Free-ranging rhesus monkeys spontaneously individuate and enumerate small numbers of non-solid portions.

Fundamental questions in cognitive science concern the origins and nature of the units that compose visual experience. Here, we investigate the capacity to individuate and store information about non-solid portions, asking in particular whether free-ranging rhesus monkeys (Macaca mulatta) quantify portions of a non-solid substance presented in discrete pouring actions. When presented with portions of carrot pieces poured from a cup into opaque boxes, rhesus picked the box with the greatest number of portions for comparisons of 1 vs. 2, 2 vs. 3, and 3 vs. 4, but not for comparisons of 4 vs. 5 and 3 vs. 6. Additional experiments indicate that rhesus based their decisions on both the number of portions and the total amount of food. These results show that the capacity to individuate non-solid portions is not unique to humans, and does not depend on structures of natural language. Further, the fact that rhesus' ability to represent non-solid portions is constrained by the same 4-item limit typically ascribed to the system of parallel individuation that operates over solid objects suggests that the visual system recruits common working memory processes for retaining information about solid objects and non-solid portions. We discuss our results with respect to theories of visual processing, as well as to the role that the human language faculty may have played in both the evolution and development of quantification.