Evidence for Semantic Communication in Alarm Calls of Wild Sichuan Snub-Nosed Monkeys.

The alarm calls of non-human primates help us to understand the evolution of animal vocal communication and the origin of human language. However, as there is a lack of research on alarm calls in primates living in multilevel societies, we studied these calls in wild Sichuan snub-nosed monkeys. By means of playback experiments, we analyzed whether call receivers understood the meaning of the alarm calls, making appropriate behavioral responses. Results showed that receivers made appropriate and specific anti-predator responses to two types of alarm calls. After hearing the aerial predator alarm call ("GEGEGE"), receivers' first gaze direction was usually upward (towards the sky), and upward gaze duration was longer than the last gaze before playback. After hearing the terrestrial predator alarm call ("O-GA"), the first gaze direction was usually downward (towards the ground), and this downward gaze duration was longer than the gaze before playback. These reactions provide evidence for external referentiality of alarm calls in Sichuan snub-nosed monkeys, that is, information about the type of predator or the appropriate response is encoded acoustically in the calls.