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!] 

What doesn't kill you

In evolutionary biology, it's always a little risky to attribute a feature to a specific selective pressure.

Why, for example, do humans have upright posture, unique amongst primates?  Three suggestions are:

• a more upright posture allowed for longer sight distance, both for seeing predators and potential prey
• standing upright freed our hands to manipulate tools
• our ancestors mostly lived by the shores of lakes, and an ability to wade while walking upright gave us access to the food-rich shallows along the edge

So which is it?  Possibly all three, and other reasons as well.  Evolution rarely is pushed in a particular pressure by just one factor.  What's interesting in this case is that upright posture is a classic example of an evolutionary trade-off; whatever advantage it gave us, it also destabilized our lumbar spines, giving humans the most lower back problems of any mammal (with the possible exceptions of dachshunds and basset hounds, who hardly got their low-slung stature through natural selection).

Sometimes, though, there's a confluence of seeming cause and effect that is so suggestive it's hard to pass up as an explanation.  Consider, for example, the rationale outlined in the paper that appeared this week in Science Advances, called "Increased Ecological Resource Variability During a Critical Transition in Hominin Evolution," by a team led by Richard Potts, director of the Human Origins Program of the Smithsonian Institution.

What the paper looks at is an oddly abrupt leap in the technology used by our distant ancestors that occurred about four hundred thousand years ago.  Using artifacts collected at the famous archaeological site Olorgesailie (in Kenya), the researchers saw that after a stable period lasting seven hundred thousand years, during which the main weapons tech -- stone hand axes -- barely changed at all, our African forebears suddenly jumped ahead to smaller, more sophisticated weapons and tools.  Additionally, they began to engage in trade with groups in other areas, and the evidence is that this travel, interaction, and trade enriched the culture of hominin groups all over East Africa.  (If you have twenty minutes, check out the wonderful TED Talk by Matt Ridley called "When Ideas Have Sex" -- it's about the cross-fertilizing effects of trade on cultures, and is absolutely brilliant.)

Olorgesailie, Kenya, where our distant ancestors lived [Image licensed under the Creative Commons Rossignol Benoît, OlorgesailieLandscape1993, CC BY-SA 3.0]

So what caused this prehistoric Great Leap Forward?  The Potts et al. team found that it coincides exactly with a period of natural destabilization in the area -- a change in climate that caused what was a wet, fertile, humid subtropical forest to change into savanna, a rapid overturning of the mammalian megafauna in the region (undoubtedly because of the climate change), and a sudden increase in tectonic activity along the East African Rift Zone, a divergent fault underneath the eastern part of Africa that ultimately is going to rip the continent in two.

The result was a drastic decrease in resources such as food and fresh water, and a landscape where survival was a great deal more uncertain than it had been.  The researchers suggest -- and the evidence seems strong -- that the ecological shifts led directly to our ancestors' innovations and behavioral changes.  Put simply, to survive, we had to get more clever about it.

The authors write:

Although climate change is considered to have been a large-scale driver of African human evolution, landscape-scale shifts in ecological resources that may have shaped novel hominin adaptations are rarely investigated.  We use well-dated, high-resolution, drill-core datasets to understand ecological dynamics associated with a major adaptive transition in the archeological record ~24 km from the coring site.  Outcrops preserve evidence of the replacement of Acheulean by Middle Stone Age (MSA) technological, cognitive, and social innovations between 500 and 300 thousand years (ka) ago, contemporaneous with large-scale taxonomic and adaptive turnover in mammal herbivores.  Beginning ~400 ka ago, tectonic, hydrological, and ecological changes combined to disrupt a relatively stable resource base, prompting fluctuations of increasing magnitude in freshwater availability, grassland communities, and woody plant cover.  Interaction of these factors offers a resource-oriented hypothesis for the evolutionary success of MSA adaptations, which likely contributed to the ecological flexibility typical of Homo sapiens foragers.

So what didn't kill us did indeed make us stronger.  Or at least smarter.

Like I said, it's always thin ice to attribute an adaptation to a specific cause, but here, the climatic and tectonic shifts occurring at almost exactly the same time as the cultural ones seems far much to attribute to coincidence. 

And of course, what it makes me wonder is how the drastic climatic shifts we're forcing today by our own reckless behavior are going to reshape our species.  Because we're not somehow immune to evolutionary pressure; yes, we've eliminated a lot of the diseases and malnutrition that acted as selectors on our population in pre-technological times, but if we mess up the climate enough, we'll very quickly find ourselves staring down the barrel of natural selection once again.

