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Problem solving often relies on generating new responses while inhibiting others, particularly prepotent ones. A paradigm to study inhibitory abilities is the reverse contingency task (Boysen and Berntson in J Exp Psychol Anim Behav Process 21:82-86, 1995), in which two different quantities of food are offered to an individual who receives the array he did not choose. Therefore, mastery of the task demands selecting the smaller quantity to obtain the larger one. Several non-human primates have been tested in the reverse contingency task. To date, only great apes and rhesus monkeys (Macaca mulatta) have succeeded in the original task, with no need of procedural modifications as the large-or-none contingency, correction trials or symbolic stimuli substituting for actual food quantities. Here, four mangabeys were presented with two stimulus arrays of one and four raisins in the context of the reverse contingency task. Three of them learned to perform the task well above chance without a modified procedure. They also reached above-chance performance when presented with two stimulus arrays of zero and four raisins, despite the initial difficulty of choosing a null quantity. After a period of 7-10 months, in which the animals were not tested on any task, all three subjects continued to perform well, even when presented with novel quantity pairs.

There is still large controversy about whether abstract knowledge of physical problems is uniquely human. We presented 9 capuchin monkeys, 6 bonobos, 6 chimpanzees and 48 children with two versions of a broken-string problem. In the standard condition, participants had to choose between an intact and a broken string as means to a reward. In the critical condition, the functional parts of the strings were covered up and replaced by perceptually similar, but non-functional cues. Apes, monkeys and young children performed significantly better in the standard condition in which the cues played a functional role, indicating knowledge of the functional properties involved. Moreover, a control experiment with chimpanzees and young children ruled out that this difference in performance could be accounted for by differences of perceptual feedback in the two conditions. We suggest that, similar to humans, nonhuman primates partly rely on abstract concepts in physical problem-solving.

Apes (Gorilla gorilla, Pan paniscus, P. troglodytes, Pong abelii) and corvids (Corvus corax, C. corone) are among the most proficient and flexible tool users in the animal kingdom. Although it has been proposed that this is the result of convergent evolution, little is known about whether this is limited to behavior or also includes the underlying cognitive mechanisms. We compared several species of apes (bonobos, chimpanzees, gorillas, and orangutans) and corvids (carrion crows and common ravens) using exactly the same paradigm: a support task with elements from the classical patterned-string tasks. Corvids proved able to solve at least an easy pattern, whereas apes outperformed corvids with respect to the complexity of the patterns solved, the relative number of subjects solving each problem, and the speed to reach criterion. We addressed the question of whether subjects based their choices purely on perceptual cues or on a more abstract understanding of the problem. This was done by using a perceptually very similar but causally different condition where instead of paper strips there were strip shapes painted on a platform. Corvids' performance did not differ between conditions, whereas apes were able to solve the real but not the painted task. This shows that apes were not basing their choices just on spatial or arbitrary perceptual cues. Instead, and unlike corvids, they must have had some causal knowledge of the task.