Kersken, V., Zhang, D., Gomez, J-C., Seed, A., & Ball, D. (2020). Capuchin monkeys individuate objects based on spatio-temporal and property/kind information: Evidence from looking and reaching measures. Animal Behavior and Cognition, 7(3), 343-364. doi: https://doi.org/10.26451/abc.07.03.06.2020

A core component of any folk physical understanding of the world is object individuation - the cognitive ability to parse sensory input into discrete objects. Whereas younger human infants use spatio-temporal information to individuate objects, they do not use property and kind information until one year of age. Some researchers propose that object individuation based on property/kind information depends on language acquisition and sortal concepts. However, there is evidence that preverbal infants and nonhuman animals also use both types of information. The present study aimed to further explore the evolutionary origins of object individuation by testing a new-world monkey species, capuchin monkeys (Sapajus spp.), using both manual search (n = 29) and looking time (n = 23) measures. In Spatio-temporal trials, subjects saw one or two objects dropped into a box, but always found (or saw) only one. In Property/kind trials, subjects saw either object A or B being dropped into a box and then always found (or saw) object A. The capuchin monkeys looked longer and searched more on inconsistent trials – with outcome differing in quantity or in kind - which suggested that they had expectations based on both spatio-temporal and property-kind representations. Looking time and search measures gave convergent results at the group but not at the individual level. Our results add to the existing evidence contradicting the linguistic hypothesis of property/kind individuation. However, contrary to recent discussions, we argue that these and related results can be explained without appealing to the notion of sortal concepts or multiple representational systems, and suggest that a full picture of the ontogeny and phylogeny of object individuation requires further empirical and theoretical research.

Object individuation, Capuchin monkeys, Property/kind information, Spatio-temporal information