Chimpanzees can recognise when others in their group are not "in the know" about a source of danger, research has shown.
Scientists found wild chimps were more likely to sound a "watch out!" alarm call about the presence of a snake when newcomers were around.
The discovery challenges the idea that only humans can spot ignorance in others and take steps to keep them informed.
Scientists placed model snakes in the path of wild Ugandan chimpanzees and watched their reactions. When individual chimps detected a snake, they sounded an "alert hoo" to warn others of their group within earshot.
As other group members arrived on the scene, those in the know repeated the alarm call to ensure the new arrivals knew a snake was about. They were less likely to repeat the call if their group-mates were already aware of the danger.
"Chimpanzees really seem to take another's knowledge state into account," said researcher Dr Catherine Crockford, from the University of St Andrews in Scotland. "They voluntarily produce a warning call to inform audience members of a danger that they do not know about. They are less likely to inform those who already know about the danger."
The findings, published in the journal Current Biology, are relevant to questions about the evolution of language, said the scientists.
"Some have argued that a crucial stage in this evolution occurred when individuals began producing vocalisations with the goal of informing and thereby reducing ignorance in others," said Dr Crockford.
The research suggested more of the factors needed to kick-start complex communication were evident in chimpanzees than experts had suspected.
Dr Crockford added: "The advantage of addressing these questions in wild chimpanzees is that they are simply doing what they always do in an ecologically relevant setting; the task is always relevant."