figure 1.
Figure 4.
Figure 4.
Additional measurements of a California sea lion whisker assuming a modulated carrier signal. All results were obtained from the sea lion whisker ZcV2. (a) Amplitude (in mV) of the signal including the carrier for one exemplary measurement at a flow speed of 53 cm s–1. (b) Frequency distribution obtained by spectral analysis of the measurement illustrated in (a) with a peak frequency at 114 Hz. (c) Signal-to-noise ratios (in dB) with the carrier frequency defined as the wanted signal.
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