stir of echoes

JuicedupjenlucAce Ventura, Jim Carrey's eccentrically zany pet detective, loves animals of all kinds, with one exception: bats. His distaste was a major source of humor in Ace Ventura: When Nature Calls, the sequel to the original smash hit, because the plot centered on a stolen sacred albino bat in Africa. True, on one level, bats are pretty much winged rodents, but from an evolutionary standpoint, they're fascinating creatures. Perhaps that's why many cultures ascribe to them a special symbolism. In Tonga and West Africa, the bat is indeed sacred, believed to be a physical manifestation of the soul. For the Chinese, bats represent longevity and happiness, and are considered lucky by the Polish, Arabs, and the inhabitants of Macedonia. Some Native American tribes deem the bat a bit of a trickster spirit. In American culture, we have the whole vampires-into-bats motifs, courtesy of the Dracula legends, not to mention certain tormented superheroes who have been known to adopt the costume of a bat for their nocturnal crime-fighting activities.

But Ace isn't alone in finding bats just a little bit icky, too. For one thing, there's that old wives' tale about bats getting entangled in people's hair. Chances are, if this happens to you, it's more likely there's an insect — like a moth, mosquito, or gnat — somewhere near the vicinity of your head (if not caught in your actual hair). Bats eat them, and they tend to hunt "on the wing" (i.e., mid-flight). While bats aren't completely blind, they do rely primarily on echolocation (sonar) to hunt for their prey: usually the aforementioned insects. That diving behavior is indicative of hunting for food, and is simply misinterpreted by more squeamish sorts as a personal attack.

I personally find the creatures fascinating; I used to visit the "bat cave" at the National Zoo in Washington, DC, on a regular basis in the summers, just to watch the fruit bats hang upside down, flit about, and occasionally emit those eerie high-pitched cries as they navigated their environment. So I was sorry to have missed an especially nifty paper at the Salt Lake City acoustics conference on something called "jamming avoidance" in big brown bats. Fortunately, Greg Edelman of the Naval Research Laboratory was at the session, and gave me the highlights during a break between sessions the next day. (The best part of scientific conferences is the informal chats after talks, wherein I get to pick peoples' brains.) 

The big brown bat is an actual species (Eptescius fuscus), not just a description of the animal. And apparently it really is possible to "jam" the frequency of a bat's ultrasonic signal. There was a time when scientists believed that tiger moths practiced a form of jamming to protect themselves from hungry bats, although the current accepted thinking is that the moths produce ultrasonic signal to warn bats that they are "chemically protected" — an attribute Wikipedia terms aposematism. (The concept reminds Jen-Luc Piquant of an anime film called Ninja Scroll, in which a female character was poisonous to the touch — a highly effective defense mechanism against things like rape, but also a curse, in that she and the man she loved could never consummate their relationship.)

Different species of bat use echolocation in slightly different ways. We all know that bats hunt and navigate by emitting ultrasonic pulses and using the returning echoes to determine the location, speed and distance of nearby objects or prey. This is called active echolocation. And several insects have evolved "bat sensitive ears" as a counter-measure, including the possibility of frequency jamming, which would disorient a bat. The big brown bat (BBB, for short) has what seems to be a unique strategy for protecting itself against jamming, according to James Simmons of Brown University.

BBBs emit "frequency modulated (FM)" signals ranging from 50 to 23 kHz. But Simmons has found that when trying to detect targets in a fairly wide band of noise (or out in the open), they tend to lengthen their sounds in such a way that the frequency modulation tapers off into what is almost a constant frequency segment. The result is that rather than having the signal's energy dispersed over the bat's full range, it is focused down to a much narrower range — between 24 and 28 kHz — which means more energy to the signal. You can hear the phase call of the big brown bat on the prowl for food here; graphically, it's represented at left, below. 

Bigbrownbatsearch And there's more! Simmons performed a series of experiments in which he tried "jamming" specific frequencies, beginning with the BBB's preferred narrowed range. He found that the bats responded by shifting their emitted frequencies up or down to  avoid the jammed frequency.  Apparently they prefer higher frequencies, but when Simmons jammed the top of their range, he found the bats shifted down to lower frequencies to compensate when necessary. It's a bit like what sometimes happens when I use that little FM transmitter device to play my iPod in the shiny new Prius. Occasionally, the chosen frequency experiences interference from an actual radio broadcast. That is, the iPod signal is jammed. I respond by switching to a different frequency that is not experiencing interference.

