Tactual discrimination of size and shape by a California sea lion (Zalophus californianus)

We analyzed the capability of a blindfolded California sea lion to discriminate 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 cm² served as stimuli. The determination of size difference thresholds (ΔS) for the discrimination of triangles revealed that the animal was capable of discriminating size differences as low as 20%. Presented with triangles and disks having identical surface areas, the sea lions’ discriminations relied on the apparent size difference of ≥34% between the longest measurable lines of both shapes (side length & diameter). When this size difference was reduced to ≤5%, the sea lion needed visual information 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 both shapes of 1.70 cm. This tactual performance comes close to that achieved by mammals with prehensile tactile organs.