Acoustic signalling of aggressive intent in the agonistic encounters of female Asian particoloured bats

The signals of animal aggressive intent can communicate aggression levels to competitors, thereby settling conflicts at an early stage and thus reducing energy costs. We tested whether aggressive vocalizations of female Asian particoloured bats, Vespertilio sinensis, encode aggressive intent during agonistic encounters that occur whenever an intruder pushes a resident to obtain a more central roosting spot. Our results showed that the number of noisy/tonal syllables of the disturbed bats met the three criteria used to define aggressive intent. First, the number and ratio of tonal syllables were significantly greater when intruders continued to push, and the number and ratio of noisy syllables were significantly greater when they stopped pushing (context criterion). Second, skin temperature related to emotional arousal during aggressive interaction generally increased when intruders stopped pushing but was unchanged or decreased when they continued pushing (predictive criterion). Finally, bats preferentially selected stimuli with more tonal syllables, and they spent more time approaching more noisy syllables in a two-choice playback experiment (response criterion). Thus, we propose that the number of noisy/tonal syllables within aggressive vocalizations of female Asian particoloured bats functions to signal aggressive intent.