Barriers and traps: great apes' performance in two functionally equivalent tasks

Tool-using tasks that require subjects to overcome the obstacles to get a reward have been a major component of research investigating causal knowledge in primates. Much of the debate in this research has focused on whether subjects simply use certain stimulus features or instead use more functionally relevant information regarding the effect that certain features may have on a moving reward. Here, we presented two obstacle tasks, a trap platform and a barrier platform, to 22 great apes. Although perceptually similar, these two tasks contain two perceptually different but functionally equivalent obstacles: a trap and a barrier. In a pre-exposure phase, subjects either experienced an obstacle task or a task without any obstacle. In the transfer phase, all subjects were presented with an obstacle task, either the trap platform or the barrier platform. Our results show that those subjects who received an obstacle task prior to the second task performed better than those who first received a non-obstacle task. The type of obstacle task that subjects received first did not have any effect on their performance in the transfer phase. We suggest that apes possess some knowledge about the effects that obstacles have on slow-moving unsupported objects.