Specialized facial muscles support sonar beam-forming by free-tailed bats

Abstract

Echolocating bats may be able to manipulate the acoustic projection pattern of their sonar pulse emissions, but the mechanism(s) for this and potential constraints are unexplored. The Mexican free-tailed bat (Tadarida brasiliensis) appears to achieve this by finely adjusting the shape of its mouth cavity (beam-forming) in a behavior akin to supralaryngeal speech motor control by humans. Flying Tadarida raise their nose and lips preceding each echolocation pulse with a hypertrophied set of specialized facial muscles (levator labii complex). We investigated 1) muscle complex activity patterns during sonar performance, 2) whether the muscles tissue displayed necessary fast-twitch specializations to accommodate echolocation, and 3) how manipulations of mouth shape altered 3D beam patterns. First, EMG recordings from awake echolocating bats confirmed the muscles were activated in precise temporal coordination with pulse emissions. Second, we describe the anatomical connectivity of the muscle complex, origin an...