Great Apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo abelii) Follow Visual Trails to Locate Hidden Food

Whether nonhuman primates understand causal relations beyond mere associations is still a matter of debate. We presented all four species of nonhuman great apes (N = 36) with a choice between 2 opaque, upside down cups after displacing them out of sight from their starting positions. Crucially, 1 of them had left a yogurt trail behind it. Great apes spontaneously used the trail to select the yogurt baited cup. Follow-up experiments demonstrated that chimpanzees distinguished trails based on the temporal order of cause and effect by ignoring trails that were already present before the reward was hidden. Additionally, chimpanzees did not select cups based on the amount of yogurt near them but instead preferred cups that signaled the endpoint of the trail. We conclude that apes' choices reveal sensitivity to a causal relation between cause (reward) and effect (trail) including their temporal order.