CLICKING AND DIVING TIGER MOTHS EVADE HUNGRY BATS

Abstract

Aaron Corcoran has had quite a few sleepless nights. The behavioural ecologist from Wake Forest University, North Carolina, has devised a setup to study the nocturnal battles between bats and moths in the wild, which involves recording moths’ evasive manoeuvres in the early morning hours. Working with William Conner, he wanted to find out just how moths achieve a great escape (p. 4278).In 2009, Corcoran and his colleagues first reported that tiger moths can jam bat sonar. ‘Tiger moths have sound-producing organs called tymbals, which produce ultrasonic clicks that disrupt bats’ echolocation, distorting the bat’s perception of where the moth is’, explains Corcoran. Having demonstrated sonar jamming in captive tethered moths, he was keen to test tiger moth escapology in a natural setting.To do this, Corcoran needed to find a way to follow individual free-flying bats and moths, record their sounds and capture split-second attack sequences on video. Working at the Southwestern Research Station in Arizona, he set up a 200 m3 observation area with high-speed video cameras to film the animals, ultrasonic microphones to record their calls and clicks, and UV lights on poles to attract the moths. To follow individual animals and create 3-D reconstructions of their flight trajectories, he used a specialised computer package to calibrate the cameras. But even with this advanced technology, Corcoran admits it was painstaking work. ‘It took 3 years to get to the point where we could start collecting data’, he says.First, Corcoran tested how effective sonar jamming is in the wild. He recorded encounters between bats and individual tiger moths – either tracked by eye from a release platform or identified by their distinctive clicks on the audio recording – and noted how many of the insects were captured and eaten. Then he silenced some tiger moths by puncturing their sound-producing organs, and released them to see how they would fare. He found that silenced moths were 10 times more likely to be caught by a bat than their clicking counterparts. ‘Sonar jamming is extremely effective’, concludes Corcoran.When he took a close look at the 3-D flight paths, he realised that moths also rely on two distinct evasive manoeuvres: diving and rapid fly-aways. ‘The most effective defence was clicking and diving at the same time’, says Corcoran, adding ‘Moths that did this always got away. It was too difficult for the bats to deal with sonar jamming while recalculating their flight trajectory to intercept a dive.’ But he was surprised to see that fly-aways were a last-ditch effort. He had reasoned that, since moths can hear bats’ echolocation calls from quite a distance, they would fly away sooner. ‘Perhaps having the security of the jamming defence allows tiger moths to get on with foraging and finding mates and waste less time avoiding predators’, he suggests.By measuring how successful different bat species were at catching their dinner in the field and in the lab, Corcoran was also able to conclude that sonar jamming is effective regardless of how well bats can fly or the echolocation frequencies they use. ‘This is pretty impressive,’ he says, ‘considering that moths are tone deaf and can’t tailor their jamming defence to the predator.’