Death metal bat

My favorite wild animals are bats.

I think the flying fox -- a large diurnal species of fruit bat -- has got to be one of the coolest animals in the world.  Think about how amazing it would be, being a flying fox.  You have great big wings and can fly anywhere you want, you get to eat figs and dates all day, and you're cute as the dickens.  What could be better than that?

Fruit-eating sky puppies, is what they are.

[Image licensed under the Creative Commons Trikansh sharma, Eye contact with flying fox, CC0 1.0]

Unfortunately, bats in general have gotten a bad name, even though they're unequivocally beneficial.  (The insectivorous kinds can eat up to a thousand small flying insects -- including disease-carrying mosquitoes -- in an hour.)   The negative reputation comes from two sources: first, an association with drinking blood (only three out of the thousand species of bats do that; all three live in South America and almost never bite humans); and second, that they carry rabies (which can happen -- but so do raccoons, foxes, skunks, feral cats and dogs, and even deer).

Bats are good guys.  They're also incredibly cool.  I did a piece last year about the wild adaptations for echolocating in nocturnal bats, an ability I still find mind-boggling.  Which is why I was so psyched to run across a paper this week in PLOS-Biology about the fact that their ability to produce such an amazing array of sounds is due to the same feature death metal singers use to get their signature growl. 

In "Bats Expand Their Vocal Range By Recruiting Different Laryngeal Structures for Echolocation and Social Communication," biologists Jonas Håkonsson, Cathrine Mikkelsen, Lasse Jakobsen, and Coen Elemans, of the University of Southern Denmark, write:

Echolocating bats produce very diverse vocal signals for echolocation and social communication that span an impressive frequency range of 1 to 120 kHz or 7 octaves.  This tremendous vocal range is unparalleled in mammalian sound production and thought to be produced by specialized laryngeal vocal membranes on top of vocal folds.  However, their function in vocal production remains untested. By filming vocal membranes in excised bat larynges (Myotis daubentonii) in vitro with ultra-high-speed video (up to 250,000 fps) and using deep learning networks to extract their motion, we provide the first direct observations that vocal membranes exhibit flow-induced self-sustained vibrations to produce 10 to 95 kHz echolocation and social communication calls in bats.  The vocal membranes achieve the highest fundamental frequencies (fo’s) of any mammal, but their vocal range is with 3 to 4 octaves comparable to most mammals.  We evaluate the currently outstanding hypotheses for vocal membrane function and propose that most laryngeal adaptations in echolocating bats result from selection for producing high-frequency, rapid echolocation calls to catch fast-moving prey.  Furthermore, we show that bats extend their lower vocal range by recruiting their ventricular folds—as in death metal growls—that vibrate at distinctly lower frequencies of 1 to 5 kHz for producing agonistic social calls.  The different selection pressures for echolocation and social communication facilitated the evolution of separate laryngeal structures that together vastly expanded the vocal range in bats.

NPR did a story on the research, and followed it up by talking to some death metal singers, all of whom were pretty fascinated to find out bats can do it, too.  "In a [masochistic] sort of way ... I think that when I can feel that my vocal cords are getting kind of shredded or beat up, that it sounds better," said Chase Mason, lead singer of the band Gatecreeper.  "You know, like, if there's a little taste of blood in the back of my throat, I think that I'm doing a good job...  A lot of people will compare you to sounding like a bear or something like that, like an animal growling or roaring even... I think it's cool.  It's very dark and gothic.  The imagery of a bat is always associated with the darker sort of things, like vampires and stuff.  So it definitely makes sense."

I'm still more favoring the Sky Puppy model of bats, but hey, I'm not arguing with a guy who can make noises like Chase Mason can.

In any case, add one more thing to the "cool" column for bats, which was pretty lengthy already.  It's incredible that however much we learn about nature, there are always ways it'll come back and surprise you.  That's why if you have a curious side, learn some science -- you'll never be short of new things to wonder at.

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The song of the bat

My favorite animal is the flying fox.

(Don't tell my dogs.)

[Image licensed under the Creative Commons Andrew Mercer (www.baldwhiteguy.co.nz), Grey headed flying fox - AndrewMercer IMG41848, CC BY-SA 4.0]

What's not to like?  They can fly, they get to eat dates and figs all day, and they have the cutest faces ever.

