SENSITIVITY OF THE MYSTACIAL VIBRISSAE OF HARBOR SEALS (PHOCA-VITULINA) FOR SIZE DIFFERENCES OF ACTIVELY TOUCHED OBJECTS

We studied the capability of one male and one female harbour seal Phoc a vitulina to discriminate diameter differences of circular discs by m eans of active touch with their mystacial vibrissae. To prevent the an imals 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 de termined for three standard discs (diameters 1.12cm, 5.04cm and 8.74 c m) by the psychophysical method of limits. While the seals achieved We ber fractions of 0.29 (male) and 0.26 (female) at the smallest standar d disc, their performance improved with increasing disc size, resultin g in an approximately constant Weber fraction of 0.13 (male) and 0.08 (female) for the two larger standard discs. The difference in performa nce between the two seals did not reflect a real difference in sensiti vity, but may best be explained by a difference in choice behaviour. A s a measure of tactile acuity, the Weber fractions obtained for the la rger standard discs indicate that harbour seals can use their mystacia l vibrissae as efficiently for active touch as monkeys use their hands .