Two weeks ago we were treated to a presentation by Dr. William Conner from Wake Forest University in which he discussed his research on the evolutionary arms race between tiger moths and one of their primary predators, bats. As most people are aware, nocturnal bats use echolocation for navigating the darkness, as well as for locating and targeting their prey. The science behind echolocation in bats is considerably more complex than most people realize. But the same can be said for the strategies that some insects use to evade hungry bats.
Bats have had to evolve strategies to successfully use sonar for navigation. More specifically they need to sort through the echoes from multiple screeches and determine what they mean. Consider a cave in which reside dozens or hundreds of flying and screeching bats all using sonar at once. Then you can begin to imagine how confusing it all could get. With so many signals overlapping one another it would be difficult to figure out what is an echo of your own call and what is your roost mate’s call or echo. Likewise, bats have very sensitive ears which they need for capturing faint echoes coming from very small objects that are moving around quickly in 3 dimensions. Also, the small ultrasonic screeches emitted by hunting bats, while too high pitched for human ears, are very loud – on the order of being near the stage at a heavy metal concert (between 120 and 140 decibels). So bats have developed physiological adaptations that prevent damage to their ears from the high intensity signals that they produce. Interestingly, the solution to these obstacles are some of the same technological breakthroughs humans have developed in the evolution of electronic sonar and radar technology. Of course, bats appear to employ them millions of years earlier.
Another problem, at least for creatures the size of bats, is the issue of energy use and conservation. Bats have among the highest metabolic rates for animals. Insectivorous bats have to eat about half their body weight in insects each night or they can risk starving. Most of vast majority of food that becomes part of a bat’s energy budget is used for flight and heat production. The remainder of the energy budget that is leftover is comparably small which makes it something that should be closely controlled. Generating the powerful, ultrasonic pulses that bats use in echolocation also requires a lot of energy, but are also highly useful to the bats. So over time bats have evolved energy conservation strategies are designed to help them last a little longer on the energy they have.
When they are just flying around, probing the night sky for possible obstacles or flying prey, bats send out occasional ultrasonic pulses – just enough to get an idea of what’s out there. But there is a trade-off in the resolution of the picture. You can think of the bat calls as a strobe light and the bat chirps as the individual flashes. Less energy is used if the strobe light pulses infrequently (say every 2 seconds), but its hard to track moving objects and it’s nearly impossible to play catch in this condition (I tested this hypothesis when I was about 11 years old). But if the light is cranked up to blink 10 times a second its easier to follow moving targets and to catch balls that are tossed to me. The trade off, however, is that this also requires 20x the amount of energy per minute. This is irrelevant in terms of the current cost of kilowatt hours, but a little energy can mean life or death to a bat. So one solution by bats is to strengthen the sonar resolution when it counts most. They do this by increasing the number of pulses they send out only when they are near moving prey or obstacles. You can hear this happening in the video below of a bat closing in on its moth prey.
Pretty cool, right? But it appears that this dance in the night sky is an old one and that some insects have developed countermeasures to this strategy. The basis for the “stop and drop” response by these moths is an evolved behavior in which they stop flying, retract their wings and fall to the ground. By falling out of the sky and into vegetation the threat posed by airborne, echolocating bats is neutralized.
But the stop and drop response to increasing bat echolocation pulses is not exclusive to moths and is known about in beetles too. It appears to be a convergent behavior that’s part of a shared, anti-predator, defense repertoire used by many nocturnal, flying insects. These insects shift through a series of defense responses depending on how loud the sound is (intensity) and how fast the strobe light-like flashes of sound are being made by the bats (frequency).
The response of a flying moth or beetle prey that senses bat echolocation is threefold and changes with the intensity and frequency of the bat’s signal. The first step is that the flying insect will change its course at the earliest signs of bat sounds in an attempt to avoid detection. If this doesn’t work and the moth determines that the bat is now tracking it, the second step is to start moving erratically and try to throw the bat off a successful trajectory (as did the moth in the picture above, top). If the echolocation pulses grow still louder and more frequent (which generally means that the bat is really close) then the moth or beetle stops flying and drops to the ground.
