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The ability to determine the mental state of others — what they know, perceive, believe, or intend — is known as "theory of mind." This ability is important to social interactions, in which one must understand another's behavior and his or her underlying intentions.Theory of mind develops during toddlerhood in humans. However, it remains unclear whether nonhuman primates also have theory of mind or whether it is a uniquely human ability. Bania and Stromberg (2013, Journal of Comparative Psychology) (PDF, 119KB) addressed this question by testing whether western lowland gorillas (Gorilla gorilla gorilla) are able to recognize another individual's attention and use this information to predict behavior. Gorillas had to gesture to one of two experimenters standing in front of them to receive a food reward. Only gestures toward the experimenter who was looking at (and therefore attending to) the gorilla were rewarded. Gorillas were more likely to choose the experimenter whose body was oriented toward them but looking away versus the experimenter whose body was oriented away but looking toward them (Figure C), suggesting that gorillas were using body orientation to guide their decisions. However, when body orientation was neutral (experimenters stood sideways; Figure B), gorillas relied on head orientation and tended to choose the experimenter facing them.