Concurrent spatial relations challenge capuchin monkeys (Sapjus libidinosus) and chimpanzees (Pan troglodytes) in an alignment task and provide implications for tool use

Aligning an object with a surface also involves producing particular spatial relations between the object and the surface just as when using a hand tool. Gibsonian perception-action theory predicts that managing two spatial relations simultaneously in an alignment problem is more difficult than managing one relation, or managing two relations sequentially. We presented 6 captive capuchin monkeys and 6 captive chimpanzees with stick objects to insert into one groove (one relation) or two grooves (two relations). Two grooves were presented parallel to each other or as a separated T (sequential placement), or as a joined T (concurrent placement). Subjects of both species needed more attempts to insert the joined T than the parallel sticks, but not the separated T. Our findings provide strong evidence that managing two concurrent spatial relations is more challenging to non-human primates than managing two sequential relations, which restricts the problems they will solve using tools.