Comparative Analysis of the Flexural Stiffness of Pinniped Vibrissae

Abstract

Vibrissae are important components of the mammalian tactile sensory system and are used to detect vibrotactile stimuli in the environment. Pinnipeds have the largest and most highly innervated vibrissae among mammals, and the hair shafts function as a biomechanical filter spanning the environmental stimuli and the neural mechanoreceptors deep in the follicle-sinus complex. Therefore, the material properties of these structures are critical in transferring vibrotactile information to the peripheral nervous system. Vibrissae were tested as cantilever beams and their flexural stiffness (EI) was measured to test the hypotheses that the shape of beaded vibrissae reduces EI and that vibrissae are anisotropic. EI was measured at two locations on each vibrissa, 25% and 50% of the overall length, and at two orientations to the point force. EI differed in orientations that were normal to each other, indicating a functional anisotropy. Since vibrissae taper from base to tip, the second moment of area (I) was lower at 50% than 25% of total length. The anterior orientation exhibited greater EI values at both locations compared to the dorsal orientation for all species. Smooth vibrissae were generally stiffer than beaded vibrissae. The profiles of beaded vibrissae are known to decrease the amplitude of vibrations when protruded into a flow field. The lower EI values of beaded vibrissae, along with the reduced vibrations, may function to enhance the sensitivity of mechanoreceptors to detection of small changes in flow from swimming prey by increasing the signal to noise ratio. This study builds upon previous morphological and hydrodynamic analyses of vibrissae and is the first comparative study of the mechanical properties of pinniped vibrissae.

Follicle Microstructure and Innervation Vary between Pinniped Micro- and Macrovibrissae

“…In addition, F-SCs should be more sensitive to water flow disturbances at a 90° angle (i.e. dorsoventrally) [Murphy et al, 2013;Ginter-Summarell et al, 2015]. These vibrissal dynamics match well with the asymmetrical innervation patterns reported in our study if axon distribution can be considered a general proxy for mechanoreceptor distribution.…”

“…The relationship between axons and mechanoreceptors is highly complex and further data are required to validate our axon-to-mechanoreceptor hypothesis. However, if we assume that axon distribution serves as a general reference point for where mechanoreceptors are located within the F-SC, our results support both the flexural stiffness and the vibration velocity hypotheses of Ginter-Summarell et al [2015] and Murphy et al [2013] since F-SCs with elliptical HSs had more pronounced axon bundles dorsally (i.e. ∼ 90° angle).…”

“…This feature may also be dependent on muscle attachment or a number of other factors, including flexural stiffness, HS morphology, the vibrissal angle of orientation, or HS curvature, none of which may be mutually exclusive. Based on current data, when vibrissae are protracted underwater, water flow initially contacts the narrow edge of the elliptical HS and flows back across the major axis of the HS [Hanke et al, 2010;Murphy et al, 2013;Ginter-Summarell et al, 2015]. In this 0° orientation, with the major HS axis oriented rostral to caudal, vibrissae exhibit their greatest flexural stiffness and possibly their lowest vibration velocity [Hanke et al, 2010;Murphy et al, 2013;Ginter-Summarell et al, 2015].…”

“…Harp seal lateral vibrissal HSs, like most pinniped HSs, are elliptical [exceptions exist, e.g. Weddell (Leptonychotes weddellii) and Ross seals; Ling, 1972;Marshall et al, 2006;Hanke et al, 2010;Ginter-Summarell et al, 2015;McGovern et al, 2015] and likely serve similar functions. However, our more circular, medial harp seal HSs may not be as efficient n = Number of cross-sections (n = 146 in total).…”

“…Among pinniped species, vibrissae vary in number, geometric arrangement, size, shape, flexural stiffness, and innervation [Ling, 1977;Marshall et al, 2006;Ginter et al, 2010Ginter et al, , 2012Hanke et al, 2013;Ginter-Summarell et al, 2015]. However, two primary vibrissal groups exist: micro-and macrovibrissae, but these groups have yet to be defined for pinnipeds.…”