Are Tonkean macaques able to make intuitive statistical inferences?

In a changing environment, making predictions about probabilistic events from few observational facts has ecological relevance for many species. Recent research has shown that preverbal infants, great apes, and capuchins (Sapajus spp.) were able to use proportions to predict the nature of a single item randomly sampled from two populations. However, studies involving macaques (Macaca fascicularis and M. mulatta) brought contradictory results about whether macaques relied on proportions or other choice heuristics to make intuitive inferences. To provide additional information on the evolutionary origins of this skill and overcome methodological concerns from previous studies, we assessed the ability of Tonkean macaques (M. tonkeana) to consider proportions in intuitive statistical inferences.

In a series of eight experimental conditions, 10 Tonkean macaques had to choose which of two jars, that differed in their relative distributions of a preferred and non-preferred food item, conferred a greater chance of obtaining one preferred item. During each test, the experimenter hid in their hands an item drawn pseudo-randomly from each of the two jars for the subject to make its choice.

Our results highlighted both between and within individual variations in their decisional framework. According to experimental conditions, while few individuals relied on proportions to make statistical inferences, most individuals relied on quantities of preferred items or other heuristics to predict the drawing outcome. Altogether, our results brought some evidence that the ability to perform intuitive statistical inferences may be shared amongst primates, but that the underlying high cognitive demands of this capacity may motivate the use of simpler heuristics in some animals' everyday decisions.