Enumeration of objects and substances in non-human primates: experiments with brown lemurs (Eulemur fulvus)

Both human infants and adult non-human primates share the capacity to track small numbers of objects across time and occlusion. The question now facing developmental and comparative psychologists is whether similar mechanisms give rise to this capacity across the two populations. Here, we explore whether non-human primates' object tracking abilities are subject to the same constraints as those of human infants. In particular, we examine whether one primate species, the brown lemur (Eulemur fulvus), also fails to represent and enumerate objects when they behave non-rigidly or non-cohesively. We presented lemurs with a series of expectancy violation studies involving simple 1 + 1 addition events in which we varied the entities to be enumerated. Like infants, lemurs successfully enumerated the two objects when those objects were rigid, cohesive individuals, but failed to enumerate similar-looking non-rigid piles of sand. In contrast to human infants, however, lemurs successfully enumerated non-cohesive objects that broke into multiple pieces. These results are discussed in light of recent theories about object processing in human infants and adults.