[Image licensed under the Creative Commons Anton 17, Lesser short-nosed fruit bat (Cynopterus brachyotis), CC BY-SA 4.0]

Fruit-eating sky puppies, is how I think of them.

I have to admit, though, that the fruit bats and flying foxes ("megachiropterans," which is Greek for "big hand-wing") are not as astonishingly weird as their cousins, the "microchiropterans" ("little hand-wing") such as the little brown bat (Myotis lucifugus) familiar to us here in the northeastern United States as a nocturnal insect hunter.  I was thinking about these fascinating animals because I'm reading the book Sensory Exotica by Howard C. Hughes, which is about animal sensory systems, and after I said, "Wow!" for the tenth time, I thought they deserved a post.

You probably know that the nocturnal insectivorous bats hunt using sonar -- they emit sounds, then by the echoes locate their prey and scoop it up.  But what you may not have considered is how stunningly complicated this is.  Here are a few things they have to be able to accomplish:

1. Use the echoes from a tiny object like an insect to tell not only what direction it is, but how far away it is.
2. Determine whether the insect is moving toward them or away from them.
3. Determine whether the insect is straight ahead, or to the right or left of them.
4. Decide if the thing they're detecting is an insect at all -- i.e., food -- or something inedible like a fluttering leaf.
5. Given that most bats live in groups -- in the case of the Mexican free-tailed bat (Tadarida brasiliensis) groups of millions in the same cave system -- they have to be able to distinguish the echoes of their own calls from the echoes (and the calls themselves) of their neighbors.
6. Since an echo is much fainter than the original noise, they have to call loudly.  Microchiropteran bats emit calls at about 130 decibels, which is louder than a nearby jet engine or an overamplified rock band.  If their calls weren't so high-pitched -- usually between 30,000 and 40,000 hertz, while even a human with excellent hearing can only detect frequencies lower than 20,000 hertz -- their noises would be deafening.  So how don't they deafen each other, or themselves?

The first one -- the prey range -- they figure out by the delay between the call and the echo.  The closer the insect is, the faster the echo comes back.  We're talking about tiny time intervals, here; for an insect 3.4 meters away, the echo would arrive ten milliseconds after making the call.  So as something gets closer, the echo and the call actually overlap, and the degree of overlap tells the bat it's heading in the right direction.

As far as whether the insect is flying toward or away from the bat, they do this by picking up the Doppler shift of the echo as compared to the pitch of the original call.  You've all heard the Doppler shift; it's the whine of a motorcycle engine suddenly dropping in pitch as it passes you.  So if the pitch of the echo is higher than the pitch of the original call, the insect is coming toward the bat; if it's lower, it's flying away.

Even more astonishing is that they can tell whether an insect is to the right, left, or straight ahead by computing the delay between the echo arriving at their ears.  If it arrives at the right ear first, the insect is the the right, and vice versa; if the echo arrives at both ears simultaneously, it's straight ahead.  Here, we're talking even smaller time intervals; the delay they're sensing is less than a thousandth of a second.

Experiments have shown that bats actually are so sensitive to the quality of the echo that they can tell not only if the sound has echoed off an insect or something else, but if it's an insect, what kind of insect it is.  Experiments have shown that horseshoe bats (Rhinolophus spp.) prefer moths over other types of nocturnal insects, and their sound analysis systems are able to tell the echo coming from the large flapping wings of a moth from the echo coming from the smaller and faster wingbeats of a mosquito or fly.

Okay, now into the part that to me, almost defies belief.  How do they detect their own calls and echoes, and distinguish them from those of their friends?  Each bat recognizes its own call because each call is tuned to a slightly different frequency, and the bat's brain learns to respond to that one frequency and no other.  They can detect a difference between sounds that are only three hertz apart (remember, their calls are in the range of thirty to forty thousand hertz).  But this engenders a problem, the solution to which is mind-boggling.

Remember the Doppler shift?  The echo changes frequency depending on whether the object they're echolocating is coming toward them or away from them.  So how does this not move the frequency of the sound outside of the range the bat is sensitive to?  Put another way, how do they tell that what they're hearing is an echo of their own voice, and not the call of a bat who vocalizes at that (different) frequency?

