Individual bats emit sonar calls in the dark, using the echo of their signature sounds to identify and target potential prey. But because they travel in large groups, their signals often "jam" each other, a problem resembling extreme radar interference. How do bats overcome this "cocktail party" cacophony to feed and survive in the wild?
A new Tel Aviv University study published in Proceedings of the Royal Society B: Biological Sciences identifies the mechanism that allows individual bats to stand out from the crowd. The research, by Dr. Yossi Yovel of TAU's Department of Zoology, finds that individual bats manage to avoid noise overlap by increasing the volume, duration and repetition rate of their signals.
According to Dr. Yovel, unlocking the mystery of bat echo recognition may offer a valuable insight into military and civilian radar systems, which are vulnerable to electronic interference.
Cocktail party chatter
"Imagine you are at a cocktail party where everyone is uttering the same word over and over again, and you are expected to recognize the echo of your own utterance to identify the location of the punch bowl," Dr. Yovel said. "Now imagine that this is tantamount to your survival. This is the bat experience. Bats often fly in groups and rely on sounds -- very similar sounds -- to find their food. They deal with two challenges: They need to detect weak echoes in a cluster of noise, and if they manage to receive the echo, they need to recognize it as their own."
Dr. Yovel and his team of TAU researchers, including Eran Amichai and Dr. Gaddi Blumrosen, tested bat responses in situations mimicking a high density of bats. They played back bat echolocation calls from multiple speakers to jam the echoes of five flying Pipistrellus kuhlii bats, simulating a naturally occurring situation of many bats flying in proximity. Under severe interference, bats emitted calls of higher intensity and longer duration, and called more often -- but they did not change the pitch of their signals, as was previously believed.
The new study builds on previous research conducted by Dr. Yovel in which he developed miniature microphones, attached to bat backs, allowing for the first-ever recording of bat frequencies in real time.
"In a study we conducted last year, we found evidence that bats do not harness any such 'jamming avoidance,' as hypothesized in the past by other scientists," said Dr. Yovel. He believes that they simply recognize their own voices.
"In another paper, published in 2009, we trained bats to crawl toward one side or another, in the direction of another bat," Dr. Yovel explained. "This indicated that they indeed differentiated between the voice of one bat and another. This also proved they could identify their own calls.
"In the current study, we trained bats to fly around a small room and land on a small object -- in the midst of a loud mixture of bat signals playing overhead. They found the object by increasing their emissions: crying louder and longer and shouting more frequently. They cried 'ahhhhhhh' instead of 'ah' twice as frequently -- every 50 milliseconds instead of the usual 100 milliseconds."
From bats to automobiles
According to Dr. Yovel, this research may provide insight into engineering used for human beings.
"We want to understand the problem," said Dr. Yovel. "The better we understand the radar interference problem, the easier it will be to solve. In the future, we will all have radar systems in our cars, and there can be hundreds of these on a stretch of highway as well. Individuality must be built into these radar codes, very clear signature codes."
Dr. Yovel is currently seeking how individuality is intrinsic to bat codes, which continues to escape scientific research.
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