The reason thousands of bats can simultaneously pour out of a narrow cave without colliding with each other has been revealed. It turns out that they use a strategy of focusing only on the nearest individual to avoid collisions amid the sound interference of the ultrasonic waves they emit while finding their own paths. This means that by avoiding just the friend in front of them, everyone can avoid collision accidents.
Researchers from the Max Planck Institute for Animal Behavior in Germany, Tel Aviv University, and the Hebrew University jointly announced their findings that bats emit high-frequency ultrasonic waves to detect the nearest individuals and avoid collisions, even in high-density group environments. This study was published in the journal Proceedings of the National Academy of Sciences (PNAS) on Nov. 1.
Bats, which have degenerated eyesight, use echolocation to identify the location of obstacles or prey. They emit ultrasonic waves from their mouths and interpret the returning echoes that bounce off surrounding objects to perceive their environment.
Kim Seon-sook, senior researcher at the National Institute of Ecology, noted that "Bats can form a kind of 'sound-drawn picture' in their minds through the information from the returning sounds, allowing them to perceive the entire space three-dimensionally" and that "they can simultaneously determine distance, texture, direction, and speed."
The problem arises when bats take flight in groups. When numerous bats emit ultrasonic waves simultaneously, their sounds interfere with each other, making it difficult to distinguish the returning echoes accurately. This situation, where crucial information is drowned out by other sounds, is referred to as the 'cocktail party dilemma,' likened to the difficulty of picking out someone's voice in a noisy banquet hall. Nevertheless, collisions among bats during actual flight are nearly nonexistent.
To analyze why there are no collision accidents among bat groups, researchers conducted experiments over two years targeting the Rhinopoma microphyllum, a species found in the Hula Valley in Israel. They attached GPS trackers and miniature ultrasonic microphones to dozens of bats, recording their movements and sound information from the moment they took off from the cave down to the second.
The analysis revealed that about 94% of the echolocation sounds from bats within a 3-meter radius of the cave exit were interfered with by the sounds of other bats. However, within just 5 seconds, the bats spread out in a fan shape and maintained their group structure while simultaneously employing a strategy of emitting shorter, weaker, and higher-frequency ultrasonic waves to reduce interference. By adjusting the length and frequency of the ultrasonic waves, the detection range is reduced, but the information about nearby targets becomes clearer.
The key is that bats concentrate only on their closest companions. By avoiding collisions with the individual right in front of them, the bats naturally widen their spacing, causing the bat group to fan out. Omer Mazar, a researcher at Tel Aviv University, explained that "From the bats' perspective, even if they miss most information due to interference, it is important to gather as much information as possible about the nearest individual."
This strategy resembles the principles of animal behavior that rely on simple information from the surroundings. For example, termites build massive nests based on a simple rule of placing soil adjacent to where dirt has already piled up. Researchers at Harvard University successfully aligned over 1,000 robots into an alphabet shape by mimicking this simple rule with the principle of 'stop when touching the robot next to you.'
In the past, scientists estimated that bats emitted ultrasonic waves at different frequencies to avoid interference. This so-called 'jamming avoidance response' was not the case for the actual bats. The researchers explained that it appeared this strategy was not used due to all individuals responding similarly.
Kim Seon-sook, senior researcher, stated that "This study well illustrates the sophisticated echolocation abilities of bats" and noted that "In nature, considering ecological characteristics, factors such as the communication method between bats known as 'social call' may also play a role along with ultrasonic waves."
References
Proceedings of the National Academy of Sciences (2025), DOI: https://doi.org/10.1073/pnas.2407810122