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Dynamic Beam Forming in Free-Tailed Bats
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There is behavioral evidence that echolocating bats can manipulate the acoustic projection pattern of their sonar pulse emissions, but the mechanism(s) for this are unknown. I hypothesized that the Mexican free-tailed bat (Tadarida brasiliensis) achieves this by finely adjusting the shape of its mouth (beam-forming) in a behavior akin to supralaryngeal speech motor control by humans. This hypothesis arose from my discovery that Tadarida brasiliensis raise their noses and lips preceding each echolocation pulse and that they possess a hypertrophied set of specialized facial muscles possibly analogous to the levator labii aleque nasi. I investigated whether this muscle complex 1) is active during sonar performance, 2) displays anatomical and histological specializations consistent with the high-speed demands of echolocation, and 3) can effectively perform beam-forming through fine manipulations of the nose and mouth. Firstly, EMG recordings from awake echolocating bats confirmed that these muscles were activated in a temporally precise coordination with pulse emissions. Secondly, I described the anatomical organization of the muscle complex, its origin and insertions, and its innervation patterns. Histochemical analyses confirmed that these were fast-twitch muscles, as expected for muscles adapted for rapid contractions for extended periods. Lastly, I directly measured how changes in face shape affected the sonar beam-width. This muscle complex allows bats to lift the nose tip to create a small aperture, producing a wide-angle beam, or to lift both the nose and the upper lips simultaneously creating a wider aperture but narrower beam. I confirmed that for a typical pulse (downward FM sweep, 50-20 kHz), raising and pulling back the lips narrowed the projection beam relative to just raising the nose tip with lips held down. These results confirm that Tadarida possesses a specialized supralaryngeal neuromuscular apparatus for sonar beam-forming.
Trent, Samantha Shea (2017). Dynamic Beam Forming in Free-Tailed Bats. Master's thesis, Texas A & M University. Available electronically from