Are aggressive vocalizations the honest signals of body size and quality in female Asian particoloured bats?

Animals make calls to give conspecifics information about themselves. Based on these calls, the receiver makes decisions that affect survival and reproduction. However, not all of the signals are reliable. Honest signals are either costly to produce or are constrained by physical or physiological factors that cannot be faked. Aggressive calls during antagonistic interactions are both costly and constrained by physiology. The female Asian particoloured bat (Vespertilio sinensis) produces aggressive displays and vocalizations when confronted by an intruder conspecific competing for roost position. We tested whether the aggressive calls of female Asian particoloured bats encode reliable information about the body size and the quality of the caller, and, subsequently, whether aggressive calls predicted a winning percentage that was calculated as the number of winning interactions divided by the total number of agonistic interactions. At the syllable level, there were significant positive relationships between the duration of various syllables and forearm length, bite force and winning percentage. In addition, frequency-related syllable parameters, such as peak frequency, minimum frequency, maximum frequency and bandwidth, were negatively correlated with immune response and winning percentage. At the call level, heavier bats that produced calls with shorter mean inter-syllable silences had higher winning percentage and healthier bats produced calls with lower maximum frequency. There was a significant positive relationship between immune response and vocal complexity, but no significant relationship was found between winning percentage and vocal complexity. These results suggested that aggressive calls in female Asian particoloured bats may be honest signals of body size and quality during the antagonistic interactions. Indeed, the reliability of acoustic communication in bats may be key to an indepth understanding of the function and evolution of social calls in bats.