Journal of Comparative Psychology (2006) Vol. 120, Issue 4, pp. 416-426. ©2006 American Psychological Association How Capuchin Monkeys (Cebus apella) Quantify Objects and Substances Kristy vanMarle, Justine Aw, Koleen McCrink, & Laurie R. Santos Yale University Humans and nonhuman animals appear to share a capacity for nonverbal quantity representations. But what are the limits of these abilities? Previous research with human infants suggests that the ontological status of an entity as an “object” or a “substance” affects infants’ ability to quantify it. We ask whether the same is true for another primate species--the new world monkey Cebus apella. We tested capuchin monkeys’ ability to select the greater of two quantities of either discrete objects or a nonsolid substance. Participants performed above chance with both objects (Experiment 1) and substances (Experiment 2); in both cases, the observed performance was ratio- dependent. This suggests that capuchins quantify objects and substances similarly and do so via analog magnitude representations. A wealth of research on numerical cognition suggests that both human and nonhuman animals represent quantity nonverbally and use this information to guide their behavior. Preverbal human infants, for example, reliably reason about numerical information in a variety of different tasks (see Feigenson, Dehaene, & Spelke, 2004 and Gallistel & Gelman, 2005 for review). .This research was supported by Yale University. This work was approved by the Yale University IACUC committee and conforms to federal guidelines for the use of animals in research. The authors would like to thank Miriam Bowring, Laura Edwards, Bryan Galipeau, Heidi Hansberry, Nathan Herring, Iris Ma, Samantha Santos, Adena Schachner, and Brandon Schneider for their help in running these studies. We are grateful to Rochel Gelman, Daniel Gottlieb, and three anonymous reviewers for helpful comments on earlier versions of this manuscript. Correspondence concerning this article should be addressed to Dr. Kristy vanMarle, Rutgers Center for Cognitive Science (RuCCS), Psych Bldg Addition, Busch Campus, 152 Frelinghuysen Road, Rutgers University-New Brunswick, Piscataway, NJ, 08854 or kvanmarle@ruccs.rutgers.edu. This article may not exactly replicate the final version published in the APA journal. It is not the copy of record. Similarly, numerical abilities have been documented in a wide range of non- linguistic species such as rats, pigeons, parrots, raccoons, ferrets, lemurs, monkeys, and apes (see Brannon & Roitman, 2003; Davis & Perusse, 1988; Dehaene, 1997; Gallistel & Gelman, 2000; and Nieder, 2005 for review). These nonverbal numerical abilities are quite general. Both human and nonhuman animals represent numerical information regardless of whether the stimuli involve auditory or visual events (e.g., Hauser, Tsao, Garcia & Spelke, 2003; Jordan, Brannon, Logothetis, & Ghazanfar, 2005; McCrink & Wynn, 2004; Meck & Church, 1983; Whalen, Gallistel, & Gelman, 1999), objects in the world (e.g., Hauser, MacNeilage, & Ware, 1996; Wynn, 1992), or actions produced by the animal (e.g., lever presses, Fernandes & Church, 1982; Mechner, 1958). In addition, rhesus macaques (Macaca mulatta ) and capuchin monkeys (Cebus apella) can order pairs of stimuli on the basis of numerosity (Brannon & Terrace, 1998; 2000; Judge,