Bats have an ambulance in their ears

Anyone who has been overtaken by a high-speed ambulance has had a physical effect called a Doppler shift: when the ambulance moves towards the listener, its movement compresses and raises the sound waves of the siren. As the ambulance moves away from the listener, the sound waves expand and the pitch decreases. An auditor wearing a headband could use this Doppler shift model to track the movement of the ambulance.

In an article published by the Proceedings of the National Academy of Sciences, the authors, Rolf Mueller, professor of mechanical engineering at the College of Engineering, and his doctoral student, Xiaoyan Yin, demonstrate that the bats' ears are delivered with an "integrated ambulance" that creates the same physical effect. Yin and Mueller think that studying the Doppler changes generated by the ear in bat biosonar could give rise to new sensory principles that could allow small but powerful sensors. An example of this type of sensor would be that of drones that can operate in dense foliage or in autonomous underwater vehicles sailing near complex submarine structures.

"The animals move their ears fast enough so that the sound waves that strike them are transformed by moving the surface of the ears and shifted to higher or lower frequencies," Mueller said. "In fact, the species of bats studied (horseback racquets and round-leaved Old World bats) can move the ears so rapidly that Doppler changes of about 350 Hz can to be created.It is seven times larger than the smallest Doppler shift observed by animals, to be able to detect. "

It has long been known that Doppler changes play an important role in the biosonar system of bats, such as the species studied by Mueller and Yin. Bats have the enviable ability to hunt in very dense vegetation, but to do this, they must solve the problem of distinguishing between a moth, their favorite prey, and the hundreds of leaves that surround it.

"The solution proposed by these two types of bats has been to take into account the Doppler changes produced by the flapping motion of their prey's wing," Mueller explained. "These" good Doppler changes "are a unique identifying feature that distinguishes prey from static distractors, such as leaves in foliage."

The researchers quickly realized that the flight movement of bats also produced Doppler shifts that would interfere with the perception of Doppler shifts induced by their prey. In the late 1960s, a solution to this riddle was discovered when it was found that rackets reduced their frequency of emission of a carefully controlled amount to eliminate exactly the "bad Doppler changes" "caused by the flight speed of bats.

"Since these revolutionary discoveries, the scientific community has generally believed that the role of Doppler changes in the biosonar systems of these animals has been fully understood," Mueller said. "The Doppler changes due to prey movements are" good Doppler changes "that the complete auditory system of the animal is optimized for detection, while the Doppler changes due to the flight movement of the bats are" bad Doppler changes "that animals eliminate by controlling their emission."

Although Mueller and Yin found speculation in the early 1960s literature that bats could produce Doppler shifts with their own ear movements, this idea was never followed by any work. experimental.

Mueller and Yin's work has carefully measured the movement of ear surfaces using stereoscopy based on high-speed video cameras, and the authors have been able to predict the speed at which surfaces are moving. in different parts of the ear. They also estimated the angle between the directions of ear movement and the bat's biosonar direction and found that velocities and movement directions were aligned to maximize the Doppler shifts produced. .

To show that the shifted Doppler signals entered the ear canal of the biomimetic pavilion and were accessible to bats, the researchers constructed a soft silicone replica of a bat ear capable of performing fast movements in bats. pulling on a tied string.

The last element of the research was to find possible uses of the Doppler changes generated by the ear.

"We have been able to show that Doppler shifts produce distinct patterns in time and frequency that can be used to indicate the direction of a target," Mueller said. "In the context of the biosonar systems of these species of bats, they focus and emit the bulk of their ultrasonic energy in a narrow frequency band, however, to indicate the direction of a target. it is generally practical to examine how multiple frequencies are transmitted.Doppler shift patterns produced by ear movements could give these bat species the ability to focus their energy in a narrow frequency band, while also being able to indicate the direction of the target. "