Relationship of Vibrissal Length and Habits in the Sciuridae

One of the most striking facial features of mammals is the presence of vibrissae, also known as whiskers or tactile hairs (the term preferred by Nomina Anatomica Veterinaria, 1983). In fact vibrissae (the term preferred by most authors) are found on almost all mammals except Homo sapiens; some mammals that lack vibrissae as adults develop them prenatally, only to lose them before birth (Cave, 1969). The near universality of vibrissae and the great variety and array in which they appear on the face and body of mammals suggests that they are important in the life of the mammal. Indeed there is great variation in vibrissae numbers among species whereas that within species is reported to be low (Dun and Fraser, 1959). Vibrissae are not merely pelage hairs that have attained greater length and thickness, but are specialized hairs that arise from a sinus hair follicle that is 5-6 times larger (longer and broader) than a pelage follicle. Each mystacial vibrissa (located on the upper lip in ordered rows) is represented by a separate barreloid in the brain stem nuclei and a barrel in the sensory cortex, giving a precise one-to-one mapping of the skin surface in the brain stem and the brain. No other sensory system is known to exhibit the discreteness of this vibrissa-to-barrel path (Van der Loos and Dorfl, 1978). Such exquisite anatomical precision internally demands an assumption of important function externally. In fact, the question seems to be not whether vibrissae are important in an animal's life and behavior, but how they function and what aspects of an animal's life are involved. Vibrissae have been implicated in a variety of functions in different species, including aggressive behavior and fighting (Flory et al., 1965), social dominance (Strozik and Festing, 1981), swimming (Ahl, 1982), protective eyeblink reflex (Fitzgerald, 1940; McGill, 1982; Van Horn, 1970), equilibrium and surface discrimination (Vincent, 1912), depth perception (Schiffman et al., 1970), maze running (Oliver, 1977; Vincent 1912), predatory behavior (Gregoire and Smith, 1975; Kemble and Lewis, 1982; MacDonnell and Flynn, 1966; Renouf, 1980), and others. Extensive reviews of vibrissae and neural function (Gustafson and FelbainKeramidas, 1977) and vibrissae and behavior (Ahl, 1986) are available for further information. However, showing how useful vibrissae are to their possessors is not easy to demonstrate. A direct attempt to test survival value of vibrissae for small rodents in the wild was made by dividing each of two species of mice (Reithrodontomys megalotis and Microtus californicus) into three groups: (1) no vibrissae clipped, (2) vibrissae clipped on one side, and (3) fibrissae clipped on both sides (Pearson, 1962). These mice were tagged, released, and retrapped 3-6 weeks later. Clipping the vibrissae did not affect the probability of a mouse's survival. However, Pearson was unwilling to conclude that vibrissae have no survival value: ". .. it is perhaps safest to follow the course of some others who have sought in vain to gather from nature experimental confirmation of evolutionary theory, and to conclude that vibrissae have survival value ... under ... circumstances other than those prevailing in the present experiment." The possible relationship of mystacial vibrissal length and life habits in mammals has long been suggested in the literature, and includes the idea that vibrissae are more highly developed in active arboreal species than in terrestrial ones (Pocock, 1914). A small vibrissal field with vibrissae extending little beyond the width of the head, has been associated with burrowing species by Kratochvil (1968). Nocturnal species, especially arboreal ones, should have especially well-developed vibrissae as their eyes are less useful than for diurnal species, making the vibrissae important for detecting objects that are near to the face. In order to establish whether mystacial vibrissal length is related to the habit of mammals, I chose to study the Sciuridae. They are similar physiologically, and have diverged into many habitats. Because most species are diurnal, differences in vibrissal development can be related to habit and not to time of activity. Additionally, the flying squirrels are both arboreal and nocturnal, thus providing a partial test of the nocturnal hypothesis. The size of most species is in the small to medium range; this becomes an important consideration because as mammals increase in size, vibrissae cannot increase proportionally for they would soon become brush brooms that would interfere with rather than aid the animal in its activities. In fact, the only very