text: Alarm signalling varies depending on the receiver's knowledge of a certain threat. Chimpanzees are significantly more likely to produce an alarm call when conspecifics are unaware of a potential threat or were not nearby when a previous alarm call was emitted. When judging if conspecifics are unaware of potential dangers, chimpanzees do not solely look for behavioural cues, but also assess receiver mental states and use this information to target signalling and monitoring. In a recent experiment, caller chimpanzees were shown a fake snake as a predator and were played pre-recorded calls from receivers. Some receivers emitted calls that were snake-related, and therefore represented receivers with knowledge of the predator, while other receivers emitted calls that were not snake-related, and therefore represented receivers without knowledge of the predator. In response to the non-snake-related calls from receivers, the signallers increased their vocal and nonvocal signalling and coupled it with increased receiver monitoring.