Understanding Solidity: Investigating Knowledge of a Functional Object Property in Brown Capuchin Monkeys (Sapajus apella) and Common Squirrel Monkeys (Saimiri sciureus)

Humans form abstract representations about the physical properties of objects, with very young infants having ‘core knowledge’ about solidity and continuity. Whether nonhuman primates also form abstract representation of physical properties is debated. Despite studies showing that some nonhuman primates can discriminate between functional and non-functional tools, whether they achieve this by recognizing an object’s physical properties or via associative learning of perceptual cues remains contested. One method for exploring an individual’s physical cognition is the trap-tube (where subjects push a reward out of a pipe whilst avoiding a trap). The trap-tube investigates whether participants understand that rewards cannot pass through solid objects (solidity) and that unsupported objects will fall (gravity). Initial research reported that chimpanzees and capuchin monkeys solved the test using associative rules; however, modifying the task by removing the tool revealed an understanding of solidity in chimpanzees. In this series of studies, we tested 12 squirrel monkeys and 14 capuchin monkeys on a modified trap-box, where the monkeys could move the reward using their fingers rather than a tool. No individual of either species passed the task at above chance levels. In Experiment 2, the same trap blocked one of two cups: both species learned to avoid it. In Experiment 3, only capuchins generalized this solution to novel materials, performing better when cues had a functional versus a non-functional relationship to the outcome. Squirrel monkeys appeared to learn via association, while results from capuchins indicated a sensitivity to the concept of solidity.