Which won't be pleasant.  I'm pretty certain that whatever happens, we're not going extinct any time soon, but the ecological catastrophe we're increasingly seeming to be facing won't leave us unscathed.  I wonder what innovations and adaptations we'll end up with to help us cope?

My guess is whatever they are, they'll be even more drastic than the ones that occurred to our kin four hundred thousand years ago.

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Have any scientifically-minded friends who like to cook?  Or maybe, you've wondered why some recipes are so flexible, and others have to be followed to the letter?

Do I have the book for you.

In Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine, by Michael Brenner, Pia Sörensen, and David Weitz, you find out why recipes work the way they do -- and not only how altering them (such as using oil versus margarine versus butter in cookies) will affect the outcome, but what's going on that makes it happen that way.

Along the way, you get to read interviews with today's top chefs, and to find out some of their favorite recipes for you to try out in your own kitchen.  Full-color (and mouth-watering) illustrations are an added filigree, but the text by itself makes this book a must-have for anyone who enjoys cooking -- and wants to learn more about why it works the way it does.

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

Viral reality

If you are of the opinion that more evidence is necessary for demonstrating the correctness of the evolutionary model, I give you: a paper by biologist Justin R. Meyer of the University of California-San Diego et al. that has conclusively demonstrated speciation occurring in the laboratory.

The gist of what the team did is to grow populations of bacteriophage Lambda (a virus that attacks and kills bacteria) in the presence of populations of two different potential food sources, more specifically E. coli that had one of two different receptors where the virus could attach.  What happened was that the original bacteriophages were non-specialists -- they could attach to either receptor, but not very efficiently -- but over time, more of them accrued mutations that allowed them to specialize in attacking one receptor over the other.  Ultimately, the non-specialists became extinct, leaving a split population where each new species could not survive on the other's food source.

Diagram of a bacteriophage [image courtesy of the Wikimedia Commons]

Pretty amazing stuff.  My response was, "If that isn't evolution, what the hell is it?"  Of course, I'm expecting the litany of goofy rejoinders to start any time now.  "It's only microevolution."  "There was no novel gene produced."  "But both of them are still viruses.  If you showed me a virus evolving into a wombat, then I'd believe you."

Nevertheless, this sticks another nail in the coffin of the anti-evolutionists -- both Intelligent Design proponents and the young-Earth creationists, the latter of whom believe that all of the Earth's species were created as-is 6,000 or so years ago along with the Earth itself, and that the 200 million year old trilobite fossils one sometimes finds simply dropped out of god's pocket while he was walking through the Garden of Eden or something.

So as usual, you can't logic your way out of a stance you didn't logic your way into.  Still, I have hope that the tide is gradually turning.  Certainly one cheering incident comes our way from Richard Lenski, who is justly famous for his groundbreaking study of evolution in bacteria and who co-authored the Meyer paper I began with.  But Lenski will forever be one of my heroes for the way he handled Andrew Schlafly, who runs Conservapedia, a Wikipedia clone that attempts to remodel the world so that all of the ultra-conservative talking points are true.  Schlafly had written a dismissive piece about Lenski's work on Conservapedia, to which Lenski responded.  The ensuing exchange resulted in one of the most epic smackdowns by a scientist I've ever seen.  Lenski takes apart Schlafly's objections piece by piece, citing data, kicking ass, and taking names.  I excerpt the end of it below, but you can (and should) read the whole thing at the article on the "Lenski Affair" over at RationalWiki:

I know that I’ve been a bit less polite in this response than in my previous one, but I’m still behaving far more politely than you deserve given your rude, willfully ignorant, and slanderous behavior.  And I’ve spent far more time responding than you deserve.  However, as I said at the outset, I take education seriously, and I know some of your acolytes still have the ability and desire to think, as do many others who will read this exchange. 

Sincerely, Richard Lenski

And if that's not spectacular enough, check out one of the four P.S.s:

I noticed that you say that one of your favorite articles on your website is the one on “Deceit.”  That article begins as follows: “Deceit is the deliberate distortion or denial of the truth with an intent to trick or fool another. Christianity and Judaism teach that deceit is wrong.  For example, the Old Testament says, ‘Thou shalt not bear false witness against thy neighbor.’”  You really should think more carefully about what that commandment means before you go around bearing false witness against others.

I can only hope that there was a mic around after that so that Lenski could drop it.

So there you have it.  Science finding out cool stuff once again, because after all, that's what science does.  The creationists, it is to be hoped, retreating further and further into the corner into which they've painted themselves.  It's probably a forlorn wish that this'll make Ken Ham shut up, but maybe he'll eventually have to adapt his strategy to address reality instead of avoiding it.

You might even say... he'll need to evolve.