Comparison of discrete ratios by rhesus macaques (Macaca mulatta)
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Perceiving and comparing ratios are crucial skills for humans. Little is known about whether other animals can compare ratios. We trained two rhesus macaques (Macaca mulatta) to choose arrays that contained the greater ratio of positive to negative stimuli, regardless of the absolute number of stimuli in each of the two choice arrays. Subjects learned this task, and their performance generalized to novel ratios. Moreover, performance was modulated by the ratio between ratios; subjects responded more quickly and accurately when the ratio between ratios was higher. Control conditions ruled out the possibility that subjects were relying on surface area, although the ratio between ratios of surface area did seem to influence their choices. Our results demonstrate that rhesus monkeys can compare discrete ratios, demonstrating not only proportional reasoning ability but also the ability to reason about relations between relations.
KeywordsRatios Proportion Numerical cognition Rhesus macaques Monkeys Relations between relations
We thank Monica Carlson, Nick DeWind, Erin Koballa, Rosa Li, James Powers, Ariel Starr, Emily Szkudlarek, and all members of Dr. Elizabeth Brannon’s and Dr. Michael Platt’s labs for assistance with data collection and helpful discussions of this study. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1106401 and the Holland-Trice Graduate Fellowship in Brain Science and Disease to CBD, and a McDonnell Scholars Award to EMB.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. All procedures were approved by Duke University’s Institutional Animal Care and Use Committee.
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