Monkey see, monkey plan, monkey do: the end-state comfort effect in cotton-top tamarins (Saguinus oedipus).

The way human adults grasp objects is typically influenced by their knowledge of what they intend to do with the objects. This influence is reflected in the end-state comfort effect: Actors adopt initially uncomfortable postures to accommodate later task demands. Although many experiments have demonstrated this effect, to the best of our knowledge its phylogenetic roots have not been investigated. In two experiments, we tested whether 9 cotton-top tamarin monkeys would show the end-state comfort effect. We did so by presenting the monkeys with a small cup containing a marshmallow. The cup was suspended in different orientations. The monkeys inhibited their natural grasping tendencies and adopted unusual grasping postures to accommodate subsequent task requirements, thus demonstrating the end-state comfort effect. This outcome provides evidence for more sophisticated motor planning than has previously been ascribed to this and related species.