Many bats scan their environment by emitting ultrasonic pulses from their mouths and then analyzing the echoes that come back. This creates a biosonar field of view that’s similar to a visual scene for us. When bats want to "zoom in" on something, a tasty bug or a refreshing pool of water, they simply open their mouths wider to focus their sonar beams. Conversely, they "zoom out" by narrowing their gape. The findings were published in Proceedings of the National Academy of Sciences last week.
Echolocating in a cramped area is different than echolocating out in the open. The volume of space that’s covered by sound pulses depends on the shape of the sensory sonar sound beam that’s emitted from the bat's mouth. Narrowing the sonar beam improves the localization of objects, eliminates undesired echoes, and increases the range by directing more energy forward. All of these benefits, however, reduce the volume of space that the bat can successfully scan. It would help to decrease the scanned volume when entering small, confined areas and increase it when flying into vast, open expanses.
To study beam-forming, a Tel Aviv University team led by Yossi Yovel conducted field experiments with a mouth-emitting bat called Bodenheimer’s pipistrelle (Hypsugo bodenheimeri) in Israel. Using 12 ultrasonic microphones, two cameras, and multiple flashes, the team recorded bats as they arrived at a small pond in the Arava Desert to drink on the fly, literally. As they descend towards the pond, the bats enter a confined, cluttered space with high, dense reeds on two sides of the banks and steep muddy banks on the other two sides.
By analyzing the bat beams and bat faces during 312 approaches, the team discovered that bats narrow and widen their sonar beam just by adjusting how wide they open their mouths. They stretched their mouths when entering the confined environment to narrow their beam width, and as they flew out into the open, they narrowed their gape to widen their beam. These changes occurred on the wing and within milliseconds.
So why do narrow mouths produce wider beams, and wider mouths produce narrow beams? These counterintuitive effects were due to diffraction, Science explains, which causes sound waves traveling through a smaller hole to spread out more.
The team repeated the experiment in a more controlled setting to make sure the mouth gapes weren't specific to drinking water. When they sent bats flying through a narrow, 1.5-meter-long tunnel, they saw similar results: The bats optimized their ability to sense their surroundings by focusing their ultrasonic beam. On the one hand, the bats could direct more energy into a narrow sector to zoom its biosonar field of view, the team writes, and on the other hand, it could widen the beam to increase the space that it senses.