The answer is that they tune their voices as they go, and do it with a pinpoint accuracy beyond what any trained opera singer could accomplish.  If they hear a sound that could be an echo or could be the voice of a nearby bat, they test it by changing the pitch of their voice.  If the pitch of the echo also changes, it's their voice, not that of another bat.  Further, they tune their voices so that the highest brain response occurs if the conditions are optimal; the echo is exactly what would indicate that it's a bug of the right species coming toward them at a particular speed.  When the frequency of the echo drops into that range, it's like Luke Skywalker using the targeting computer in his X-wing fighter.  Target locked in!  Bam!

Lunchtime.

If you think that's wild, consider the last one.  How does a bat not deafen itself, if its calls are loud enough to create an echo from a tiny object that its sensitive ears can pick up?  Seems like it's a self-limiting system: if the calls aren't loud enough, the echo is too faint; if the calls are sufficiently loud, the call itself will be disastrously loud for the bat's own ears.

This is solved by an ingenious mechanism.  When the bat vocalizes, a set of tiny muscles connected to the bones of the inner ear (which are the same as ours, the hammer, anvil, and stirrup) pull on the bones and move them away from each other, temporarily diminishing the bat's ability to hear.  As soon as the call is made, the muscles relax and the bones move back together, restoring the bat's hearing.  This needs to happen in an astonishingly short amount of time; recall that the time between call and echo is measured in milliseconds.  But this is what they do -- induce deafness for a fraction of a second, and restore hearing in time to pick up the echo!

So that's our look at the astonishing coolness of nature for the day.  We should appreciate bats; not only are they not the bad guys depicted in horror fiction, they are fantastic predators on animals a lot of us don't like -- nocturnal insects.  Microchiropteran bats can eat on the order of three hundred insects an hour, all night long; at night, hunting is pretty much all they do.  The aforementioned enormous colonies of Mexican free-tailed bats are estimated to eat five hundred thousand kilograms of insects every night.

Which, I have to admit, puts even my favorite fruit-eating flying foxes to shame.

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Saber-toothed tigers.  Giant ground sloths.  Mastodons and woolly mammoths.  Enormous birds like the elephant bird and the moa.  North American camels, hippos, and rhinos.  Glyptodons, an armadillo relative as big as a Volkswagen Beetle with an enormous spiked club on the end of their tail.

What do they all have in common?  Besides being huge and cool?

They all went extinct, and all around the same time -- around 14,000 years ago.  Remnant populations persisted a while longer in some cases (there was a small herd of woolly mammoths on Wrangel Island in the Aleutians only four thousand years ago, for example), but these animals went from being the major fauna of North America, South America, Eurasia, and Australia to being completely gone in an astonishingly short time.

What caused their demise?

This week's Skeptophilia book of the week is The End of the Megafauna: The Fate of the World's Hugest, Fiercest, and Strangest Animals, by Ross MacPhee, which considers the question, and looks at various scenarios -- human overhunting, introduced disease, climatic shifts, catastrophes like meteor strikes or nearby supernova explosions.  Seeing how fast things can change is sobering, especially given that we are currently in the Sixth Great Extinction -- a recent paper said that current extinction rates are about the same as they were during the height of the Cretaceous-Tertiary Extinction 66 million years ago, which wiped out all the non-avian dinosaurs and a great many other species at the same time.  

Along the way we get to see beautiful depictions of these bizarre animals by artist Peter Schouten, giving us a glimpse of what this continent's wildlife would have looked like only fifteen thousand years ago.  It's a fascinating glimpse into a lost world, and an object lesson to the people currently creating our global environmental policy -- we're no more immune to the consequences of environmental devastation as the ground sloths and glyptodons were.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!] 

Tuning in and tuning out

Mexican free-tailed bats (Tadarida brasiliensis) are remarkable animals.  They live in staggeringly huge colonies.  The largest known, in Bracken Cave in Comal County north of San Antonio, Texas, has twenty million bats.  I got to see a smaller (but still impressive) colony, in Carlsbad Caverns National Park, New Mexico, when I was a kid, and I'll never forget the sight of the thousands and thousands of bats streaming out of the cave mouth at dusk.

