Wild Diana monkeys are preyed upon by leopards and chimpanzees. These two predators differ in their hunting tactics and Diana monkeys use two distinct antipredator strategies to defend themselves. After detecting a leopard, Diana monkeys respond by giving loud, conspicuous alarm calls that function both to warn others and to signal to the predator that it has been detected. Leopards tend to leave the area once they have been discovered. In contrast, upon detecting a chimpanzee, male Diana monkeys do not vocalize at all, while females give only a few quiet alarm calls and flee quickly to hide in the forest canopy, chimpanzees have sophisticated climbing skills that would allow them to pursue monkeys, even in the high strata of the forest canopy. Choosing an antipredator strategy appropriate to chimpanzees is complicated for Diana monkeys by the fact that chimpanzees themselves also fall prey to leopards. When encountering a leopard, chimpanzees give loud, conspicuous alarm calls. To escape successfully from leopards and chimpanzees, therefore, Diana monkeys must distinguish between, and respond differently to, chimpanzees alarm calls and chimpanzee vocalizations simply signal the presence of a leopard and should elicit the monkeys' loud, conspicuous alarm calls. To investigate Diana monkeys' understanding of such cause-effect relationships involving predators, Zuberbuhler examined the monkeys' responses to chimpanzee vocalizations in two different types of experiments. In the first, he played tape recording of either chimpanzees' alarm calls or their social vocalizations and noted the monkeys' responses. Diana monkeys differed in their response to chimpanzees' alarm calls. In some groups, monkeys behaved as if they recognized that these alarm calls signaled the potential presence of a leopard: they responded to chimpanzees' leopard alarm calls by giving leopard alarm calls themselves, in contrast to their normal response to the presence of chimpanzees. Groups of monkeys living in the core area of the resident chimpanzee community were more likely to behave in this way than were peripheral groups. In the second set of experiments, Zuberbuhler played leopard growls to Diana monkeys shortly after exposing them to recording of either chimpanzees' alarm calls or their social vocalizations. After first hearing chimpanzees' alarm calls, some monkeys failed to respond to the subsequent recording of leopard growls, even though this stimulus normally elicited a strong vocal response. These monkeys behaved exactly like the Diana monkeys in a comparison group, who also gave many leopard alarm calls to an initial recording of leopard growls but no longer called to the second recording of leopard growls five minutes later. This similarity in behavior suggests that these monkeys had some knowledge of the causal factors underlying the production of chimpanzees' alarm calls.