There's a lot of questions that remain unanswered, most notably, why does the BBB do this frequency shifting in the first place? Okay, perhaps to counteract any signal jamming ability its prey might have evolved, although the evidence for that is contested. And is there a physiological mechanism at work here that we might be able to emulate? Because apart from the biological and neurological interest, Simmons has a real-world application in mind: naval submarines use sonar for navigation, and are vulnerable to the same kind of jamming. If scientists can figure out precisely how bats manage to do this, they might find improved countermeasures for that kind of sonar interference.

Another species of gleaning bat — called the pallid bat (Antrozous pallidus) — uses active echolocation for navigation, but employs a passive approach for hunting, to better elude detection by its prey. Basically, it listens for noise generated by insects, and thus has both external and internal "ears" — what amounts to two parallel auditory systems. A recent paper on the topic appeared in the Journal of Comparative Neurology.

The passive approach might have evolved in response to its prey's sensitivity to ultrasound — itself probably an evolved ability to evade hungry bats. For instance, David Yager of the University of Maryland has studied the interactions of bats (predator) and praying mantises (prey). Yager made his name in the field while still a graduate student at Cornell University, when he discovered the praying mantis has a single ear located in the center of its chest, that is highly sensitive to ultrasound in the frequency ranges commonly used by bats.

Researchers had assumed that the ultrasound disoriented the insect, since it tended to respond by swirling and diving erratically in flight. But Yager found that those movements were a defense mechanism, not a vulnerability. When a bat locks onto a target, it begins emitting its pulses very rapidly, a kind of climactic "feeding buzz." It makes sense: in order to get the most precise information possible about its prey's speed, distance and location, the bat needs as much feedback as possible, so it sends out many more pulses. The mantis senses the "feeding buzz," realizes it's been locked on as a target, and performs a series of evasive maneuvers similar to what jet pilots would execute to evade an incoming missile. A species of moth, Noctuidae, employs a similar defensive strategy. They're also equipped with a special hearing organ sensitive to bat sonar; the muscle twitches in response to the ultrasonic pulses, and the moth embarks on a series of evasive aerial maneuvers, just like the praying mantis.

There's always a danger of misinterpretation when studying living creatures. (Edelman mentioned a famous experiment by B.F. Skinner concerning the behavior of pigeons; I wasn't familiar with the story, but Skinner apparently got things very wrong.) It's not like the bats can help clarify their behavior by answering probing questions. So I don't quite know what to make of another paper at the ASA meeting by Robert Dooling (University of Maryland) — which has already been featured on Science Daily and mentioned by Afarensis, with more coverage no doubt to come — on what frequency ranges dinosaurs could hear.  I mean, we have a hard enough time figuring out why bats and insects do the things they do; how much more difficult is it to decipher the behavior of creatures long extinct?

But that's exactly what Dooling and his cohorts are attempting to do, starting with the knowledge that the inner ears of birds, crocodiles and dinosaurs are very similar in structure, most notably when it comes to a part called the basilar membrane. They found a correlation between body mass of a species and the size of that membrane. Smaller, lighter creatures, like birds, can hear higher frequencies that larger, heavier species, like elephants or dinosaurs.

Based on their analysis, Dooling's team concluded that dinosaurs probably heard things in the lower range of the frequency spectrum, extending, at most, to about 3 kHz (equivalent to a conventional telephone). They probably couldn't hear higher frequencies, such as the high-pitched sounds of birds. Which is ironic, since current paleontological theory holds that today's birds are probably the closest living relatives of the extinct dinosaurs. And all this time, the frustrated birds were convinced the dinosaurs were just ignoring them. It's also good news for any Young Earth Creationists who believe man and dinosaurs existed at the same time. Human voices fall around 8 kHz (although we can hear up to about 20 kHz). Sometimes it's a good thing not to be heard.

