Sensitivity of the mystacial vibrissae of harbour seals (Phoca vitulina) for size differences of actively touched objects.

We studied the capability of one male and one female harbour seal Phoca vitulina to discriminate diameter differences of circular discs by means of active touch with their mystacial vibrissae. To prevent the animals from perceiving visual information, they were blindfolded during trials. In a two-alternative forced-choice procedure, the seals were required to choose the larger of two Perspex discs. Weber fractions c (the ratio of the lowest diameter difference detected by the seals on 75% of occasions to the starting disc diameter D, delta D/D = c) were determined for three standard discs (diameters 1.12cm, 5.04cm and 8.74cm) by the psychophysical method of limits. While the seals achieved Weber fractions of 0.29 (male) and 0.26 (female) at the smallest standard disc, their performance improved with increasing disc size, resulting in an approximately constant Weber fraction of 0.13 (male) and 0.08 (female) for the two larger standard discs. The difference in performance between the two seals did not reflect a real difference in sensitivity, but may best be explained by a difference in choice behaviour. As a measure of tactile acuity, the Weber fractions obtained for the larger standard discs indicate that harbour seals can use their mystacial vibrissae as efficiently for active touch as monkeys use their hands.