Was the food on the left, or on the right? (Image: Josep Call) When in doubt (Image: Gerry Ellis/Getty)

THERE goes another sophisticated mental feat once thought to be uniquely human. Apes may be sufficiently self-aware to doubt their own knowledge.

Josep Call of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, put food in one of two opaque plastic pipes and had watching bonobos, chimpanzees, gorillas and orangutans pick the one with the food. If they were made to wait, the apes sometimes forgot where the food was, but by and large they did well …