'Bag of Chips' Effect: Bats Eavesdrop on Other Bats to Find Food The calls of other bats tell them there's food nearby, much like opening a bag of chips in a movie theater draws the interest of one's seatmates.

Greater mouse-tailed bats listen to each other's calls to locate prey.

PHOTOGRAPH BY JENS RYDELL

for National Geographic

PUBLISHED JANUARY 8, 2015

When I hear my husband rummaging in the pantry, I often walk over to see if he's found anything good. It turns out bats do something similar by using sound to direct them to the best places to find food.

Like all bats, the greater mouse-tailed bat(Rhinopoma microphyllum) uses echolocation—a type of built-in sonar—to navigate and find prey. When it comes close to an insect, the bat sends out calls that bounce off its prey, helping the predator zero in. But something else happens when these calls go out, a new study says: They serve as a general signal telling other bats there's a meal nearby. (See "Bats Make Calls to Jam Rivals' Sonar—First Time Ever Found.")

Yossi Yovel of Israel's Tel Aviv University, the biologist who led this study—which was published January 8 in Current Biology—calls it the "bag of chips effect."

"If I'm sitting in a dark movie theater and I open a bag of chips, everyone around me knows that I've got something to eat," Yovel said. "It's the same with these bats."

Into the Bat Caves

For the study, Yovel created tiny GPS chips outfitted with a microphone that recorded the bats' high-frequency calls. He tested them by tagging greater mouse-tailed bats—a highly social species that spends the summer in Israel.

Yovel and colleagues caught bats living in caves near the Sea of Galilee (map) and attached the GPS chips to their bodies with surgical glue. The glue naturally disintegrated after a week, allowing the chips to fall off without harming the bats. (Also see "To Know Bats Is to Love Them.")

After confirming that the chips could record the bats' supersonic calls and track their location, Yovel realized he had the opportunity to answer a novel question about bat-foraging behavior: Could the mammals use each other's signals to help find food?

Dinner Bell

So he attached more GPS units to more bats. Yovel and colleagues could retrieve only 40 percent of their data recorders because to find the chips, the researchers had to clamber into caves or climb into the mountains where the bats came to rest. But the researchers still ended up with information on 1,100 interactions between the bats.

Yovel found that when another bat made the stereotypical "homing in on food" call within earshot—roughly 328 feet (100 meters)—of other bats, the eavesdroppers moved toward where they heard the call. (SeeNational Geographic's best bat pictures.)

Yovel emphasizes that the bats aren't making this call to communicate—rather, the bats have simply learned that this signal means there's something good to eat over here.

"These bats are essentially eavesdropping on the sounds of other bats," he said.