Estimating and operating on discrete quantities in orangutans (Pongo pygmaeus)

This study investigated the ability of 3 male orangutans (Pongo pygmaeus; 1 subadult, 2 adults) to estimate, compare, and operate on 2 sets of small q uantities (1-6 cereal bits). Experiment I investigated the orangutans' abil ity to choose the larger of 2 quantities when they were presented successiv ely as opposed to simultaneously, thus being perceptually unavailable at th e time of choice. Experiment 2 investigated the orangutans' ability to sele ct the larger quantity after the original quantities were augmented or redu ced. Orangutans were capable of selecting the larger of 2 quantities in Exp eriment I. There was also some evidence from Experiment 2, albeit weaker, t hat orangutans may mentally combine quantities (but not dissociate) to obta in the larger of 2 quantities. This study suggests that orangutans use a re presentational mechanism (especially when comparing quantities) to select t he larger of 2 sets of items.