In the eternal war between bats and insects, bats’ primary weapon is echolocation – biological sonar that lets them detect, track and attack flying insects in total darkness. Insect-eating bats emit very brief, high-frequency calls (mostly beyond the range of human hearing), then analyze the returning echoes to “see” objects as fine as a human hair. 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 cries of bats. That early-warning system sparks evasive aerobatics that can be quite effective. But a more active defense had long been suspected for at least some moths. My colleagues and I confirmed that hypothesis in 2009 by demonstrating conclusively that certain tiger moths (a group of moths with over 11,000 species) do indeed produce ultrasonic clicks that disrupt, or “jam,” bat echolocation.
Other tiger moth species have been shown to use similar clicking sounds in a quite different way: to warn predators that they feed on toxic plants and are distasteful, just as some diurnal animals, such as poison-dart frogs, use bright colorations as a bad-taste warning. Using controlled laboratory experiments, my colleagues and I showed that the jamming moth, Bertholdia trigona, is not toxic and bats clearly find it tasty. These moths produce about 10 times as many clicks as other moths that had been studied. When attacked by a bat, Bertholdia responds with a barrage of clicks (up to 4,000 per second) that cause the bat to alter its echolocation behavior and narrowly miss the prey.
For the next step of my research, I wanted to study sonar jamming in the natural environment, where I could uncover new details of these predatory interactions. In preparing for this work, I was surprised at how many key questions regarding bat-insect interactions remained unanswered. Most previous studies were conducted in flight cages with captive bats and insects. These include our own sonar-jamming research, which took place in an indoor room covered in sound-absorbing foam and moths suspended on lines from the ceiling. This was perfect for carefully controlling experiments, but too far removed from the natural world to see bats and moths dueling in all their glory. Studying aerial, fast-flying predators and prey in darkness, however, is a daunting chore.
Prior to my work, field studies typically involved scientists watching bats attacking insects at streetlights and noting the outcomes. The most sophisticated studies of bats and moths were done two decades ago using stroboscopic flash photography, with high-intensity bursts of light emitted many times per second. I 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 learning experience.
My fieldwork was conducted at the American Museum of Natural History’s Southwestern Research Station in the Chiricahua Mountains of Arizona. This area, where BCI conducts field-training workshops each year, is home to some 20 bat species. To attract bats and moths to my observation site, I hoisted two ultraviolet lights on 10-foot (3-meter) poles in an open field.
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. The moths’ sharp turns and dives were matched by the bats in what looked almost like synchronized swimming in the night sky. It resembled a full-scale battle rather than the individual combat I wanted to examine. But before long, I realized that the cloud of moths thinned considerably as the evening hours passed, until I could finally begin teasing apart individual attacks – one bat versus one moth.
Then the real challenge began. In order to convert videos of bats and moths into precise, three-dimensional flight trajectories, the 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. Normally, that’s done at a much smaller scale, typically in areas the size of a Ping-Pong table rather than the tennis-court expanse that I needed.
Calibration usually involves placing objects with known dimensions in view of multiple cameras, then capturing video that is analyzed with special software. For my oversized project, I experimented with placing large objects made of plastic pipe in the field, where I surveyed their locations as precisely as possible. Unfortunately, I had no training in surveying and was naively using the wrong equipment. Months of trying produced no useful calibrations.
I spent the off-season researching alternate techniques and eventually came up with a solution: the relative-orientation method, known as “wand calibration.” This involves placing a reflective ball at each end of a rod (or wand), which is mounted atop a pole and twirled around the space you are calibrating. The wand in this process is usually less than a yard (about a meter) long, but for my outdoor calibration, I scaled it up to 8 feet (2.4 meters) and attached it to a 16-foot (4.9-meter) pole. Repeatedly raising and twirling this behemoth throughout the area covered by our cameras was a sweaty workout, but after two months of testing, we achieved centimeter-level accuracy. And we still had two weeks left of our second summer to begin collecting data.
Our study organism – Bertholdia trigona – is the only moth species known to jam bats. It is a beautiful little pink and orange tiger moth with small clear windows on each of its forewings. In Arizona, the moths emerge as adults from their pupal cases as the summer monsoon rains begin around the middle of July. They remain active for only three or four weeks, and fortunately, they were abundant during the brief time left for our research that summer.
We set up blacklight sheets (bed sheets illuminated by ultraviolet lights that attract night-flying insects) around the research station to collect Bertholdia moths. We allowed half the moths to retain their ability to produce sounds. With the other half, we conducted a small surgery with miniature tweezers to remove the moths’ tymbals – the organs they use to make their jamming clicks.
Each night after setting up our equipment in the field, doing the calibration, and preparing the moths, we began our experiment. With help from volunteers at the station, we carefully placed each moth on a small heating pad on a platform near our setup. This was to ensure the moths would stay warm and ready to fly on cool nights. Then the bats arrived. We watched each moth take off from the platform and into the airspace of an attacking bat, recording each encounter in video and ultrasound. Later, in the laboratory at Wake Forest University, I meticulously analyzed these data, including converting the videos into 3-D flight trajectories using our calibrations and special software. It took a third field season to collect all the data we needed, and our results were analyzed and published in the Journal of Experimental Biology in December 2012.
The results were amazing. Moths that couldn’t make sounds (those with their tymbals removed) were caught by bats 10 times more often than moths that could jam the bats’ echolocation. This makes jamming the most effective defense against bats ever documented in nature.
And Bertholdia doesn’t rely on jamming alone. It also employs two evasive maneuvers in combination with jamming: it either flies away from the approaching bat or alters its flight by diving sharply. Diving and jamming proved to be the most effective combination. Not one of the 24 moths we recorded with both behaviors was caught.
We also found that jamming was effective against various bat species with different types of echolocation and flight behavior. The moths had hit upon a generalized jamming signal – one that works regardless of who is being jammed.
But perhaps most importantly, through years of study, trial and error and fine-tuning, we 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.
AARON CORCORAN, Ph.D., is a Post-Doctoral Fellow at the University of Maryland. More information about his research can be found at www.sonarjamming.com. These studies were conducted as part of his Ph.D. research at Wake Forest University in North Carolina and funded by the National Science Foundation.