Insect-eating bats use a kind of sonar to detect their prey in the dark. A new study shows some moths can interfere with that sonar to avoid getting eaten.
Wake Forest University biology professor Bill Conner studies the evolution of animal communication and the predator-prey interactions of bats and moths. He says bats have an interesting system for finding their prey, called echolocation, or biological sonar.
"They send out high-frequency cries, and they listen carefully for the echoes," he explains.
From those echoes, bats can construct an acoustic map of their surroundings - including the exact location of their flying insect prey. In response, many insects, including moths, have evolved sensitive ears that can hear the bats' ultrasonic chirps.
Connor says that when moths hear a bat closing in, they take evasive action.
"They simply turn away, or they'll go into spirals and power dives, and it results in a real aerobatic contest between the bats and the moths."
What is all that noise about?
But bats can still catch moths, in spite of those evasive maneuvers. So some moths have taken their defense strategy one step further. As Conner puts it, "These moths answer back."
Conner's graduate student Aaron Corcoran says scientists have proposed three hypotheses to explain what these moth sounds might be doing to the bats. The first is that the sounds startle the bats.
"Just by being loud and unexpected, the bats are taken off guard, and in a crucial moment, the moth is able to escape," Corcoran says.
The second hypothesis, Corcoran says, is that the moths are communicating to the bats, "Don't eat me, I'm poisonous!" The third possibility is that the sounds are interfering with the bats' echolocation system, in a way that keeps the bats from being able to detect and capture the moths.
Most previous research has supported the second hypothesis: that toxic moths use the sounds as a warning to would-be predators. But then Conner and Corcoran became interested in one particular species of tiger moth, Bertholdia trigona, because, says Corcoran, "it makes a whole lot of sound." As many as 450 clicks in one-tenth of a second, in fact.
"This seems like much more sound than is needed to startle or to warn a bat."
Moths go on the defensive
So Corcoran brought bats and moths together in a laboratory. He expected the bats to behave differently, depending on whether they were being startled, warned or jammed. If the moths were startling the bats, the bats should avoid the moths at first, but then get used to their sounds and start catching them. If the moths were warning the bats that they were toxic, the bats should catch the moths at first, then learn to associate the sounds with the bad taste, and stop trying to catch the moths altogether.
"But that's not what we saw. When the bats were attacking the moths that were clicking, they had a really hard time catching them. They only caught the clicking moths maybe one in five times."
The moths' defense was effective right away, and it continued to work through all seven days of the experiment. Corcoran says the sounds the bats were making also suggested that the moths were jamming their sonar. What exactly does a confused, sonar-jammed bat sound like?
"Well, in order to understand that, you have to know what kind of sounds they make when they're not confused."
When a bat goes after an ordinary moth, its echolocation chirps start out slow, then speed up as the bat hones in on its victim. But when the bats in Corcoran's experiment tried to catch a tiger moth, just as the bats would normally have been speeding up their sonar, "the moth clicks came on full blast, and then the bats slowed down their echolocation series - the opposite of what they would normally be doing. And then they sped up again, trying to make a catch, but there was no time, and the attacks were mainly unsuccessful."
Moths ahead - for now - in the evolutionary arms race
Corcoran's advisor Bill Connor says their work is the first to show that one species can disrupt the biological sonar system of another. Corcoran emphasizes the evolutionary significance of their findings.
"This study showed a new level of escalation in the bat-moth evolutionary arms race."
He says that the fact that a very small moth is able to produce enough sound to overcome such a sophisticated acoustic predator shows "a level of sophistication that's rarely seen in nature."
Aaron Corcoran is continuing his work at a research station in southeastern Arizona, where 18 species of bats battle it out every night with 30 species of clicking tiger moths. He'll be looking to see if any of the bats have found a way to overcome the moths' sonar jamming defenses: what he calls the next level of escalation in the bat-moth evolutionary arms race. His study is published in the journal to a slowed-down recording of the echolocation chirps of a big brown bat as it attacks and catches a silent moth. Audio provided by Aaron Corcoran.
to a slowed-down recording of the echolocation chirps of a big brown bat and the defensive clicks of a tiger moth as it successfully evades attack. Audio recording copyright Science/AAAS.
The first video shows a tiger moth escape the attack of a big brown bat by making a series of loud clicks that jam the bat's sonar. The video and audio have been slowed down by a factor of 15. Video copyright Science/AAAS
The second video shows a big brown bat catching a tiger moth whose sound-producing organs, or tymbals, have been pierced so that the moth can no longer make its characteristic defensive clicks. The video and audio have been slowed down by a factor of 15. Video copyright Science/AAAS