TACTUAL DISCRIMINATION OF SIZE BY MEANS OF MYSTACIAL VIBRISSAE IN HARBOR SEALS - IN AIR VERSUS UNDERWATER

In the present investigation the tactual discrimination capabilities o f Harbour seals in air and underwater was determined. In a two alterna tive forced choice procedure the seals were required to choose the lar ger of two simultaneously presented circular discs. Perspex discs with a diameter (empty set) range from 30 mm to 50 mm (in steps of 0.5 and 1.0 mm) were used as stimuli. The standard stimulus (D) was a 40 mm e mpty set disc. for which the upper (comparison stimuli in descending o rder, 50 --> 40 mm empty set) and lower size difference threshold (com parison stimuli in ascending order, 30 --> 40 mm empty set) was determ ined by a modified method of constant stimuli. Both seals were capable of detecting diameter differences as low as 2.0 mm (= 5% size differe nce). Compared to tactual systems of other mammals the high-resolving power of the mystacial vibrissae of Harbour seals for size differences of actively touched objects comes close to that of the human hand. Th is may give some idea as to the biological importance of tactile infor mation for these animals.