In the eternal war between bats and insects, the bats’ primary weapon is echolocation – the biological sonar that lets them detect, track and attack flying insects in total darkness. It is so effective that scientists estimate bats eat enough insects to save American farmers more than $3.7 billion a year in reduced crop damage and pesticide needs.
In response, many insects have evolved countermeasures, especially biological “bat detectors:” ears tuned to hear the high-frequency calls of bats. That early-warning system sparks evasive aerobatics that can be quite effective.
|A Townsend’s big-eared bat chases a moth in Arizona. Photo courtesy of Aaron Corcoran
But another, more active alternative has been proposed. Graduate student Aaron Corcoran of Wake Forest University in North Carolina and his colleagues confirmed in laboratory experiments a few years ago that at least one tiger moth, Bertholdia trigona, produces ultrasonic “clicks” that actually disrupt, or jam, bats’ echolocation.
For the next step of his research, Corcoran wanted to study sonar jamming in the natural environment in hopes of uncovering new details of predator-prey interactions. Most previous studies, he said, were conducted in flight cages with captive bats and insects.
He planned to use the latest technology – infrared lighting and multiple video cameras – to accurately reconstruct bat attacks in full 3-D, without altering their behavior with bright, flashing lights. That turned into quite a chore.
He conducted fieldwork at the American Museum of Natural History’s Southwestern Research Station in the Chiricahua Mountains of Arizona. To attract bats and moths, he hoisted two ultraviolet lights on 10-foot poles in an open field. Sure enough, an hour after dark, a dense cloud of bugs swarmed around the lights and a half dozen bats took turns diving into this pool of prey.
Then the real challenge began. In order to convert videos of bats and moths into precise, 3-D flight trajectories, his three cameras had to be calibrated so their exact positions and viewing angles could be used to triangulate the locations of the bats and moths for each frame of the videos. The problem there was that he was working in a much, much larger area than such calibrations usually involve. He experimented for months without getting any usable calibrations.
So he did some serious research during the off-season and returned the next year with a “wand calibration” plan – basically spinning an eight-foot rod with a reflective ball on each end – that proved successful. After two months of testing and fine-tuning, he was ready to collect data.
Corcoran captured Bertholdia moths and removed the “tymbals” that are used to generate sound from half of them. The other half retained their clicking abilities. Each night, the moths were placed on small, heated platforms until the bats arrived. Then each moth took off, flying into the airspace of an attacking bat as Corcoran and his team recorded the encounter in video and ultrasound. The data were later analyzed in the lab.
“The results were amazing,” Corcoran said. Moths that could not make sounds were caught by bats 10 times more often than those that could jam the bats’ echolocation. This, he concluded, “makes jamming the most effective defense against bats ever documented in nature.”
And this tiger moth also employs evasive maneuvers in combination with jamming: it either flies away from the approaching bat or alters its flight by diving sharply. Not one of 24 moths recorded both diving and jamming was caught. And the jamming appeared to be effective against a number of bat species that use varied types of echolocation and flight behavior.
But perhaps most importantly, Corcoran, who’s now a post-doctoral fellow at the University of Maryland, says he has “worked out the kinks of an effective system for recording bats and moths in their natural environment. This opens a door to addressing dozens of questions that still surround the life-and-death battles between bats and moths, and for discovering still-unknown behaviors. One thing is certain: the more we look, the more we discover that bats – and the prey they pursue – are capable of remarkable feats that we can barely imagine.”
BCI Members can read the whole story of bats and sonar-jamming moths in the Fall 2013 issue of BATS magazine.