Once a bat has locked its beam of sound onto a moth victim, there's little hope for the fluttering insect. As the bat closes in, it switches its call pattern, triggering the victim to fly erratically in a last attempt to elude the predator as it homes in. And all the time, the moth is listening to the attacker's ultrasonic calls. James Fullard is fascinated by moths' simple ears, which have mostly evolved to defend the moth from a single predator, the bat. He explains that moths hear their bat infested world through simple ears. Some species have as few as one sound receptor to detect the incoming threat,but how does this simple auditory system respond during the closing seconds of a bat attack? Working with his colleagues Jeff Dawson and David Jacobs,Fullard began playing recordings of a closing bat's cries and recording how the moth's ear sound receptors responded. The team expected to see the nerve impulses produced by the receptors rocket as the bat recording closed in, but were astonished when the moths' gave up warning of the impending attack, and continued firing as if the bat had suddenly retreated(p. 281)!
How bats and moths interact acoustically has fascinated scientists since the late 1950s, when Kenneth Roeder began analysing moths' responses to bats in his back yard. He discovered that noctuid moths, such as the underwing moths, have two auditory cells, called A1 and A2, while notodontid moths lack the A2 receptor, so they pick up the whole attack with the A1 cell alone. Roeder had suggested that the moth's vain attempts to escape capture were triggered by signals from the A2 cell, but how would the notodontids evade attack when they lack the A2 receptor?
This wasn't clear, and it was impossible to simulate the sound of an incoming bat from a moth's perspective, until James Simmons made some unique bat recordings; he trained a bat to attack a microphone in the lab. Fullard and his team decided to replay the final two seconds of Simmon's remarkable recordings to noctuid and notodontid moth ears, and recorded their nerve signals to see what the moths really heard.
As expected, the A1 cell burst into action as the sounds closed in, firing with increasing intensity as the bat approached. The team kept waiting for the A2 cell to begin firing, but the cell didn't respond strongly during the insect's final moments, so the A1 receptor tracks the attack from start to finish. Even more surprisingly, within the moth's final fifth of a second of life, the A1 cell suddenly gave up firing rapidly, and returned to signalling intermittently, as if the attack had failed and the bat was some distance away!
Fullard explains that there are two possible reasons for the moth's sudden deafness. The most obvious explanation is that there's little point in registering the final assault, as the moth has no chance of escape once the bat is within striking range.
But as predation by bats is the major evolutionary pressure on moth ears,Fullard has an alternative theory. He suggests that the hunters could have selected for failure of the A1 receptor, so that the moths final moment of peace improves the bat's chances of success.