Prick up your ears
Simple is the hearing organ of the descendant. Too simple, one thought, for the high-frequency ultrasound of a bat attacking. But one species doesn't fall on deaf ears for the nocturnal hunter.
There is probably nothing better for a bat lover than sitting outside on a warm summer night and listening spellbound to the ultrasonic calls of the night hunters flying by at the bat detector. For prey insects, possession of such a device would be downright life-saving: if they could hear the attackers, they would have a real chance of dodging the attack.
In fact, it has been believed for over fifty years that some moths can detect low-frequency ultrasonic calls. But shortly before the final attack, the bat rapidly increases the pitch and thus – so it is assumed – makes the calls just as inaudible for the insect as for us humans. Nobody believed that the simple folded ears could cover a sufficiently large frequency range.
It has not been clarified with certainty whether the bat lets the pitch rise in order to get a better spatial resolution, or whether it also wants to conceal its approach. In any case, it would be surprising if the insect world did not counteract this with suitable countermeasures - after all, both species have always been in an evolutionary clinch. And as it turns out, science has actually underestimated at least one species of moth.
James Windmill and his colleagues from the University of Bristol used a laser to measure the vibration behavior of the housemother's eardrum (Noctua pronuba), a common owl moth. Even the first tests revealed something surprising: the hearing organ reacted to simple tone impulses in a more complex way than would be expected from a simple vibration system.
But it only got really exciting when they sonicated the organ with simulated bat calls. Within fractions of a second, the researchers registered an increase in the resonant frequency of the eardrum, with the result that the high frequencies of the final approach also suddenly became audible to the moth. This effect was absent in dead animals, so Noctua pronuba is actively manipulating the pitch range of its ear.
The researchers had not expected that the moth would show such a feat. After all, its hearing organs, with a tiny eardrum and three motion-sensitive neurons, are among the simplest in the animal kingdom. Two of the nerve cells are used to transmit acoustic signals, but it is not possible to say with certainty what the task of the third one is - perhaps it is responsible for precisely this acoustic fine-tuning.
Further investigations showed that the volume of the ultrasound plays the decisive role. If it is as high as that of a bat ten meters away, there is no lightning-fast adaptation. If, on the other hand, the robber is only about three meters away, the folding ear resets itself immediately. Noctua pronuba even keeps its pricked ears for a few minutes – probably in case the unsuccessful bat turns around and launches a new attack. So are there equal opportunities in the night sky after all? That depends on what other tricks the opponents have in store. Some moths, for example, are known to stop flapping their wings as soon as they hear ultrasonic sounds. Some bats, on the other hand, prefer to forgo the last phase of target location and pounce "blindly", so to speak, on the unsuspecting prey. If you get to deal with such an animal, then of course even the finest hearing will no longer help.
