Emergence density and stream frequency spectra of Tadarida brasiliensis

Abstract

Bats depend on echolocation to maneuver through their environment. Mexican Free-Tailed bats (Tadarida brasiliensis) form large maternal colony roosts and emerge in large, dense groups. Previous work found that the T. brasilensis bats adjust their call structure during emergence, which may help the bats avoid flight collisions. The limited physical space between bats in the emerging stream is not only problematic with flight collisions, but also with echolocation interference. In this study, we tested how the density of emergence affects echolocation. Specifically, we tested the hypothesis that the density of flying bats affects echolocation characteristics of the entire stream. We determined six emergence density categories from thermal imagery, and randomly selected 20, 500 ms audio samples corresponding to each density. For each audio sample, we calculated the frequency spectrum, and analyzed how the stream frequency spectrum changed according to population density. Across densities, the stream frequency spectra all have the same shape and peak frequencies, but the variance for certain frequencies differs. This change in variance suggests that these bats are attempting to avoid jamming in dense emerging streams by altering their frequencies from bat-to-bat, irrespective of density.