Two bottlenosed dolphins taught to classify pairs of three-dimensional obje
cts as either same or different were tested with novel stimulus sets to det
ermine how well their classification abilities would generalize. Both dolph
ins were immediately able to classify novel pairs of planar objects, differ
ing only in shape, as same or different. When tested on sets of three objec
ts consisting of either all different objects or of two identical objects a
nd one different object, both dolphins proved to be able to classify 'all d
ifferent' sets as different and 'not all different' sets as same, at levels
significantly above chance. These data suggest that dolphins can use knowl
edge about similarity-based classification strategies gained from previous
training to perform successfully in a variety of novel same-different class
ification tasks. Visual classificatory abilities of dolphins appear to be c
omparable to those that have been demonstrated in primates. (C) 2000 Elsevi
er Science B.V. All rights reserved.