Buzz
Bats rule the night when it comes to snacking, relying on echolocation to send out ultrasonic chirps and track their prey through the darkness. However, this bio-sonar technique has one major drawback—it’s susceptible to interference.
Acoustic Warfare
In the 1960s, researchers discovered that when bats used echolocation to hunt tiger moths, the moths emitted their own ultrasonic clicks that disrupted the bats' sonar. Years of research led to three possible explanations for this behavior: it either startled the bats, gave the moths a chance to escape, signaled that certain moths were toxic, or interfered with the bats' bio-sonar.
In 2008, biologist Aaron Corcoran began investigating this question for his PhD research at Wake Forest University. Fortunately, his advisor had already discovered the ideal moth for testing the jamming hypothesis: Grote's tiger moth (Bertholdia trigona), a species native to the American Southwest that is not only a common bat target but also particularly loud when being hunted.
“With Bertholdia we could rule out the warning hypothesis as long as we were certain that the bats we used weren’t already conditioned (subconsciously) to associate clicking moths with toxicity,” Corcoran says on his website. “Bertholdia also produced about ten times more noise than other moths. In other words, if any moth had the ability to disrupt a bat’s sonar, this was the one.”
When Corcoran pitted the moths against big brown bats (Eptesicus fuscus) in a flight chamber and recorded both audio and video of the encounters, he found that bats had no trouble catching and eating moths that had been silenced by removing their tymbals, the organs responsible for creating the clicks. However, when the moths were free to make noise, the bats “only caught about two out of every ten.”
The bats also behaved strangely when the moths clicked. “Bats usually increase the speed of their echolocation calls during attacks so they can receive echoes faster,” Corcoran writes. If the bats took the moth’s clicks as a warning, they would immediately abort the chase. But Corcoran’s bats did neither. They persisted with the attack but “spent more time listening after each call, as if they had trouble hearing the echoes returning from the moths,” leading Corcoran to conclude that the moths were indeed jamming the bats' sonar. With a new study, he discovered that moths are not the only creatures capable of disrupting bat sonar. Some bats also interfere with each other’s echolocation while competing for the same prey.
It’s Sabotage!
Since publishing the results of his moth study in 2009, Corcoran has continued to explore the jamming defense—its mechanics, its evolution in moths, and how the insects know when to activate the jamming. One evening, while recording big brown bats and Grote’s moths along the Arizona-New Mexico border, he realized there were other creatures involved. Mexican free-tailed bats (Tadarida brasiliensis) were also flying nearby and emitting their own calls. Upon reviewing the recordings, Corcoran found that some of the free-tailed bats' calls sounded strikingly similar to the moths' jamming clicks.
Perhaps, he thought, the bats were using a similar strategy as the moths, disrupting each other's sonar. Mexican free-tailed bats live in colonies that can number in the millions (for example, Bracken Cave near San Antonio, Texas, hosts around 20 million bats during the summer), and when they hunt at night, they are known to listen in on each other’s feeding calls to locate prey. If a bat is hunting at the same time as millions of its fellow bats, it wouldn’t be surprising if they tried to sabotage each other's hunts to gain an advantage.
To test the theory that the strange call—which Corcoran named the sinusoidal frequency-modulated (sinFM) call—was interfering with other bats’ sonar, Corcoran teamed up with biologist William Conner to record bats hunting in the wild. They filmed the bats and used an array of microphones to track their locations when they called, then created a 3D model of their flight paths to analyze the timing and nature of their calls.
The model revealed that the bats only produced the sinFM call when another bat made its “feeding buzz,” a rapid sequence of echolocation pings used to track prey in the final moments of a chase. Corcoran suggests the sinFM call is particularly effective at jamming a rival bat closing in on prey because it overlaps with the feeding buzz, filling the “listening window” between calls with noise, making it more difficult for the targeted bat to pinpoint its prey. As expected, when a sinFM call was played, the nearby bats missed their targets by a large margin, reducing their insect capture rate by 75 to 85 percent.
Next, Corcoran and Conner attracted bats with moths tied to a string and played recordings of the sinFM call and other sounds while the bats swooped in to catch the bait. Again, when the sinFM call was played during the bats' feeding buzz, they mostly missed the target, catching the moths less than a quarter of the time. However, when they played a different noise during the feeding buzz or the sinFM call just before the bats' buzzed, the bats had no trouble catching the moths. The call only caused them to miss when it was played at precisely the right moment.
The big brown bats Corcoran had studied previously also use a specialized social call known as the “frequency-modulated bout” (FMB), which essentially signals to other bats to back off and claims a bug as their own. It’s possible that Mexican free-tailed bats do the same, but it’s unlikely. The bats disrupted by the sinFM call didn’t fly away or abandon their prey like the brown bats that are deterred by the FMB. Instead, they circled back and tried again to catch the insects. The fact that the bats didn’t give up after hearing a sinFM call and that the call only affected them during the feeding buzz suggests that the call’s purpose isn’t to claim the prey, but rather to jam rivals and delay them long enough for the bat making the call to seize it for itself.
How the call works is still not entirely clear, but Corcoran believes it interferes with a buzzing bat’s ability to locate its prey by overlapping with the feeding buzz, filling in the gaps of sound and confusing the bat's auditory processing.
Quoth the Raven...
A new study shows that bats aren’t the only creatures undermining their competitors. Ravens also keep their rivals in check—not through sensory disruption, but through social manipulation. Researchers in Austria discovered that top-ranking ravens—bonded breeding pairs that control territory and access to food—will attack and disrupt lower-ranking birds when they try to form bonds and pair up. The researchers believe these attacks aim to prevent other ravens from building alliances and climbing the social ladder, thus protecting the top birds' future access to resources by keeping competition at bay.
