Free-Tailed Bat Song

Brazilian free-tailed bats (Tadarida brasiliensis), are small insectivorous molossids that are widely distributed from the central United States through central South America. T. brasiliensis are believed to be the largest mammalian congregations other than humans roosting in immense colonies with estimates of 1 to 20 million individuals. Recently, we documented and analyzed the behavioral contexts of 16 vocalizations produced by T. brasiliensis, including male song and revealed a complex, potentially information rich communication system. We discovered a diverse vocal repertoire with huge variation in acoustic features indicative of a potentially enormous quantity of information that can be encoded simply through differences in syllable acoustics. However, even with such syllable diversity we found that in some cases instead of individual syllables varying with behavioral contexts, either temporal emission patterns or syntax (how the syllable was combined with others) did. Thus, information is encoded in three dimensions-by the number of elements, by the temporal emissions, and by syntax-indicating a highly complex vocal system.

Most recently I have been focusing on the songs of T. brasiliensis males. During the mating season males establish territories, which they vigorously and aggressively defend against encroaching males, but they allow multiple females to enter and reside. During this period songs are easily evoked from territorial males when a bat (either male or female) approaches their territory (in fact I have recently found that males will respond with songs instantaneously simply by playing recorded echolocation sequences near their territories). Males also produce songs while performing wing-flapping displays.

We have recorded songs from over 30 bats at two locations, Austin and College Station. All T. brasiliensis songs are composed of three types of phrases (chirp, trill and buzz) and four types. Both syllable and phrase types are distinct, discrete and easily identified across individuals and regions.

Songs of two free-tailed bats. Chirp phrases are enclosed by dashed boxes, trill phrases by ovals, and buzz phrases by solid boxes. “A” and “B” denote examples of type A and type B syllables. Click image to hear song. (wav file)

An analysis of phrase order using Markov models revealed non-random combinations and specific rules for song construction.

Model of phrase construction of songs. Songs were not randomly constructed. All songs began with chirps, buzzes were never followed by trills and buzzes occurred more frequently than expected at the end of songs. Line thickness represents the frequency of occurrence and + signs denote deviation from random expectations.

Our most interesting finding, however, is that even though song construction and phrase order follow basic (likely species-specific) patterns, the number of syllables within phrases, and the number and order of phrases in songs varied greatly both within and among individuals. Thus, our results overwhelmingly indicate that male free-tailed bats dynamically vary syllable number, phrase order, and phrase repetitions across song renditions. We are currently examining how behavioral context affects song construction and trying to identify special features of the male free-tailed bats’ brain that underlie singing behavior.