Whiskers are tactile sensory hairs found in almost all mammals. They are typically longer and stiffer than normal body hairs and grow outward in an ordered grid-like arrangement. Each whisker is connected to many sensory nerve cells at its base beneath the skin. These nerve cells can detect small deflections in the whisker as it physically interacts with its surroundings, relaying this information to the brain. The harbor seal, and other aquatic mammals, can even sense and analyze changes in water flow caused by prey fish or other seals. With whiskers that are sensitive to displacements of 1μm or less, harbor seals need some way to reduce whisker vibrations that can occur as the hairs move through water. Remarkably, harbor seals accomplish this with the unique form of their sensitive whiskers.
Taking a closer look at harbor seal whiskers shows that they have an undulating or wavy surface structure. The cross section of the whisker is an ellipse, but the size of it changes along the hair. This creates peaks and troughs every 1-3 mm along the hair. Normally, dragging a bluff object like a whisker through water creates vortices, or swirls of water, that would vibrate the whisker as they trailed off behind it. However, the shape of the whisker on its leading edge alters the flow of water over and behind the whisker as the seal swims. The whisker’s wavy leading edge creates a trail of swirling water behind the whisker (also called the wake) with equal pressure on either side and a gap between the whisker and vortices. This significantly lowers forces on the whisker and thus prevents vortex-induced vibrations. Having the whiskers as still as possible, even when swimming, enables harbor seals to sense small changes in water movement. This gives them the ability to search for and sense prey fish, other seals, or predators.
Check out this related strategy explaining how harbor seal whiskers are tuned to detect water movements generated by their fish prey.
This summary was contributed by Leon Wang (Research Assistant ASU Biomimicry Center).
“Harbor seals (Phoca vitulina L.) use their mystacial vibrissae to identify objects by active touch and for sensing water movements caused by prey fish or by other seals (Dehnhardt and Kaminski, 1995; Dehnhardt et al., 1998a; Dehnhardt et al., 2001; Hanke and Bleckmann, 2004; Hanke et al., 2000; Schulte-Pelkum et al., 2007). They are able to follow the path of these water disturbances over a distance that can by far surpass the range of vision or hearing (hydrodynamic trail following). While swimming and scanning the water for flow patterns, harbor seals keep their whiskers in an abducted position, largely perpendicular to the swimming direction. However, the flexible vibrissae bend easily, and the mechanoreceptors at the follicle are sensitive to displacements of the hair of 1 micron or less at their best frequencies (Dehnhardt et al., 1998a). These findings raised the question concerning how harbor seals cope with the flow resistance on their vibrissae and with vortex induced vibrations (VIVs) – that is, the oscillations due to vortex shedding that occur when an object is dragged through the water.” (Hanke et al. 2010:2665)
“Analyzing the 3-D structure of the wake of the vibrissa (Fig.5, right), we find that the separation of primary vortices (Kármán vortices) is replaced by a complex 3-D vortex structure in which various states of vortex separation occur simultaneously on different locations in a spanwise direction. Furthermore, the region of vortex formation is considerably shifted downstream compared with the circular or elliptic cylinder wake, thus opening a gap between the vibrissa surface and the region with fluctuating flow (Figs 5, 6). Therefore, the resulting pressure imposed on the complete vibrissa is more symmetric compared with that on a circular or an elliptic cylinder. The three features together – the reduction of the primary vortex separation, the gap between the vibrissa and the first vortices and the symmetry of the pressure field – prevent large periodic forces on the vibrissa originating from its own wake and thus prevent vortex-induced vibrations. As a result, lift forces on the vibrissa calculated from CFD data are reduced by more than 90% (Fig.7), and the mean drag force is reduced by approximately 40% (Fig.8) compared with those on a circular cylinder with an identical hydraulic diameter.” (Hanke et al. 2010:2669)
“Here we show that harbor seal vibrissae have evolved a structure that largely reduces vortex induced vibrations. We conclude that the seals, by reducing these vibrations, keep their vibrissae as still as possible while searching for, or detecting, relevant hydrodynamic signals and thus enhance the signal-to-noise ratio (cf. Fish et al., 2008). A similar vibrissal structure is present in 15 out of the 18 species of true seals (Phocidae), demonstrating the high adaptive value of the effects described here.” (Hanke et al. 2010:2670)
“The structure of the harbor seal vibrissa demonstrates a highly efficient mechanism for the suppression of forces owing to vortex shedding from rod-like objects with drag reduction. Numerous applications in biomimetic designs are conceivable, including flow
sensors for underwater vehicles or parts of buildings and offshore facilities with reduced vibrations from wind or water flow. Inspiration from nature has so far been lacking in this field of research. Particularly challenging is the suppression of vortex induced vibrations in flexible structures with low mass (Owen et al., 2001), as observed here.” (Hanke et al. 2010:2671)