The ability to reason about probabilities has ecological relevance for many species. Recent research has shown that both preverbal infants and non-human great apes can make predictions about single-item samples randomly drawn from populations by reasoning about proportions. To further explore the evolutionary origins of this ability, we conducted the first investigation of probabilistic inference in a monkey species (capuchins; Sapajus spp.). Across four experiments, capuchins (N = 19) were presented with two populations of food items that differed in their relative distribution of preferred and non-preferred items, such that one population was more likely to yield a preferred item. In each trial, capuchins had to select between hidden single-item samples randomly drawn from each population. In Experiment 1 each population was homogeneous so reasoning about proportions was not required; Experiments 2–3 replicated previous probabilistic reasoning research with infants and apes; and Experiment 4 was a novel condition untested in other species, providing an important extension to previous work. Results revealed that at least some capuchins were able to make probabilistic inferences via reasoning about proportions as opposed to simpler quantity heuristics. Performance was relatively poor in Experiment 4, so the possibility remains that capuchins may use quantity-based heuristics in some situations, though further work is required to confirm this. Interestingly, performance was not at ceiling in Experiment 1, which did not involve reasoning about proportions, but did involve sampling. This suggests that the sampling task posed demands in addition to reasoning about proportions, possibly related to inhibitory control, working memory, and/or knowledge of object permanence.
We would like to thank the RZSS Living Links staff who gave permission to conduct research and provided invaluable support and assistance. Thanks to Keith Haynes at the University of St Andrews for constructing the cubicle window. Thanks also to Natalia Robert-Nunez for secondary coding of data.
This study was funded by an Economic and Social Research Council Future Research Leaders Grant (ES/K009540/1) to D.B.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest
Human and animal rights
All procedures were in accordance with UK law and the ASAB Guidelines for the Treatment of Animals in Behavioural Research and Teaching. The study was approved by the University of St Andrews School of Psychology and Neuroscience Ethics Committee and the Research Committee at Living Links.
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