The Greater mouse-tailed bat flies through the night sky, searching for insects. Credit: Photo by Jens Rydell.

When emerging at night by the thousands, bats avoid colliding by changing how they move and echolocate; how is a now solved decades-old mystery.

Aya Goldshtein, Omer Mazar, and Yossi Yovel have spent many evenings standing outside bat caves.

Even so, seeing thousands of bats erupting out of a cave and flapping into the night, sometimes in densities so high that they appear liquid, astounds the scientists every time.

But until recently, the bat biologists were even more baffled by what they didn’t see.

Goldshtein, from the Max Planck Institute of Animal Behavior, said: “The bats don’t run into each other, even in colonies of hundreds of thousands of bats all flying out of a small opening.”

How bats don’t fatally crash into each other every night when they squeeze out of caves to forage was a scientific mystery.

Many bats perceive their world mostly through echolocation: they emit a call and listen for the reflected echo, which in turn allows them to ‘see’ what is around them.

But if many bats are echolocating at once — such as when a whole colony emerges from a cave in the space of a few minutes — then the calls of others should drown out the important echoic information that bats need.

Scientists call this loss of acoustic information ‘jamming’ and they expect that bats should collide because of it.

And yet, aerial accidents outside caves are so rare, “you’re almost excited when you witness one,” said Goldshtein.

Ambient chatter

For decades, scientists have tried to figure out how bats solve this mystery in which ambient chatter deafens you to the voice you need to hear.

For example, they examined how bats echolocate in groups. In the laboratory, scientists observed that individual bats in a small group each echolocated at a slightly different frequency, which in theory should reduce jamming. Was this the solution?

Yovel says that past studies like these are important stepping stones, but they have fallen short of providing a compelling answer to the cocktail party mystery because of a crucial missing piece: “No one had looked at this situation from the point of view of an individual bat during emergence.

“How can we understand a behaviour if we don’t study it in action?”

For the first time, Goldshtein and colleagues have collected data from wild bats emerging from a cave at dusk.

They used a combination of high-resolution tracking, developed by Ran Nathan and Sivan Toledo, ultrasonic recording, and sensorimotor computer modelling — all of which allowed the researchers to step into the bats’ sensory world as the animals squeezed out of the cave opening and flew through the landscape to forage.

The team, led by scientists from Tel Aviv University, studied greater mouse-tailed bats in Israel’s Hula Valley.

Over two years, they tagged tens of bats with lightweight trackers that recorded the bats’ location every second.

Some of these tags also included ultrasonic microphones that recorded the auditory scene from the individual bat’s point of view.

Each year, data was collected on the same night that bats were tagged.

A caveat: the tagged bats were released outside the cave and into the emerging colony, meaning that real data were missing at the cave opening when density is highest.

The team filled in this gap with a computational model that was developed by Mazar and simulated emergence.

The model incorporated data collected by the trackers and microphones to recreate the full behavioural sequence starting from the entrance of the cave and ending after bats had flown two kilometres through the valley.

Mazar said: “The simulation allows us to verify our assumptions of how bats solve this complex task during emergence.”

Remarkable

And the picture that emerged was remarkable.

When exiting the cave, bats experience a cacophony of calls, with 94% of echolocations being jammed.

Yet, within five seconds of leaving the cave, bats significantly reduced the echolocation jamming.

They also made two important behavioural changes: first, they fanned out from the dense colony core while maintaining the group structure; and second, they emitted shorter and weaker calls at higher frequency.

The researchers suspected that bats would reduce jamming by quickly dispersing from the cave. But why did bats change their echolocation to a higher frequency?

Wouldn’t more calling only increase the problem of jamming and therefore collision risk?

To understand that result, the authors had to approach the scene from a bat’s point of view.

Mazar said: “Imagine you’re a bat flying through a cluttered space. The most important object you need to know about is the bat directly in front.

“So you should echolocate in such a way that gives you the most detailed information about only that bat.

“Sure, you might miss most of the information available because of jamming, but it doesn’t matter because you only need enough detail to avoid crashing into that bat.”

In other words, bats change the way they echolocate to gain detailed information about their near neighbours — a strategy that ultimately helps them to successfully manoeuvre and avoid collisions.

The authors emphasise that this unexpected result for how bats solve the cocktail party dilemma was made possible by studying bats in their natural environment as they perform the relevant task.

Goldshtein said: “Theoretical and lab studies of the past have allowed us to imagine the possibilities, but only by putting ourselves, as close as possible, into the shoes of an animal will we ever be able to understand the challenges they face and what they do to solve them.”

The study is published in Proceedings of the National Academy of Sciences.