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 鈥渟eeing鈥� 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.
鈥淲hen 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,鈥� says Yossi Yovel from Tel Aviv University in Israel.
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Yovel鈥檚 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 鈥� the bats鈥� main prey 鈥� 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鈥檚 way. 鈥淲e see by analysing their sonar calls that they respond to the presence of a nearby bat as if it was an obstacle,鈥� says Yovel. 鈥淭here鈥檚 a trade-off.
鈥淏ats 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 鈥� you will have to stop tracking the fly.鈥�
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鈥檚 sonar to prevent rivals from reaching prey.
Yovel doesn鈥檛 think that鈥檚 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 .
Unjammable bat signal
鈥淚 believe it鈥檚 very difficult to jam a bat. We have tried doing so in the lab and it鈥檚 almost impossible. Bats are highly tuned to recognising their own signal,鈥� he says.
The role and significance of eavesdropping has been difficult to study, as it鈥檚 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.
鈥淭his interesting study basically puts us for the first time virtually on a bat鈥檚 back,鈥� says Stefan Greif at the Max Planck Institute for Ornithology in Seewiesen, Germany. 鈥淓avesdropping might well be more widespread than we know at the moment and accumulating evidence is supporting this idea.鈥�
Bats could also be eavesdropping to learn , or .
Yovel and his team are now studying five other bat species to see how they forage.
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