Hearing people munching on a snack and rustling the packet in a cinema is annoying, but for some bats the sound of other bats about to eat is a vital clue to locating food.
Detecting tasty insects by echolocation limits bats to "seeing" to a distance of nearly 10 metres. But if they plug into the sonars of bats around them, they can hear when another bat has found insects up to 160 metres away. This is because loud bat hunting calls carry much further than sound reflecting off a tiny insect.
"When a bat is attacking prey it emits a typical sequence of calls so any other bat within 100 metres knows that someone found food. I call this the bag of chips effect, because when someone in a dark cinema opens a bag of chips everybody knows it," says Yossi Yovel from Tel Aviv University in Israel.
Yovel's team mounted tiny GPS devices and ultrasonic recorders to track greater mouse-tailed bats (Rhinopoma microphyllum), which roost in colonies of hundreds or even thousands. They found that the bats spent about 40 per cent of their time hunting within 150 metres of other bats, even though getting too close hampered their chance of catching prey as they then had to focus on collision avoidance instead.
Why then do they hunt in such close proximity? To test this, the team modelled bat behaviour and found that the most likely explanation is the bag of chips effect. The queens of flying ants – the bats' main prey – are so sporadically clustered that it may be much easier for a single bat to locate them by eavesdropping on others in a group, essentially expanding their echolocation range more than tenfold.
Trade-off
But there is a point at which this strategy becomes a hindrance. When the number of bats in the group becomes too large, they start to get in each other's way. "We see by analysing their sonar calls that they respond to the presence of a nearby bat as if it was an obstacle," says Yovel. "There's a trade-off.
"Bats might suffer as they constantly need to track other bats, and this makes tracking food difficult. Imagine that you are tracking a fly and a baseball is thrown in your direction – you will have to stop tracking the fly."
And some bats may be using sonar calls even more selfishly. A recent study found that Mexican free-tailed bats (Tadarida brasiliensis) could be intentionally jamming each other's sonar to prevent rivals from reaching prey.
Yovel doesn't think that's the case with the greater mouse-tailed bat. An earlier study he carried out suggests that even when two bats were operating at the same frequency, they could probably tell their signals apart.
Unjammable bat signal
"I believe it's very difficult to jam a bat. We have tried doing so in the lab and it's almost impossible. Bats are highly tuned to recognising their own signal," he says.
The role and significance of eavesdropping has been difficult to study, as it's tough to investigate bats in the wild. In the new study the researchers say they used the smallest GPS trackers available, but still found they were prone to falling off.
"This interesting study basically puts us for the first time virtually on a bat's back," says Stefan Greif at the Max Planck Institute for Ornithology in Seewiesen, Germany. "Eavesdropping might well be more widespread than we know at the moment and accumulating evidence is supporting this idea."
Bats could also be eavesdropping to learn what food to eat, or to directly detect hidden prey.
Yovel and his team are now studying five other bat species to see how they forage.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2014.11.010
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