Great Himalayan leaf-nosed bats modify vocalizations to communicate threat escalation during agonistic interactions

Abstract

Bats vocalize extensively within various social contexts. Nevertheless, studies of agonistic interactions, associating vocalizations signalling the emotional state of a caller with individual signatures during aggressive vocalizations remain scarce. Here, we examined whether male Great Himalayan leaf-nosed bats (Hipposideros armiger) modify their aggressive vocalizations during agonistic interactions depending on the level of aggression, and whether these vocalizations encode individual signatures. We applied a cost-benefit analysis to audio-video recordings of 50 dyadic agonistic interactions to categorize displays into two levels of aggression intensity (low aggression: bared teeth, slightly pulled up body and/or wings; high aggression: rapidly flapping wings, punching, biting). Male H. armiger used graded visual agonistic displays accompanied by bent upward frequency modulation (bUFM) vocalizations to defend their roosting territories. At high aggression intensities, males decreased the minimum frequency of aggressive calls and increased the frequency bandwidth. Males also transferred energy from the second harmonic to the first harmonic as the threat escalated. These systematic modifications of acoustic parameters as aggressive intensity fluctuated corresponded to prosodic modifications in human speech, indicating that emotion-related acoustic cues are a common feature of acoustic communication in mammals. In addition, we found that the aggressive calls of eight adult males encoded discriminable signatures, and that males could discriminate among individuals based on these aggressive calls. Such discrimination is probably useful for individual identification among rival neighbours.