How Nontoxic Moths Jam Bat Sonar with Ultasonic Pulses

Sep 22, 2011
Albert Burchsted

Bat Hunting Moth - Jessica Nelson

Nontoxic moths have no chemical defenses against bats, but produce clicks that jam bat sonar instead of mimicking toxic moths.

Many tiger moths are toxic to bats and, when they hear a bat approaching, emit ultrasonic clicks that inform the bat of their toxicity. After detecting these clicks, the bats leave the moths alone. When researchers first studied the ability of nontoxic tiger moths to emit clicks and avoid being captured by bats, they found many of these edible moths mimic the toxic moths. Recent work has shown that this may not be the case for all non toxic tiger moths, as at least one species, **, is able to jam the bat's sonar and escape predation because the bat cannot locate the moth.

Three Hypotheses

There are three hypotheses that could explain the manner in which the click pulses of nontoxic moths function in evading predation by bats.

Auditory mimicry where the pulses mimic the toxic moths' warnings.
The pulses serve to startle the bats as they approach the moth.
The pulses jam the bat's sonar.

These hypotheses can be tested by:

Measuring the tonal range, loudness, timing of presentation, and frequency of the pulses.
Presenting a series of nontoxic moths to naive bats, and watching the bats' responses. Naive bats are ones that probably had not attempted to feed on nontoxic moths, but which had fed on toxic moths.

There should be several differences between the moth signals and the responses by the bats:

Pulses That Mimic Toxic Moths' Warnings

Pulses should be similar in tone, loudness, timing, and frequency of production to the distinctive pulses of the toxic moths. A naive bat's response to a repeated series of presentations of nontoxic moths should show a gradual increase in captures over time as the bat learns that the pulses are false alarms. The bats should track the moths in much the same manner that they track the toxic moths, veering away from the moth just before contact.

Pulses That Startle Bats

The pulses should differ in tonal range from the toxic moth pulses, be louder, and should be delivered as a short burst with about the same timing (just before the bat contacts the moth) as pulses of a toxic moth. A naive bat's response to a repeated series of presentations should demonstrate an increase of capture rate over time as the bat becomes habituated to the startle stimulus. The bats should track the moths in much the same manner that they track the toxic moths, veering away from the moth just after the moth's pulses.

Pulses That Jam Bat Sonar

To mask the moth's location, these pulses should differ in frequency from the toxic moth pulses, be louder, and be delivered earlier and for a longer time than the toxic moth pulses. An individual bat's capture rate should remain relatively constant across time as the bat's sensory mechanism should not adapt significantly to a noisy signal. The bats should become confused as their prey becomes lost in a burst of noise and stop tracking the moth.

Findings

Toxic moths' click pulses are loud, of constant pitch, and delivered at a relatively slow rate. Corcoran's research found that pulses of B. trigona are very loud, fluctuate in pitch, and delivered at rates up to 4500 per second. Bats that approach moths emitting these clicks often pause in flight and/or veer away from the moth (first video in article) rather than continuing to pursue it. A bat's capture rate remains consistently low over a week's time.

Moths whose sonar-generating organs were silenced were readily captured by bats (second video in article).

Discussion

If a bat homing in on a moth hits a wall of noise, all echoes from its target will be lost in the noise and the bat will not be able to determine direction of flight, distance from, or speed of the target. Although a bat is much more resistant to injury than a moth, the bat would not take the chance of contacting an object that it was not prepared to capture. Even a moth could break a fragile wing bone and incapacitate the bat. Additionally, the bat would not know the identity of the prey and could be capturing a toxic species. Thus, it would be in the best interests of a bat to ignore the target and try for another.

The sounds generated by these moths are quite different from those predicted by either of the first two hypotheses. A burst of loud noise varying in pitch and generated at a high rate of speed amounts to a burst of white noise: a sound barrier through which a bat would not want to fly. If a bat homing in on a moth hits a wall of noise, all echoes from its target will be lost in the noise and the bat will not be able to determine direction of flight, distance from, or speed of the target. Although a bat is much more resistant to injury than a moth, the bat would not take the chance of contacting an object that it was not prepared to capture. An undetected twig, large moth, or even an undetected bat could break its fragile wing bones and incapacitate the bat. Additionally, the bat would not know the identity of the prey and could be capturing a toxic species. Thus, it would be in the best interests of a bat to ignore the target and try for another.

In response to the moth's sound pulses, the behaviors of the bats that approach them indicate that this scenario is probably going through the bat's mind:

Where did my target go? - The bat slows down in flight.
What am I flying into? - The bat stops active flying.
I'd better go someplace else. - The bat turns to a direction other than the last direction the moth was detected.

If the moth's sound burst is produced too early, too late, or for too short a time, the bat may be able to refocus on the moth and capture it. In most cases, however, it is better to skip a relatively easy meal than to be injured and become unable to fly, and the bats presented with prey of this kind constantly exhibit a lowered capture rate.

References

"The Adaptive Function of Tiger Moth Clicks Against Echolocating Bats: An experimental and synthetic approach." John M. Ratcliffe and James H. Fullard. 2005. J Exp Biol 208: 4689-4698.

"Tiger Moths Jam Bat Sonar" Aaron J. Corcoran, Jesse R. Barber, William E. Conner. 2009. Science 325: 325-327.