Bats are very intelligent mammals who navigate with the aid of a sonar system – so why do they crash into large walls even though they detect them? 

Researchers at Tel Aviv University (TAU) were curious about this phenomenon and recently reached the conclusion that the collision results from an error in their acoustic perception rather than from a sensory limitation. They believe that the combination of a large wall and a weak echo disrupts the bats’ sensory perception and causes them to ignore the obstacle. They also discovered that acoustic perception is not inborn but rather acquired through experience – bat pups did not “hit the wall.” 

The study was led by Dr. Sasha Danilovich, then a doctoral student in the lab of Prof. Yossi Yovel, head of the Sagol School for Neuroscience and faculty member at the School of Zoology at TAU’s Faculty of Life Sciences. Other participants included Dr. Arian Bonman and students Gal Shalev and Aya Goldstein of the sensory perception and cognition laboratory at the School of Zoology and the Sagol School of Neuroscience. The paper was published in the prestigious journal Proceedings of the (US) National Academy of Science (PNAS) under the title “Echolocating bats detect but misperceive a multidimensional incongruent acoustic stimulus.” 

The zoologists released dozens of bats in a corridor blocked by objects of different sizes that were made of different materials. To their surprise, the researchers discovered that the bats collided with large sponge walls that produce a weak echo as if they did not exist. The bats’ behavior suggested that they did this even though they had detected the wall with their sonar system, indicating that the collision did not result from a sensory limitation, but rather from an acoustic misperception.

The researchers hypothesize that the unnatural combination of a large object (wall) and a weak echo disrupts the bats’ sensory perception and causes them to ignore the obstacle, just much like people who bump into transparent walls who bang their heads or break their noses. 

At the next stage of the experiment, the researchers methodically changed the features of the echoing objects along the corridor in terms of size, texture and echo intensity. “We systematically manipulated

intensity and aperture by changing the materials and width

of different reflectors, and we conclude that a coherent echobased

percept is created only when these two acoustic dimensions

have certain relations which are typical for objects in nature (such as

large and intense or small and weak reflectors),” the team wrote. 

They concluded that the bats’ acoustic perception depends on a coherent, typical correlation of the dimensions with objects in nature – for example: a large object-strong echo and a small object – weak echo.

“Bats excel in acoustic perception. They are able to detect objects as tiny as mosquitoes, using sound waves,” explained Yovel. “Using echolocation they can calculate the three-dimensional location of both small and large objects, perceiving their shape, size and texture. To this end, a bat’s brain processes various acoustic dimensions from the echoes returning from the object (such as frequency, spectrum and intensity). This perception is based on several senses that combine many different dimensions, such as color and shape.”

In addition, the TAU researchers discovered that bats are not born with this ability. Repeating the experiment with young bats they found that they do not fly into walls.  The study, which does not currently have any practical applications but solve an enigma that has interested scientists around the world, also found that adult bats can quickly learn the new correlations among the dimensions.

“By presenting the bats with objects whose acoustic dimensions are not coherent, we were able to mislead them, creating a misconception that caused them to repeatedly try to fly through a wall, even though they had identified it with their sonar. The experiment gives us a peek into how the world is perceived by these creatures, whose senses are so unique and different from ours,” concluded Danilovich.