Intuitions about gravity and solidity in great apes: the tubes task.

We investigated whether great apes, like human infants, monkeys and dogs, are subject to a strong gravity bias when tested with the tubes task, and--in case of mastery--what the source of competence on the tubes task is. We presented 22 apes with three versions of the tubes task, in which an object is dropped down a tube connected to one of three potential hiding places and the subject is required to locate the object. In two versions, apes were confronted with a causal tube that varied in the amount of perceptual information it provided (i.e. presence or absence of acoustic cues). The third version was a non-causal adaptation of the task in which a painted line 'connected' dropping and hiding places. Results indicate that apes neither have a reliable gravity bias when tested with the tubes, nor understand the causal function of the tube. Even though there is evidence that they can integrate tube-related causal information to localize the object, they seem to depend mainly on non-causal inferences when searching for an invisibly displaced object.