The latest controversial discovery in bat foraging
Mexican free-tailed bats congregate in the largest colonies of any mammal on earth. One colony can eat upwards of 250,000 tons of insects in one night. With such intense competition, these bats have developed a new hunting tactic: one Mexican free-tailed bat will jam the echolocation call of another in order to steal his mark.
The first bat doesn’t take this thievery lying down. He will simply jam the second bat right back, and this battle royale can continue for as many as 5 rounds. According to Aaron Corcoran, the Ph.D. student who first realized what was happening, combat ends when one bat decides it would be easier to find food elsewhere, with less competition.
Jamming works as signal scrambler. Bats scour their environment for tiny moths by sending ultrasonic waves out from their voice boxes and listening for the waves that bounce back. The returning echo strikes the sensitive inner ear receptors, creating a map of the bat’s surroundings. Jamming occurs when a call of a slightly different but very similar frequency hits the first call and forces a mistranslation. Since bat calls are frequency modulated – meaning the pitch of the calls goes up and then back down like a police siren – any noise affects it. The mistranslation happens when the second bat’s call alters the echo, causing the bat to see a different environment and ultimately miss his target.
Bat jamming is nothing new. Moths click in the direction of their incoming predator in a way that hides their location. They act as ventriloquists, essentially throwing their position to the left or to the right a few inches by sending their own frequency-modulated click. The click throws the hunter off just enough that his moth-location gauge miscalculates, preventing capture. Even human noises can jam a bat call. White noise, for instance, is a broad sound that covers a lot of frequencies. It can jam a bat as effectively as a moth. The same is true with nighttime construction sounds. The novelty of Corcoran’s discovery is that it’s the first example of jamming within the same species.
This video shows a bat closing in on his prey. The sounds you hear are the bat's echolocating calls - the way he traverses his habitat at night. The sounds have been altered so we can hear them. More videos like this one can be found on Aaron Corcoran's site.
Corcoran found these calls by accident while on a research expedition at the Southwestern Research Station in Portal, Arizona back in 2013. He left North Carolina for this sunny state to record the jamming clicks of Bertholdia trigona, also known as Grote’s tiger moth. Moth clicks and bat call frequencies sit well above those audible by people, so Corcoran used a computer program to convert the sounds. Back in the lab, he was listening to the clicks when he noticed something different about the bat calls.
“I only ever heard this new sound when a second bat was also present and making the feeding buzz sound,” said Corcoran. “I had never seen that before.”
Corcoran supported his hypothesis with more experiments and more controls out in the field. He predicted that if a recording of the new call were played, the bat in question would miss its target. He changed the speed and frequency of the signal-jamming call to show that it wasn’t just any sound throwing these bats off their game, but that particular call at that particular frequency. Only the original had an effect, and it caused the capture rate to fall from 85 percent to 15.
Not everyone is as smitten with this new discovery as Corcoran is, however. Some chiropterologists felt there wasn’t enough evidence to say the bats were in fact jamming each other.
“There’s no doubt that there are bats doing this weird behavior,” says fellow Wake Forest University Ph.D. student Nick Dowdy, who also studies bat communication. “What’s in question is the ‘why.’ The details just have not been addressed yet.”
Dowdy doesn’t question the existence of these interesting new sounds, he just doesn’t think the reasons behind them have been proven. He thinks an alternative theory proposed by Cindy Moss at Johns Hopkins University has some merit. She says the bats could just be staking their claim for a particular moth. In other words, a bat sees a plump moth flutter through his territory, so he calls dibs, expecting other bats to respect the first-come- first-serve policy. Corcoran’s so-called battles may only occur when two bats try to call dibs on the same moth. Moss doesn’t believe these bats are jamming the calls of the other, they are simply arguing over who called dibs first.
Kirsten Bohn, a behavioral biology research professor also at Johns Hopkins University, who’s been studying Mexican free-tailed bats for years, says she’s on the fence. “The only reason I would question it would be that it doesn’t seem like the biggest payoff,” she says. “It seems like it would be more prudent to go find another insect.” Emitting feeding buzzes takes so much energy that Bohn thinks that no one moth would be worth it.
Other bat researchers are also in Bohn's camp. Ela Warnecke, a psychology and Brain Science Ph.D. student at Johns Hopkins who studies bat jamming with Cindy Moss, describes bat calls as voices. She says those voices tend to be similar within the same species. “Like you and me talking right now,” she says. “We have very similar voices. If we were bats in the same room, we would change our voices. I might talk a little higher and you a little lower, so we can distinguish between the frequencies more easily.” Bats usually avoid competing with each other by altering their calls, as Warnecke describes. That’s what makes Corcoran’s finding so intriguing. The calls he detects seem to show bats competing for one prey item instead of avoiding each other.
Corcoran’s 2014 Science paper didn’t address the potential relationships between the jamming bats, which is one of Dowdy’s main concerns. The two competing bats could be mother and offspring, which would offer another explanation for the strange calls. Mother bats teach their brood how to hunt properly by accompanying them outside the cave. The calls could be a teaching tool – upping the ante for the juvenile bat and making it more difficult to find the moth.
“I should say that it is true these calls are produced when two or more bats are competing for the same food item, and it is it true that it reduces capture success,” says Dowdy. “But the headline is ‘Bats jamming Bats,’ and that’s what we don’t know for sure.
Warnecke suggests running experiments in the presence of both too much prey and too little, which would reveal whether the bats only use the call among limited resources. She says that could test the jamming idea. In the presence of ample prey bats wouldn’t need to compete or use the energy needed to emit jamming calls. The opposite would hold true in conditions of scarcity.
Corcoran says most scientists agree with his theory and think his experiments back up his findings. Dowdy, Bohn, and Warnecke are among the few who remain unconvinced. The results of Corcoran’s paper challenge previously accepted theories and facts about bats, but it also inspires discussion. This new Mexican free-tailed bat call further proves that these animals have more in their repertoire than scientists previously thought. These furry, winged creatures are complex hunters, but maybe that’s just the beginning of their intelligence.
This article is also featured on the Boston University News Service website, along with more of my classmate's work.