Welcome back! This week, we dive into the underwater world as perceived by vibrissae yet again, but this time we will be discussing pinnipeds: seals, sea lions, and walruses. California sea lions are able to discriminate differences in sizes to a very small scale, but they can not detect differences quite as well as manatees (Denhardt 1994). Some interesting statistics are that seals have 10x as many myelinated nerve fibers of the deep vibrissal nerve than cats, and otters have 4x as many as cats, showing a trend from most to least in aquatic mammals, semi-aquatic mammals, and terrestrial mammals (Hyvarinen et al. 2009). Another difference from manatees is that pinnipeds do not move their vibrissae; instead they sweep their whole heads laterally, which is more like the whisking motion we talked about a few weeks ago in rodents (Dehnhardt 1994). They are careful not to touch objects with their muzzles while sensing with their whiskers (Dehnhardt 1994). They do, however, use their whiskers in social displays (Hyvarinen et al. 2009). Generally, they use their longest whiskers to locate an object, but then switch to more anterior, shorter whiskers to identify objects (Dehnhardt 1994). Pinnipeds synergistically combine tactile information acquired from their vibrissae with kinaesthesis, which is the sense of the location of your own body, to perceive size differences between objects (Dehnhardt 1994). Pinniped vibrissae are more greatly innervated than those of any terrestrial mammal vibrissae (Dehnhardt 1994). As expected, the somatosensory cortex is characterized by a strong central representation of these vibrissae (Dehnhardt 1994). Pinniped follicle sinus complexes contain several kinds of mechanical receptors, some of which adapt quickly and others of which are slow-adapting so that vibrations and static stimuli can be detected (Dehnhardt 1994).
One similarity to cats is that the rate of impulses in the slow-adapting afferent (going toward the brain) fibres of the trigeminal nerve rises with increased angle of whisker deflection (Dehnhardt 1994). Otherwise, pinniped vibrissae are pretty different. Pinnipeds shed their pelage annually, but the whiskers are shed and regrown continuously so they are never without their main sensory system (Ginter et al. 2012). Their vibrissae are the largest among mammals, with Antarctic fur seals with whiskers as long as 48 cm, and some seals even have beaded vibrissae rather than smooth (Ginter et al. 2012). The shortest vibrissae of seals with beaded whiskers are smooth, and it is unknown when in development the beaded morphology appears (Ginter et al. 2012). The evolution of the vibrissal sense has been called the “most important adaptation of the sensory system to the aquatic environment,” (Hyvarinen et al. 2009). The beaded shape reduces vibrations while swimming and can be used to detect speed while on deep dives (Ginter et al. 2012). Some seals may have evolved this shape because they dive to greater depths than sea lions, so they must hunt with less ambient light (Ginter et al. 2012). Blindfolded harbor seals with beaded vibrissae are much more accurate at following the path of a swimming fish than blindfolded California sea lions, suggesting that seals rely more heavily on their vibrissae for foraging (Ginter et al. 2012). Gray seal vibrissae fall somewhere in between beaded and smooth, and this is likely because their head morphology is different from that of other seals, so their whisker morphology may have evolved differently (Ginter et al. 2012). Data show that the whiskers of seals with smooth vibrissae, such as the bearded seal, are different from the smooth whiskers of sea lions; one of these differences is that bearded seal vibrissae are rectangular in cross section, while sea lion whiskers have an oval cross section shape (Ginter et al. 2012). Another notable adaptation to underwater sensation that the ring sinus, the main sensory area at the base of the whisker, is deeper under the skin; this prevents bending of the whisker from water moving past the animal and, along with blubber between the whiskers, maintains a temperature suitable for sensing (Hyvarinen et al. 2009).
Summary: ~The whiskers of seals, sea lions, and walruses are highly adapted to the aquatic environment.
~Some seals have beaded whiskers which are good for reducing vibrations while swimming quickly on deep dives.
~Pinniped whiskers are very long, more highly innervated than any terrestrial mammal’s, and deeply rooted so that the main sensory area is about 1 cm below the surface of the skin for optimum rigidity and temperature.
~Dehnhardt G. 1994). Tactile size discrimination by a California sea lion (Zalophus californianus) using its mystacial vibrissae. J Comp Physiol A. 1994(175): 791-800.
~Ginter CC, DeWitt TJ, Fish FE, Marshall CD. 2012. Fused traditional and geometric morphometrics demonstrate pinniped whisker diversity. Plos one. 7(4): e34481.
~Hyvarinen H, Palviainen A, Strandberg U, Holopainen IJ. 2009. Aquatic environment and differentiation of vibrissae: Comparison of sinus hair systems of ringed seal, otter and pole cat. Brain Behav Evol. 2009(74): 268-279.
Next week, we will wrap up with some ideas for experiments using all we have learned so far!