Rhesus monkeys, Macaca mulatta, know what others can and cannot hear

Animals from numerous taxa use auditory information to functionally deceive other individuals, either by producing or withholding various vocal signals. However, little empirical evidence has demonstrated that any nonhuman species recognizes how manipulating auditory information can alter the knowledge state of other individuals. We explored whether one monkey species, the rhesus macaque, understands the connection between hearing and knowing. Monkeys were presented with the opportunity to take grapes from a human competitor who was looking away. One of two grapes was placed inside a silent container, and the other was placed inside a noisy container. We predicted that subjects would selectively choose the silent container over the noisy container, because the noisy container might alert the human competitor to the subject's actions. As predicted, subjects reliably took the grape from inside the silent container when the competitor was not looking. In contrast, subjects chose randomly when the competitor was looking and therefore already knew about the subject's approach. These results demonstrate that monkeys preferentially attempted to obtain food silently only in conditions in which silence was relevant to obtaining food undetected.