Context-dependent alarm calls in the Mauritian flying fox, an island-endemic fruit bat

Alarm calls enable animals to warn conspecifics of imminent threats and coordinate collective responses. Despite bats’ rich social vocal behaviour and the predation pressures faced by many species, no experimental evidence of such calls has been reported in this order. Here, we report alarm calls in Chiroptera, in the Mauritian flying fox ( Pteropus niger ), an island endemic species that evolved without natural predators yet now faces intense anthropogenic pressures. In captive colonies, an experimenter entered the enclosure either empty-handed ("No-Net") or carrying a capture net ("Net"), a more salient threat cue. The calls emitted in both conditions were distinguishable from other social calls and likely represent alarm calls. Alarm calls from the two conditions were also acoustically distinguishable, and in one colony bats called twice as often under "Net". Alarm call production was context-dependent: calls were more frequent during the day, when roosting bats are less mobile and potentially more vulnerable, whereas at night the same stimuli mostly elicited flight. Alarm calls emerged only when group size reached at least four individuals. These findings broaden the known communicative repertoire of bats, highlight the need to reassess alarm signalling across Pteropodidae , and provide a basis for comparative work on its origins.