Primate researcher and psychologist Sally Boysen performed an experiment in which she placed two chimpanzees in adjacent cages. It was arranged so that one of the chimpanzees could see a veterinarian in hiding, who was aiming a tranquillizer dart at the other chimpanzee. The chimpanzee screamed out to alert its companion to the danger. However, the chimpanzees did not scream when the veterinarian was visible to both of them.
In other words, chimpanzees have the ability to realize that their companions are unaware of a danger that they can perceive, and try to notify them in order to help.
■References (Books, papers, Web articles, etc.)
『家族進化論』 第5章・第4節 心の理論と利他的行動 山極寿一 （東京大学出版会、2012）
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