Bats’ dinner conversation may go over your head
Male big brown bats use ultrasonic sounds to tell others to back off
by Bethany Brookshire
4:28pm, March 27, 2014
On summer evenings in North America, entire conversations go on above your head. Mixed in with the evening birdcalls and droning of insects, there are calls just beyond the range of our pathetic human ears: the vocalizations of bats. We usually think of bats as preternaturally quiet, but it turns out that when these little insect eaters stealthily swoop through the night, they aren’t silent at all.
Scientists at the University of Maryland have shown that when male big brown bats (Eptesicus fuscus) compete for food, the loudest bat gets the worm. These big boy bats produce calls that warn other bats away from their meal. The vocalizations are entirely unlike the noises bats produce to echolocate, and suggest that there is an entire dialogue taking place above us that we have only recently recognized.
A big brown bat is not the leathery monster you might picture. About 10 centimeters (four inches) long and weighing a bit less than a fun-size Snickers, these bats have a wingspan of about 30 centimeters (one foot). They snap up insects over most of North America and have a tendency to end up in our attics. They hunt via echolocation, producing ultrasonic pulses that the bats use to detect prey.
Genevieve Spanjer Wright and her colleagues at the University of Maryland were interested in whether bats might also emit sounds to communicate. In a previous study, the group reported that bats make what appear to be six different types of social calls. Some appeared to be associated with times when the bats were foraging for food. But it’s very hard to tell in the wild what chirp belongs to which bat. “There’s a huge amount of stuff that we don’t know yet because of the problem of observing and tracking bats outdoors,” says James Simmons, a neuroscientist at Brown University in Providence, R.I.
To find out more, Wright’s group flew captive bats (obtained when someone puts in a panicked call about a bat in their attic) in pairs in a large arena, with a single helpless mealworm tethered to the floor. The two-bats-one-bait scenario produced a competitive situation. It was also a tough task, as bats aren’t used to attacking a tethered meal. Bats exposed to the arena for the first time flew hungrily and helplessly — and silently.
But when bats had been trained to pick up the tethered mealworm, the situation changed. In results published March 27 in Current Biology, Wright and her colleagues showed that experienced bats began to emit a series of calls called frequency-modulated bouts, or FMBs, three to four pulses followed by a “feeding buzz.” These were longer and lower in frequency than the pulses that big brown bats use for echolocation. The sounds were highly individual, and the scientists were easily able to trace them to a particular bat.
When two experienced bats faced off against each other, the most gregarious bat frequently took home the worm. When bats emitted the FMBs, the other bat would back off.
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