Spontaneous representations of small numbers of objects by rhesus macaques: examinations of content and format

The project of comparative cognition benefits from common measures across species. We report here on five experiments using the violation of expectancy looking time measure with free-ranging rhesus macaques (Macaca mulatta), each designed to build on current knowledge concerning spontaneous representations of number. Each subject, tested in only one experimental condition, watched as eggplants were placed behind a screen one at a time, after which the screen was removed revealing an outcome that either matched or did not match the number placed there. Subjects looked longer at impossible than possible outcomes in 1+1=2 or 3, 1 small+1 small=1 big or 2 small, 2+1=2 or 3, and 2+1=3 or 4 conditions. They failed in 2+1+1=4 or 3 or 5 and in 1+1+1=2 or 3 conditions. This pattern of results closely matches that observed across several previous studies of human infants. The data allow us to test among four different proposals concerning the format and content of the mental representations underlying looking in these experiments. Object file representations are favored over: (i) low-level perceptual representations, (ii) representations of continuous variables such as volume or surface area, and (iii) analog magnitude representations of number. We conclude by considering exactly how the object tracking system revealed in these and other related experiments does and does not represent number, and how it might be one evolutionary precursor of the human specific system of number representations.