TACTUAL DISCRIMINATION OF SIZE AND SHAPE BY A CALIFORNIA SEA LION (ZALOPHUS-CALIFORNIANUS)

We analyzed the capability of a blindfolded California sea lion to dis criminate objects differing in size and/or shape by active touch with its mystacial vibrissae. In a two-alternative forced-choice procedure, equilateral triangles and disks with surface areas ranging from 60 to 0.5 cm(2) served as stimuli. The determination of size difference thr esholds (Delta S) for the discrimination of triangles revealed that th e animal was capable of discriminating size differences as low as 20%. Presented with triangles and disks having identical surface areas, th e sea lions' discriminations relied on the apparent size difference of greater than or equal to 34% between the longest measurable lines of both shapes (side length > diameter). When this size difference was re duced to less than or equal to 5%, the sea lion needed visual informat ion about the shapes before it was able to discriminate them tactually , When the size of these shapes was gradually reduced, the animal was able to make the discrimination down to a longest measurable line of b oth shapes of 1.70 cm. This tactual performance comes close to that ac hieved by mammals with prehensile tactile organs.