Scientists have known that animals alter their vocalizations based on the information they perceive during a predator encounter, but this is the first study to show animals adapting their calls based on the specific knowledge—or ignorance—of others. The discovery helps shed light on the evolution of human language and communication.
"We think this ability to produce flexible warning calls is uniquely human," said Alex Thornton, lead author of the study and an assistant professor of anthropology at CU Boulder. "But this study suggests the roots of this cognitive adaptation may lie deeper in our evolutionary history than we thought."
The study was published in the journal Scientific Reports on Thursday.
The research team conducted the field portion of the study over three years in the Goualougo Triangle in the Republic of Congo, where they live with and observe a group of roughly 100 habituated chimpanzees.
The researchers staged 20 simulated encounters between a chimpanzee and a life-size leopard model, which is a major predator of the primates in that area. Some encounters involved the chimpanzees discovering the model for the first time, while others involved a second chimpanzee watching a companion discover the model.
The researchers expected that chimpanzees would modify their warning calls depending on whether they themselves discovered a predator, or whether they watched another chimpanzee discover it. However, the study revealed a greater level of social complexity—chimpanzees modified their calls based on the specific chimpanzee that saw another chimpanzee discover the predator.
When a subordinate male observed another male, of equal or higher rank, discover the leopard, the first male made warning hoots that are typically heard when discovering the predator for the first time. When a subordinate male saw a lower-ranked male discover the leopard, they changed their vocalization patterns, making longer, tonal hoots that communicate the specific object or type of danger they observed.
"By doing so, the subordinate males are conveying the type of threat they observed, and signaling that they were knowledgeable about the risk and are paying attention," Thornton said.
The study has implications for how scientists think about the origins of human language. Researchers now believe that the complex, grammatical language used by modern humans arose relatively recently in our evolutionary history. However, the foundations for language may have evolved earlier and taken shape in part by the need for our ape ancestors like chimpanzees to adjust their communicative behavior based on the social dynamics within the group.
"Complex language probably originated as a by-product of our complex social behavior," Thornton said. "These results show that the precursors to some important language components may be deeply rooted in our primate past."
The research was funded by the National Science Foundation, Leakey Foundation, Wenner-Gren Foundation for Anthropological Research, the University of California, Berkeley, and CU Boulder.
