Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Tuesday, May 25, 2021

The song of the bat

My favorite animal is the flying fox.

(Don't tell my dogs.)

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!


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.


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!] 

1 comment:

  1. mating might be another activity that bats do at night. At least it was for the pair of bats that me and my friends found one night after work. One of my friends was almost dive bombed by the pair. They ended up on the ground and my friend went over to investigate what it was. He claims that it was 2 bats mating. I didn't get a good look before they flew off, but it was certainly an interesting experience.