Bats are the masters of late-night snacking thanks to echolocation, and use ultrasonic chirps and the resulting echoes to find their prey in the dark. The bio-sonar has one big drawback, though. It’s vulnerable to interference.
In the 1960s, scientists noticed that when echolocating bats hunted tiger moths, the insects emitted ultrasonic clicks of their own that threw the bats off their trail. Decades of research on the behavior led to three ideas, not mutually exclusive, about its purpose: that it startled the bats and bought the moths time to flee; that it acted as a warning signal to tell bats that some moths were toxic; and that it jammed the bats’ sonar.
In 2008, biologist Aaron Corcoran started studying the question for his PhD research at Wake Forest University. Lucky for him, his advisor had already found the perfect moth to test the jamming hypothesis: Grote's tiger moth (Bertholdia trigona), a species native to the American Southwest that’s both a favorite prey of bats and especially noisy when it’s being hunted.
“With Bertholdia we could exclude the warning hypothesis as long as we knew the bats we used weren't already trained to think (unconsciously) of clicking moths as toxic moths,” Corcoran says on his website. “Bertholdia also made about ten times the noise as other moths. In other words, if any moth could jam a bat, this was the one.”
When Corcoran pitted the moths against big brown bats (Eptesicus fuscus) in a flight room and recorded audio and video of the attacks, he found that bats easily caught and ate moths that had been rendered mute by cutting away their tymbals, the organs used to produce the clicks. When the moths were free to make a racket, though, the bats “only caught about two out of every ten.”
The bats acted funny when the moths clicked, too. “Normally bats make echolocation calls at faster and faster rates when they attack insects so they get information back more quickly,” Corcoran writes. And if they take the moth clicks as a warning, they quickly abort their attack. Corcoran’s bats did neither. They kept pressing the attack, but “spent more time listening after each call, as though they were having difficulty hearing the echoes returning from the moths,” leading Corcoran to conclude that the moths were indeed jamming the bats’ sonar. Now, with a new study, he’s found that moths aren’t the only animals that jam bats’ sonar like this. Some bats also interfere with each other’s echolocation as they vie for the same prey.
Since he published the results of his moth study in 2009, Corcoran has been doing more research on the jamming defense—how it works, how it evolved in the moths, and how the bugs know when to start jamming. One night, he was recording big brown bats and Grote’s moths around the Arizona-New Mexico border when he realized they weren’t alone. Other night flyers, Mexican free-tailed bats (Tadarida brasiliensis), were also flitting around and making their own calls. When he listened to the recordings he made, he noticed that some of the free-tailed bats’ calls were strikingly similar to the moths’ jamming clicks.
Maybe, he thought, the bats were doing the same thing as the bugs, and messing with each other's sonar. Mexican free-tailed bats live together in colonies that can number in the millions (Bracken Cave, near San Antonio, Texas, is home to an estimated 20 million bats during the summer), and when they take to the skies at night to hunt, they’re known to eavesdrop on each other's feeding calls to locate prey. If a bat is trying to find dinner at the same time as a million or more of its hungry, eavesdropping roostmates, it wouldn’t be surprising that they’d try to get a leg up on the competition by sabotaging their hunts.
To test the idea that the strange call—which Corcoran dubbed the the sinusoidal frequency-modulated (sinFM) call—interfered with other bats’ sonar, Corcoran went back into the field with biologist William Conner to record the bats as they hunted. They captured the bats on video and used an array of microphones to pinpoint their locations when they called, and then used that information to make a 3D model of their flight paths showing what calls they made and when.
The model showed that the bats only made the sinFM calls when another bat made its “feeding buzz,” a rapid series of echolocation pings used to hone in on prey in the final seconds of a chase. The sinFM call appeared to be well suited for jamming a rival bat that’s closing in on something, says Corcoran, because it overlaps with the feeding buzz and fills in the “listening window” between the calls with noise, making it harder for a buzzing bat to figure out where its prey is. Sure enough, when a sinFM call was let loose, nearby bats missed their targets big time, and their insect catch rate dropped by 75 to 85 percent.
Next, Corcoran and Conner lured bats with moths tethered on a string and played recordings of the sinFM call and other sounds as they swooped in to grab the bait. Again, when the call was played as the bats made their feeding buzz, they mostly missed their target, and caught the moths less than a quarter of the time. When the researchers played another noise during the feeding buzz, or the sinFM call right before the bats buzzed, though, they had no problem picking the moths off. The call only made them miss when it was played at just the right time.
The big brown bats that Corcoran had previously been studying also throw their competitors off a bug’s trail with a specialized social call called the “frequency-modulated bout” (FMB), which essentially tells other bats to back off and calls “dibs” on a prey item. It’s possible that that’s what the free-tailed bats were doing, too, but not very likely. The bats foiled by a sinFM call didn’t fly off or give up on their prey like the brown bats that get shooed away by the FMB do. Instead, they circled around again and made another attempt at catching the bugs. The fact that the bats didn’t abandon the hunt after hearing a sinFM call, and that it only had an effect during the feeding buzz, suggests that the call isn’t for claiming a bug, but jamming rivals and keeping them from finding it long enough for the jammer to steal it for themselves.
How the call does that isn’t exactly clear yet, but Corcoran thinks that it interferes with a buzzing bat’s ability to locate its prey is by overlapping with its feeding buzz, filling in the sonic spaces and confusing its auditory neurons.
Quoth the Raven...
Another new study shows that bats aren’t the only animals that undermine their competition. Ravens also keep each other in check—not with sensory sabotage, but political maneuvering. Researchers in Austria found that ravens at the top of the social ladder—bonded, breeding pairs of birds that control territory and access to food—will attack and interrupt lower birds when they attempt to bond and pair up. The researchers think that these interventions are meant to keep the other birds from forming alliances and climbing the ranks, so that they can’t compete for the power couples’ resources in the future.