TACTILE SIZE DISCRIMINATION BY A CALIFORNIA SEA LION (ZALOPHUS-CALIFORNIANUS) USING ITS MYSTACIAL VIBRISSAE

The capability of a blindfolded California sea lion (Zalophus californ ianus) to discriminate diameter differences of circular discs by means of active touch with its mystacial vibrissae was studied. Using a for ced choice paradigm the sea lion was required to choose the larger of two simultaneously presented perspex discs. Absolute difference thresh olds (Delta D) were determined for 3 standard discs (1.12 cm empty set , 2.52 cm empty set, 8.74 cm empty set) by the psychophysical method o f constants. Increasing disc size resulted in an increase in the absol ute difference threshold from 0.33 cm for the smallest disc size to 1. 55 cm for the largest disc size. The relative difference threshold (We ber fraction) remained approximately constant at a mean value of 0.26. According to a video anal ysis the sea lion did not move its vibrissa e when touching the discs. Instead, it performed precisely controlled lateral head movements, with the touched disc located centrally betwee n the vibrissae of both sides of the muzzle. Since the extent of these head movements was identical at discs to be compared, discs of differ ent size must have led to different degrees of deflection of vibrissae involved in the tactile process, resulting in quantitatively differen t mechanical stimulations of mechanoreceptors in the follicles. This s uggests that the accuracy of the sea lion's size discrimination was de termined by the efficiency of two sensory systems: the mechanosensitiv ity of follicle receptors as well as kinaesthesis.