Ambient temperature does not affect the tactile sensitivity of mystacial vibrissae in harbour seals

Vibrissae provide pinnipeds with tactile information primarily in the aquatic environment, which is characterized by its high thermal conductivity and large potential cooling power. Since studies of thermal effects on human tactile sensitivity have revealed that cooling below normal skin temperature impairs sensitivity, the present study investigates the tactile sensitivity of the vibrissal system of harbour seals at varying ambient temperatures. Using plates bearing gratings of alternating grooves and ridges, the texture difference thresholds of two adult seals were determined under water. We took advantage of the natural difference in ambient temperature between summer and winter. Mean water temperature was 1. 2 degreesC during the winter and 22 degreesC during the summer. During the cold season, the thermal status of both seals was examined using an infrared-sensitive camera system. The texture difference threshold of both seals remained the same (0.18 mm groove width difference) under both test conditions. The thermographic examination revealed that the skin areas of the head where the mystacial and supraorbital vibrissae are located show a substantially higher degree of thermal emission than do adjacent skin areas. This suggests that, in the vibrissal follicles of harbour seals, no vasoconstriction occurs during cold acclimation, so that the appropriate operating temperature for the mechanoreceptors is maintained.