The sonar-jamming discovery
Is it possible a moth -- that little fuzzy thing that flies in circles around your porch light at night -- has the capabilities of human military technology not invented until World War II? Answering this question eluded scientists since the possibility first arose with experiments conducted in the mid-1960s showing that bats abort attacks on prey if the sounds of tiger moths (of which there are around 11,000 species) were played from a speaker at just the right moment. Evidence for the jamming hypothesis has mounted since that time, however evidence against it mounted just as quickly and the scientific debate continued until I took on the question for my PhD research in 2008.
One of the primary difficulties in proving jamming (the idea that moth clicks somehow interfere with bat echolocation) was eliminating other possibilities. Other possible functions of moth clicks -- and one in particular -- had gained favor since those early experiments. The moth clicks may serve as a message to bats that the moths are distasteful. It has been known for some time that many tiger moth caterpillars (or wooly bears) feed on toxic host plants. They have immunity to the toxic compounds and amazingly save the toxins until adulthood, where they make the moths distasteful to bats and other predators. Moth clicks could have the same function as the bright coloration of other distasteful animals like poison-dart frogs and Monarch butterflies -- a warning to their predators. Bright colors would be ineffective at night, but sounds could easily get a bat's attention. This hypothesis -- termed acoustic aposematism or simply warning-- has now been proven many times over for several species of tiger moth. But showing one function for moth clicks does not eliminate other possible functions. Moths could be warning and jamming, or some moths might warn while others might jam.
For my PhD I wanted to test this hypothesis once and for all. Fortunately, before I even began, my advisor Dr. Bill Conner and his former graduate student Jesse Barber (now a professor at Idaho State University) had found the ideal moth for testing jamming -- Grote's tiger moth, Bertholdia trigona (the scientific name). Bertholdia had two key characteristics for testing jamming -- bats found them tasty, and they produced more noise than any other moth measured to date. While a distasteful moth could still be jamming bats, it makes it much more difficult to test jamming, because a bat could be avoiding the moth because of acoustic warning or because of jamming. We would have no way of knowing. With Bertholdia we could exclude the warning hypothesis as long as we knew the bats we used weren't already trained to think (unconsciously) of clicking moths as toxic moths. Bertholdia also made about ten times the noise as other moths. In other words, if any moth could jam a bat, this was the one.
When I arrived at Wake Forest University I conducted an experiment pitting Bertholdia against big brown bats in an indoor flight room equipped with special microphones capable of recording the extremely high-pitched sounds of bats and moths (sounds we normally can’t hear) and infrared video cameras to record video of attacks in complete darkness. One of the most difficult parts of the experiment was that we needed bats who we knew had never tried to catch clicking toxic prey. The only way could know this was if we got the bats before they had learned to fly and catch insects on their own. So we brought baby bats and their mothers into our flight room and gave them new homes. First we fed them a special baby bat formula, and eventually we raised them, teaching them to eat solid foods (mealworms) fly, and eventually capture moths off a fishing line in our flight room.
We also had to get the moths, which are only active about four weeks out of the year, and the only place we knew where to get them was south-east Arizona (where I now do much of my research). Jesse led a field crew in Arizona to collect the moths and ship them overnight to me at Wake Forest (in North Carolina), where I had to have the bats ready to go. The timing was perfect. Just as the bats had learned to fly and catch training moths, the experimental Bertholdia moths started arriving and I conducted controlled experiments for nine nights with each bat.
Amazingly, the bats behaved just as we had predicted according to the jamming hypothesis. Despite never having an opportunity to learn that clicking moths might be toxic (bats have to learn this, it is not innate), the bats only caught about two out of every ten Bertholdia when they clicked, and caught almost every moth on the fishing line that didn’t click, including some Bertholdia that we manipulated to not be able to click. We also found a very unique behavior of the bats. Normally bats make echolocation calls at faster and faster rates when they attack insects so they get information back more quickly. But when the bats heard the moth clicks, they did the opposite, they spent more time listening after each call, as though they were having difficulty hearing the echoes returning from the moths. This never happened in other experiments where bats attacked moths that clicked as a warning, so we were able to conclude that Bertholdia is the first animal ever shown to jam the echolocation (or sonar) of its predator.
As with any scientific discovery, this brought up many questions. The biggest one on my mind was, how does this work? What do the moth clicks do to the bat’s brain and echolocation to cause it to break off? You can read about my next study, entitled “How do tiger moths jam bat sonar?” (Coming soon).
This research was funded by National Science Foundation Grant # 0951160
Could a moth have the jamming technology of a military aircraft?
The dogbane tiger moth feeds on toxic plants as caterpillars, saving the toxins into adulthood. Its clicks warn bats of its distastefulness.
Bertholdia trigona was the perfect moth for testing jamming -- bats ate them if they got a chance, eliminating warning as a function of the clicks, and they produced the most noise of any moth measured.