Free-tailed bats echolocate, which you probably already knew; they navigate in the dark by vocalizing and then listening for the echoes, creating a "sonic landscape" of their surroundings accurate enough to snag an insect out of the air in pitch darkness.  But this engenders two problems, which I honestly never though of until they were brought up by the professor of my Vertebrate Zoology course when I was in graduate school:

1. If these bats live in groups of millions of individuals, how do they tune in to the echoes of their own voices, distinguishing them from the cacophony of their friends and family all vocalizing at the same time?
2. In order to echolocate, they must have exquisitely sensitive hearing.  They're picking up the faint echoes of their own calls with an accuracy that allows them to detect the contours and motion (if any) of the object they're sensing.  To create an audible echo, they have to vocalize really loudly.  So how does the original vocalization not deafen those sensitive ears?

The answer to the first was discovered by some research at the University of Tübingen back in 2009.  Using recordings, scientists found that bats are sensitive not only to the echoes themselves, but can pick out from those echoes enough information about the sonic waveform that they can recognize their own voices.  Each bat's voice has a distinct, if not unique, sonic "fingerprint" -- much like human voices.

The answer to the second is, if anything, even more astonishing.  Just as humans do, bats have three tiny sound-conducting bones in their middle ear -- the malleus, incus, and stapes (commonly known as the hammer, anvil, and stirrup) -- that transmit sound from the eardrum into the cochlea (the organ of hearing).  Bats have a tiny muscle attached to the malleus, and when they open their mouths to vocalize, the muscle contracts, pulling the malleus away from the incus.  Result: dramatically decreased sound transmission.  But even more amazing, as soon as they stop vocalizing, the muscle relaxes -- fast enough to bring the malleus back in contact with the incus in time to pick up the echo.

Bats, it turns out, aren't the only animals to experience these sorts of problems.  The reason this whole topic comes up is because of some research that was published last week in The Journal of Neuroscience.  In a paper called "Signal Diversification is Associated with Corollary Discharge Evolution in Weakly Electric Fish," by Matasaburo Fukutomi and Bruce Carlson of Washington University, we learn about a group of fish called mormyrids (elephant fish) that have, in effect, the opposite problem from bats; they have to find a way to tune out their own communication so they can sense that of their neighbors.

Long-nosed elephant fish (Gnathonemus petersii)  [Image licensed under the Creative Commons spinola, Elefantenrüsselfisch, CC BY-SA 3.0]

Mormyrids communicate by electrical signals; the long "trunk" is actually an exquisitely-sensitive electrical sensor.  They not only use it to pick up electrical signals given off the nerves and muscles of the insect larva prey they feed on, they use it to pick up those sent by other members of their own species.  In effect, they talk using voltage.

Here, though, they have to be able to ignore the voltage shifts in the water around them given off by their own bodies.  It's as if you were in a conversation with a friend, and instead of doing what most civilized friends do -- taking turns talking -- you both babble continuously, and your brain simply stops paying attention to your own voice.

They do this using a corollary discharge, an inhibitory signal that blocks the higher parts of the brain from responding to the signal.  The researchers found that corollary discharges only occurred in response to voltage changes from the individual itself, and not to those from other individuals.

In other words, just like the bats, mormyrid fish can recognize their own communications.  "Despite the complexity of sensory and motor systems working together to deal with the problem of separating self-generated from external signals, it seems like the principle is very simple," said study co-author Bruce Carlson, in an interview with Science Daily.  "The systems talk to each other.  Somehow, they adjust to even widespread, dramatic changes in signals over short periods of evolutionary time."

So there you have it.  Another natural phenomenon to be impressed by.  It reminds me of the wonderful TED talk by David Eagleman called, "Can We Develop New Senses for Humans?" that talks about an animal's umwelt -- in essence, how it perceives the world.  What must the world seem like to a fish that gathers most of its information from electrical signals?

Staggers the imagination, doesn't it?

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Fan of true crime stories?  This week's Skeptophilia book recommendation of the week is for you.

In The Poisoner's Handbook:Murder and the Birth of Forensic Medicine in Jazz Age New York, by Deborah Blum, you'll find out about how forensic science got off the ground -- through the efforts of two scientists, Charles Norris and Alexander Gettler, who took on the corruption-ridden law enforcement offices of Tammany Hall in order to stop people from literally getting away with murder.

In a book that reads more like a crime thriller than it does history, Blum takes us along with Norris and Gettler as they turned crime detection into a true science, resulting in hundreds of people being brought to justice for what would otherwise have been unsolved murders.  In Blum's hands, it's a fast, brilliant read -- if you're a fan of CSI, Forensics Files, and Bones, get a copy of The Poisoner's Handbook, you won't be able to put it down.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]