Understanding of and reasoning about object-object relationships in long-tailed macaques?

Abstract

Diagnostic reasoning, defined as the ability to infer unobserved causes based on the observation of their effects, is a central cognitive competency of humans. Yet, little is known about diagnostic reasoning in non-human primates, and what we know is largely restricted to the Great Apes. To track the evolutionary history of these skills within primates, we investigated long-tailed macaques' understanding of the significance of inclinations of covers of hidden food as diagnostic indicators for the presence of an object located underneath. Subjects were confronted with choices between different objects that might cover food items. Based on their physical characteristics, the shape and orientation of the covers did or did not reveal the location of a hidden reward. For instance, hiding the reward under a solid board led to its inclination, whereas a hollow cup remained unaltered. Thus, the type of cover and the occurrence or absence of a change in their appearance could potentially be used to reason diagnostically about the location of the reward. In several experiments, the macaques were confronted with a varying number of covers and their performance was dependent on the level of complexity and on the type of change of the covers' orientation. The macaques could use a board's inclination to detect the reward, but failed to do so if the lack of inclination was indicative of an alternative hiding place. We suggest that the monkeys' performance is based on a rudimentary understanding of causality, but find no good evidence for sophisticated diagnostic reasoning in this particular domain.

Fig. 1

Illustrations of the five different conditions presented in Experiment 1, shown from the perspective of the monkeys. The objects are shown as orientated after the hiding of the reward. In Condition A, the reward was under each of the two objects in 50 % of the trials. In Condition B, the reward was under the board that had been flat before the hiding (the board on the right side in this example). In Condition C, the reward was under the inclined board that rests on a wooden block. In condition D, the reward was under the board and in Condition E, it was under the cup

Fig. 2

Performance of the monkeys in Experiment 1. Capital letters below the x axis denote the conditions. The two objects used per condition are shown below the x axis. The illustration shows the objects after the hiding of the reward. For conditions B–E, the top object is rewarded and the lower object is not rewarded. In the control condition A, each object was rewarded on 50 % of the trials. Boxplots show median, 25th and 75th quartile, whiskers show 10th and 90th percentiles, dots represent outliers. The solid horizontal line represents the 50 % chance level, the vertical dotted line separates control and test conditions. Asterisk shows significant deviation from chance according to a one-sample t test

Fig. 3

Performance of the monkeys in Experiment 2. Capital letters below the x axis denote the conditions. The objects used per condition are shown below the x axis. The illustration shows the objects after the hiding of the reward. For conditions B–D, the top object is rewarded and the lower object is not rewarded. In the control condition A, each object was rewarded in 50 % of the trials. Boxplots show median, 25th and 75th quartile, whiskers show 10th and 90th percentiles, dots represent outliers. The solid horizontal line represents the 50 % chance level, the vertical dotted line separates control and test conditions. Boxes marked with small letters above the x axis differ significantly from each other, based on a repeated measure ANOVA with post hoc Holm–Sidak tests. Asterisk shows significant deviation from chance according to a one-sample t test

Fig. 4

Performance of the monkeys in Experiment 3. Boxplots show median, 25th and 75th quartile, whiskers show 10th and 90th percentiles, dots represent outliers. The solid horizontal line represents the 50 % chance level. Boxes marked with small letters above the x axis differ significantly from each other, based on a Friedman test with post hoc SNK-tests. Asterisks show significant deviation from chance according to a one-sample t test or Wilcoxon test