Of course, dropping into the darkness comes with its own dangers, particularly in the tropics. For a moth that falls to earth, death can just as easily come from falling into a pond and drowning or contact with soilborne diseases or dropping onto a predator’s dinner table (like ants, rodents, amphibians, etc.). To avoid this and to gain another layer of protection, some moths have developed another strategy for defense in the final stage of attack. Dr. Conner refers to this brief period of time as “the moment of truth,” which includes a second or two of rapid chirps from the bat and ends with a strike. Or a miss. But what could cause these warm-blooded, sonar-guided, finely-honed hunting machines to miss their mark?
As it turns out, its jamming technology. The moth are basically interrupting the bat’s ultra-sensitive sonar. Some of the tiger moths studied in the Conner lab possess sound producing structures called tymbals. The tymbals, like the rest of the moth’s exoskeleton, are made from chitin, a light and durable material. It’s also flexible like plastic. In fact, if you’ve ever played with an empty, plastic soda bottle you have probably mimicked the way tymbals make sound. By pressing in a side of a soda bottle you can make a popping sound by changing its shape. Then by letting the bottle pop back into place you make another pop. This principle is true for the tymbals on a moth, which are attached to muscle groups that contract and relax in rapid succession and popping the tymbals in and out of place. This creates a series of snapping sounds that resembles a “buzz”… at least to animals with ultrasonic hearing. The moths use sound from these, in conjunction with pheromones, to inform potential mates of their species and suitability as a mate. They also use these high-pitched buzzers to jam bat sonar signals.
Recall the first video in this post and how as the bat swooped in it ramped up its sonar signal and captured the tethered moth. Now compare that to the video above which shows “the moment of truth” and the bat coming in for the kill. You can hear the bat’s chirping becoming more rapid the closer it gets, and then loud buzzing sound which is created by the moth. This sound throws off the bat’s orientation, causing it to miss the target. It’s the bat equivalent of a left fielder loosing a pop fly ball in the sun. It’s pretty effective too with most attacks on buzzing moths failing to result in a capture. Dr. Conner discussed how one female big brown bat that was tested had failed in its attempts to capture a buzzing moth 38 consecutive times. For the 39th attempt the moth’s tymbals were silenced and the bat captured the moth straightaway.
Of course this is an evolutionary arms race. So when one adaptation is developed by moths, a counter-adaptation is evolved by bats. So it stands to reason that there is, somewhere, some behavior in bats that overcomes the “sonar jamming” and “stop and drop” behaviors of moths. And there is. Whispering bats (Barbastella barbastellus) have evolved echolocation pulses that are quieter allowing a bat to get closer to flying prey. This quieter call fails to trigger the defensive behaviors in moths that are in response to loud bat pulses.
Not to be left out, parasites of moths are also involved in this arms race. Obviously, if you live on a moth it’s important that your host survives, making your fates intertwined. It’s also fairly obvious that if you’re intent on having your moth live through nightly bat attacks where the only warnings are sounds, you need make sure your moth’s ears are in good, working order. This is no easy task if you’re a moth ear mite. Back in 1957, the acarologist Asher E. Treat published a paper on his observations of moth ear mites, with a surprising finding. Treat found that these mites only lived in one of the moth’s two ears. It was true for every single moth he sampled. Each case was the same. Female mites need to break through the ear drum to lay eggs inside, which causes complete deafness in that ear. However, any ear mites on a moth would sooner move to another moth than damage the second ear. Treat and others surmised that this was an adaption to prevent complete deafness in the host moths, which would render them prone to bat attack and decrease their chances of mating. Apparently the costs of riding on a deaf moth outweigh the benefits and behavior of using the second, perfectly good eardrum as a nursery.
Continuing study of this evolutionary arms race is fascinating in itself. It’s also fascinating to consider the convergent examples in human sonar and radar technology. In much the same way, this research is predictive of what likely exists in other biological systems that incorporate sonar. For example, less is known about echolocation in marine mammals and what countermeasures have evolved in their prey. In all likelihood they will be very similar to the behaviors in the bat and insect system.