The Sensitive Whiskers of Sea Otters
Sea Otters are Hairy Marine Mammals
Sea otters are the smallest and most recent marine mammal to return to the ocean. Only mammals possess hair and sea otters are exceptional in this aspect. The density of body hairs of sea otters is one of the highest for any mammal, up to 150,000 hairs/sq cm. Sea otters use this thick, dense coat of hair to stay warm since they don’t have blubber like other marine mammals. However, sea otters also have a different and exceptional type of hair called vibrissae, commonly known as whiskers. Most mammals have vibrissae, including your pet dog or cat.
What Are Whiskers?
Whiskers are specialized sensory hairs that relay tactile information from the environment to the brain. This is accomplished by movement of the hair-shaft either through direct (active) touch with an object, or by movement from air or water. Whiskers differ from other types of hairs in their construction. They are comprised of a hair-shaft surrounded by flexible tissue, nerves and several blood-filled sinuses. These structures and the blood-filled sinuses are enclosed within a tough collagen capsule that makes up the hair follicle. Their technical name is “Follicle-Sinus Capsule” or F-SC for short. Each F-SC is supplied by a deep vibrissal nerve. This nerve is comprised of many axons that connect to receptors (called mechanoreceptors) within the inner tissues of the F-SC. Movements of the hair-shaft stimulate the mechnorectors, which then send information (via axons) regarding its movement to the brain where sensation is received. The sensitivity of each F-SC can be estimated by counting the number of these axons.
Whiskers of Marine Mammals Are Different
The whiskers of marine mammals are quite different from terrestrial mammals. Seals, sea lions and walruses possess the largest whiskers of any mammal. Whiskers of terrestrial mammals have two blood sinuses and are supplied by ~100-200 axons. In contrast, the whiskers of seals have three blood sinuses, are longer in length, and are supplied by roughly 1350 axons. Seals have ~7-10 Xs more axons per whisker than terrestrial species. Seals use their vibrissae for either “hydrodynamic trail following” or “active touch”. “Hydrodynamic trail following” is the ability to detect and follow a trail of turbulent water, such as those formed in the wake of a swimming fish, using whiskers alone. Harbor seals and California sea lions, and perhaps others, have this ability. “Active touch” sensation is used by many seals, but is enhanced in benthic foraging specialists (feeding on the sea floor) such as bearded seals and walruses. Bearded seals and walruses use their senstive whiskers to detect prey as they dig into the seafloor where vision is not an available sense.
Sea Otter Whiskers
Like bearded seals and walrues, sea otters are also benthic foragers that use “active touch.” Although they are known for using their hands to collect food, their whiskers are also important. But we didn’t know how important until recently. Since sea otters are closely related to terrestrial mammals such as weasles (mustelids), we asked, “Do sea otters have whiskers that are more like terrestrial mammals, more like seals, or have they evolved something different entirely?” The close relationship of sea otters to terrestrial mammals, and the recent re-invasion of the ocean by sea otters suggests that their whiskers are more like terrestrial mammals. Alternatively, whiskers with greater sensitivity (like those of seals) would be advantageous for benthic foraging, where the sense of touch is supreme. Our recent study, published in Frontiers in Neuroanatomy, show that sea otters have an average 120 whiskers on their muzzle, which is similar to most seals. The internal anatomy of their whiskers is identical to seals and sea lions; they have the three blood sinus system, elastic tissues and lots of nerves. Their whiskers are supplied by ~1340 axons, about the same as found in several seals. This is quite surprising since the number of axons for river otters, a closely related but semiaquatic otter, is ~500 axons per whisker. This work shows that the array of whiskers on the sea otter’s muzzle are equally important for benthic foraging identifying prey items. It is likely that the sensitivity level of sea otter whiskers is similar to that of seals and sea lions. Quite a feat for our smallest and most recent marine mammal.
Christopher D. Marshall, Ph.D.
Departments of Marine Biology, and Wildlife & Fisheries Sciences
Texas A&M University
Dr. Marshall is the Principal Investigator of the Ecomorphology & Comparative Physiology Laboratory at Texas A&M University – Galveston. His work focuses on integrating morphology, physiology, and behavioral performance of feeding in marine vertebrates to provide information regarding an organism’s foraging ecology, and their conservation. This work includes investigations of natural history and comparative neurobiology of sensory systems of marine vertebrates.
Follow Dr. Marshall’s work on Twitter: @EcoMorphLab
Abstract: Sea otters (Enhydra lutris) are the most recent group of mammals to return to the sea, and may exemplify divergent somatosensory tactile systems among mammals. Therefore, we quantified the mystacial vibrissal array of sea otters and histologically processed follicle-sinus complexes (F – SCs) to test the hypotheses that the number of myelinated axons per F – SC is greater than that found for terrestrial mammalian vibrissae and that their organization and microstructure converge with those of pinniped vibrissae. A mean of 120.5 vibrissae were arranged rostrally on a broad, blunt muzzle in 7–8 rows and 9–13 columns. The F-SCs of sea otters are tripartite in their organization and similar in microstructure to pinnipeds rather than terrestrial species. Each F-SC was innervated by a mean 1339 ± 408.3 axons. Innervation to the entire mystacial vibrissal array was estimated at 161,313 axons. Our data support the hypothesis that the disproportionate expansion of the coronal gyrus in somatosensory cortex of sea otters is related to the high innervation investment of the mystacial vibrissal array, and that quantifying innervation investment is a good proxy for tactile sensitivity. We predict that the tactile performance of sea otter mystacial vibrissae is comparable to that of harbor seals, sea lions and walruses.