[UPDATE: An eagle-eyed commenter rightly says "Gotcha!" The 8 kHz refers to overtones in the human voice; the actual frequency range is much broader, and part of it would indeed fall within the range og what the researchers think dinosaurs may have been able to hear. And that's bad news for the YECs. I'd advise them to speak in high-pitched falsetto… you know, just in case.]

13 thoughts on “stir of echoes”

  1. “Human voices fall about 8 kHz”?
    Gotcha, Jennifer. We can make OVERTONES that high, but most of the sound pressure level and information transmission in a human voice occur between 80 and 1100 Hz, not kHz. (Wikipedia citation). The highest note sung in opera is F6, 1397 Hz.
    As someone who can’t hear much above 2000 Hz without my hearing aids, I can testify that human speech is easily understood in the frequencies below 2 kHz.
    And 20 kHz is the upper limit of detection for those with excellent hearing. Primarily kids.

  2. Thanks for the correction! I briefly considered checking that number, but it was late, I was exhausted, and why deprive my eagle-eyed readers of the chance to weigh in? 🙂

  3. There is the famous essay by Thomas Nagel “What Is It Like to Be a Bat”, which has more to do about conscious perception than bats, and an interesting essay by Scigliano called “Through the Eye of An Octopus”.
    It is always good to deflect anthropomorphic solipsism, and consider our
    other inhabitants on this planet.

  4. “The concept reminds Jen-Luc Piquant of an anime film called Ninja Scroll, in which a female character was poisonous to the touch –”
    Hmmm–I thought that was a generic female trait..;)

  5. Jennifer wrote “Well around my house it’s all about the Litte Brown Bat (Myotis lucifugus) — another actual name.”
    Some years ago a Little Brown Bat took up residence between the screen and glass of a window in our bedroom — there was a hole in the screen I hadn’t noticed but which it found. Once the bat took up residence, of course, I couldn’t repair it. In the morning it would be tucked up in there sleeping. In the evening it would wake, stretch, yawn (pointy little teeth!) groom, and wiggle out to go hunting. Watching it was like watching a miniature cat. It stayed for a month or so, and then disappeared. 🙁

  6. “more like a flying weasel”…They aren’t rodents!
    Hmmm no, more like a Chiroptera Suidae Sus domestica 😉

  7. Or, from “Calvin and Hobbes”:
    “What am I going to do about this report on bats? You’ve got to help me, Hobbes! OK, … um, first let’s make a list of what we know. Yeah! That’s a good way to start! Great! Number one: what are bats? They’re bugs, aren’t they? Yeah, put that down. #1 BATS = BUGS Are you sure? They fly, right? They’re ugly and hairy, right? C’mon, this is taking all day!”…
    ” Calvin- “Hi Susie! Did you write your report? Yeah, I spent all last evening on it. Did you? Well, when you know as much as I do, it doesn’t take as long. Mine took about 15 minutes. 15 minutes? Let’s see. I guess you won’t be setting the grade curve THIS time, Susie! Read it and weep. “Bats: the big bug scourge of the skies.” Note the professional clear plastic binder.
    Susie – “Bats aren’t BUGS!”

  8. BTW, There is a “Bat cave” in Toronto’s Royal Ontario Museum (ROM) based on Jamaica’s St. Clair Cave. Unfortunately, it’s only a diorama, so there’s no pungent smell of guano.
    My question is about vegetarian bats such as Fruit bats and other pollinators. Do they use their Echolocation purely for travel? Surely they wouldn’t need sonar to detect flowering plants?
    Nevermind, I looked it up…they have a great sense of sight and smell. (So not all bats are carnivorous)

  9. My hearing was tested up to 40 KHz. Ultrasonic anti-theft systems in stores used to drive me nuts. Now, i just don’t go to those places. There’s a belle curve, and i live just outside it.

  10. Aren’t humans just a sort of big omnivorous non-flying bat?
    Nearly one-fifth of all mammal species are bats.

  11. Thank you for teaching me something I didn’t know – and I work with bats all the time. The ‘jamming frequency’ is very interesting. I wish more people where like you and took an interest in bats, and appreciated the good they do. Too many people fear and hate these beautiful creatures that share the planet with us.

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