Original Paper: Corcoran, A. J. and W. E. Conner. 2012. Sonar jamming in the field: Effectiveness and behavior of a unique prey defense. Journal of Experimental Biology 215: 4278−4287.

Bats prey on nocturnal moths using echolocation. A bat pursuing prey emits ultrasonic signals that reflect off flying moths and echo back to the bat, enabling it to home in on the prey. Moths can “hear” ultrasonic bat signals through paired auditory organs (tympana) that respond to both low- and high-intensity sound waves. The moths have evolved evasive behaviors to evade capture.

Some moths use rapid changes in flight behavior, such as turning away or rapid dives, to evade capture. Others use unique tymbal organs, located in the thorax, to produce ultrasonic clicks. Researchers hypothesized that these clicks serve as echolocation-jamming mechanisms. To test this hypothesis they used tiger moths (Bertholdia trigona). One group of moths (N = 41) had their tymbal organs punctured, making them incapable of producing sounds (silenced), and another group (N = 38) were left with intact tymbal organs (clicking). Both groups were released into bat feeding areas. The fates of the released moths were documented with video and audio recordings. Results of the experiment are shown in the figures.

In Figure A, numbers over the bars show the number of attacks when silenced or clicking moths exhibited a particular evasive behavior. Statistically, there was no difference between behaviors or between silenced moths and clicking moths.

In Figure B, numbers over the bars represent the number of captures that were made when silenced or clicking moths exhibited a particular evasive behavior. In all three categories the difference between silenced moths and clicking moths was statistically significant.

Questions

Question 1

According to the data, moths with disrupted tymbal organs displayed the same types of evasive maneuvers as moths with intact tymbal organs. There is no statistical difference in the evasive maneuvers made by silenced versus clicking moths. This finding demonstrates that the moths with the disrupted tymbal organs are still capable of “hearing” the echolocation signals coming from predatory bats and of responding with evasive maneuvers. If a significant difference had been seen in Figure A, this could indicate that tymbal organ disruption impeded a moth’s ability to either hear the bat signals or respond with an evasive behavior.

Question 2

According to the data, moths with intact tymbal organs were captured significantly less often than moths with disrupted tymbal organs. This is true for all silenced moths no matter what type of evasive maneuver they used to escape bat predation. If evasive maneuvers alone were responsible for moth escapes, we would not expect the number of captures between clicking and silenced moths to be significantly different. The fact that they are significantly different supports the hypothesis that the clicking of the tymbal organ serves as an echolocation-jamming device, which when paired with evasive maneuvers results in fewer captures.

Question 3

Different frequency sound waves cause flexion of the basilar membrane at different locations because the basilar membrane varies in thickness and stiffness. At the base it is most stiff and thick and is only flexed by high frequency pressure waves in the cochlear canals. At the apical end it is thinner and less stiff and is flexed by low frequency pressure waves in the cochlear canal. The bat can hear higher frequency sounds than humans can and therefore their basilar membranes must be thicker and stiffer at the basal end than are the basilar membranes of humans.