Phylogenetic roots of quantity processing: Apes do not rely on object indexing to process quantities

There is substantial evidence for human infants' use of 2 different number processing systems, Object-File System and Approximate Number System; however, the majority of studies of non-human primates reveal processing via Approximate Number System only. The present study investigated whether apes use the Object-File System when tested with an entity-first design, often used in infant studies. Apes' performance was consistently ratio-dependent and showed no sign of an Object-File System, regardless of entity type (rigid and flexible cohesive objects vs. non-cohesive powder and liquid). After ruling out other low-level explanations, our results suggest that great apes relied solely on an Approximate Number System. These results are discussed in light of a possible developmental difference between human infants and adult great apes.