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http://www.newscientist.com/article/mg20627566.300-apes-found-suffering-selfdoubt.html

Apes found suffering self-doubt

18 April, 2010

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 on the task.

To test if the apes doubted their own decisions, Call gave them the option to 
peek into the end of the pipes before they chose one. He found that the apes 
were more likely to check the pipes if they had to wait before picking one 
(Animal Cognition, DOI: 10.1007/s10071-010-0317-x). Call says this suggests 
that the apes had begun to doubt their memory.

Earlier studies have shown that apes and other mammals can be aware that they 
do not know the answer to a test. However, Call claims that the doubt 
apparently revealed by his trials represents a subtle thought process not 
previously seen beyond humans.