Hanke, Wolf, and Guido Dehnhardt. “Vibrissal touch in pinnipeds.” Scholarpedia of Touch. Atlantis Press, 2016. 125-139. doi:10.4249/scholarpedia.6828
For mammals without fingertips, there are whiskers (vibrissae). You know, those adorable little hairs at the end of cat and dog snouts? Whiskers have a much more important role than just aesthetics though; at their bases there are receptors that make them great for feeling. Little information is available about the neurophysiological system of whiskers, especially for pinnipeds, flipper-footed marine mammals like sea lions, seals, and walruses. Part of this limitation is attributed to the difficulties of observing whisker use in the natural environment, i.e. underwater.
Observations so far have taught scientists that whiskers on pinnipeds are above the eyes, above the snout, and on true seal species, they are also above the nostrils (Figure 1). They count between 15-350 per side depending on the species; table 1 is basic summary of the number of whiskers on different pinnipeds.
Whiskers are anchored in a nerve heavy housing called the follicle-sinus complex (F-CS; see figure 2 in the original article for a well labeled diagram). The F-CS is a skin capsule with blood filled sinuses and a hair follicle. The sinuses supply nutrients and influence the whiskers ability to sense.
FC-Ss in pinnipeds are unique from other mammals, with the exception of the sea otter, because they have three sinuses: a ring sinus, a lower cavernous sinus, and an upper cavernous sinus. The ring sinus, and the lower cavernous sinus are common; the upper cavernous sinus is not well studied but is believed to protect sensory receptors from cooling by surround water.
Additional unique features of FS-C structures in pinnipeds are that ten times more nerve fibers are found in the deep vibrissal nerve than in terrestrial mammals, and the cross sectional shape of the hair shaft in pinnipeds is oval while in terrestrial mammals it’s round. The shape of the long cross section is variable between pinnipeds, either smooth and waved (undulated), depending on the species (Table 1; Figure 2).
Sinuses and cross sectional shapes both play an important role in whisker sensitivity because they influence elastic deformation. For harbor seals the unique undulated cross sectional shape decreases the vibrations brought on by swimming and causes the elasticity of the whiskers to decrease when submerged in water, both resulting in enhanced sensory receptions in the FC-S. Although neural responses in all pinnipeds were observed regardless of position, length, or cross section of the whisker, which suggests that they are all important for sensing.
Apart from social interaction on land, whiskers are typically reserved for underwater use so observations are difficult to make. Previous work concluded that evident from a lack of a developed biosonar (sensing by sound- like bats and dolphins) physiology and the presence of whiskers suggest that they are a vital part of sensing.
Direct touch experiments have been used to investigate whisker ability further. The experiment was designed so that at the sound of a buzzer a trained, blindfolded sea lion started to investigate objects. From this experiment scientists learned that pinnipeds explore an object by moving their heads while the whiskers are in contact with said object. Similar experiments have been used to observe pinnipeds distinguish between shapes and surface structures. They appear to distinguish by size based on the number of whisker in contact with an object.
Hydrodynamic experiments were used to access the whiskers ability to sense movement in the water column. In these experiments a trained, blindfolded seal followed a remote controlled submarine. The seal was able to track the trail if either it started at the same place as the submarine, or if the seal intercepted the submarines trail. Harbor seals are particularly sensitive to hydrodynamic vibrations because of their undulated whisker shape. They are able to detect between differences in vibrations depending on the size, shape, direction, and movement of their sources. They use this type of sensing to feed, follow their young, and to center in air holes in iced regions (Figure 3).
A hot feature about the F-CS is that they generate abundant heat so ambient temperatures do not influence sensation, i.e. when the water temperature is very cold, the whiskers sensitivity does not change. This is important for seals living in cold water because it enables them to hunt just as well as seals in warmer waters. Pinnipeds with undulated whiskers (like some true seals) tend to live in low visibility temperate and arctic waters (because they are able to hunt in low visibilty), and pinnipeds with smooth whiskers are either benthic feeders or live in clear subtropical/tropical waters (Figure 4). It is noted that in bottom feeding pinnipeds, like bearded seals and walrus, tend to have a greater number of whiskers over all, an observation attributed to the need for enhanced touch sensation.
All in all you may get the feeling that whiskers are important to pinniped survival. They are used to feel out their surrounding and to hunt. Future research on the neurological response to whisker sensitivity will help scientists understand pinniped behavior.