Dialect formation in ghost bats: genetic, geographic and morphological drivers of social and echolocation call divergence

Geographical patterns of vocal dialects in bats are poorly understood, despite growing evidence of complex vocal communication systems. We investigated variation in vocalisations recorded at five ghost bat colonies in the Northern Territory, Australia. We calculated genetic and morphological distances among individuals and correlations with geographic distance. We then determined variation within three social vocalisations (‘chirp-trill’, ‘squabble’, ‘ultrasonic social’) and the ‘echolocation’ call using seven spectrographic measurements. Finally, we tested whether acoustic distance could be explained by genetic, geographic or morphological distance. Geographic and genetic distance were highly correlated, suggesting the occurrence of isolation by distance. All measures of morphological distance were consistent with Bergmann's Rule, except noseleaf shape, which is likely constrained by its role in echolocation. Geographic variation was evident within each of the three social vocalisations and the echolocation call, with the patterns of geographic variation differing among the four vocalisation types. The degree of difference was surprising, given the ghost bat's long-range seasonal dispersal. Acoustic distance in chirp-trill and squabble calls was marginally significantly correlated with genetic (and geographic) distance, but these relationships were not significant after controlling for geography. In contrast, ultrasonic social and echolocation calls varied among colonies but showed no significant associations with other metrics, apart from a weak correlation between ultrasonic social distance and forearm length. This supports the view that these ultrasonic calls are under stabilising selection due to their role in foraging and orientation. This study provides the first evidence of dialect formation in megadermatid bats. It highlights the importance of considering multiple vocalisation types and investigating multiple processes in signal evolution. Overall, we found genetic, geographic and morphological distances accounted for some of the variation in acoustic differences among colonies, but further work is needed to investigate other processes that may also contribute to dialect formation in ghost bats.