Structure and Function of Pinniped Vibrissae

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Murphy, Christin Taylor
Pinniped; Sensitivity; Vibrissae; Vibrotactile; Whiskers
thesis / dissertation description
The vibrissal system of pinnipeds relies on sturdy, specialized vibrissae and supporting neural architecture apparently designed for the reception of waterborne disturbances. Although it is known that pinnipeds can use their vibrissae for fine-scale tactile discrimination and hydrodynamic detection, many aspects of vibrissal function remain poorly understood. The present work examined the adaptive significance of vibrissal structure, the sensitivity of the vibrissal system, and the signals received by this system. All of these points were considered with respect to their function in hydrodynamic reception. Four methods of study: laser vibrometry, computed tomography (CT) scanning, psychophysical testing and animal-borne tagging were used to investigate the functioning of this sensory system.Laser vibrometer recordings were used to investigate the effect of vibrissal surface structure and orientation on flow-induced vibrations in excised vibrissae. Vibrations were recorded from the shaft of excised vibrissae exposed to laminar water flow in a flume tank. Samples from three pinniped species were tested: the harbor seal (Phoca vitulina), northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus). The vibrissae of the seals had an undulated surface structure, while the vibrissae of the sea lion had a smooth surface. No significant difference between species, and therefore surface structure, was observed. However, when vibrissae were tested at three angles of orientation to the water flow, a strong effect of orientation on vibration frequency and velocity was observed across species. CT scanning data revealed that the vibrissae of all the species tested had flattened cross-sectional profiles. This cross-sectional flattening could account for the observed orientation effects. Furthermore, this morphological characteristic may represent an adaptation for improved functioning in the aquatic environment by reducing self-induced-noise from swimming and potentially enhancing detection of signals from other planes.Psychophysical testing was conducted with a trained harbor seal in order to investigate the sensitivity of the vibrissal system of this species. A behavioral procedure was used to measure absolute detection thresholds for sinusoidal stimuli delivered to the vibrissae by a vibrating plate. Thresholds were measured at 9 discrete frequencies from 10 to 1000 Hz. The seal's performance in this stimulus detection task showed that the vibrissal array was sensitive to directly coupled vibrations across the range of frequencies tested, with best sensitivity of 0.09 mm/s at 80 Hz. The velocity thresholds as a function of frequency showed a characteristic U-shaped curve with a gradual low-frequency roll-off below 80 Hz and a steeper high-frequency roll-off above 250 Hz. The thresholds measured for the harbor seal in this study were about 100 times more sensitive than previous in-air measures of vibrissal sensitivity for this species. The results were similar to those reported by others for the detection of waterborne vibrations, but show an extended range of frequency sensitivity.Animal-borne tagging methods were used to investigate the signals received by the vibrissae and better understand the relevant signal components involved in hydrodynamic detection. A novel tagging system, wLogger, was developed to record vibrations directly from a vibrissa by means of an accelerometer coupled to the vibrissal shaft. Laboratory testing using excised whiskers in a water flume confirmed that the tag is capable of recording vibrational signals without hampering the natural movement of the vibrissa. In addition, the tag successfully measured vibrations from the vibrissae of a harbor seal during active swimming and hydrodynamic detection. Live animal testing, along with the supplemental recordings from excised vibrissae, revealed that interaction with hydrodynamic disturbances disrupted the vibrational signal received by the whisker. When exposed to a hydrodynamic signal, whisker vibrations increased in bandwidth, spreading energy across a wider range of frequencies. This finding suggests that modulation of the vibrational signal may play a key role in the detection of hydrodynamic stimuli by the seal.The results of this dissertation research provide insight into the functioning of the vibrissal system in pinnipeds and establishes the groundwork for future pathways of investigation. By investigating the vibrissal system from the focal points of structure, sensitivity and received signals, a more comprehensive understanding of this refined sensory modality is emerging.