Perceptual strategies in the estimation of physical quantities by orangutans (Pongo pygmaeus).

The perceptual strategies used by 4 orangutans (2 subadults, 2 adults) when choosing the larger of 2 volumes in a Piagetian conservation task were investigated. Three possible perceptual strategies were investigated: (a) direct perceptual estimation of the container's content independent of its shape, (b) use of the spatial and temporal cues provided by the pouring of liquid from one container to another, and (c) ability to initially identify the larger volume and track it across transformations disregarding misleading perceptual cues. Results indicated that the direct perceptual estimation strategy was the best candidate to explain the orangutan's systematic choice of the larger of 2 quantities.