An essay about caves and the origins of echolocation

Caves and the origins of echolocation

Imagine that you are in a cave, a very familiar cave, but with no light. Do you think you could collect information about your location by shouting or clapping and listening to the echoes? Would a large chamber sound different than a tight passage? Sound ridiculous? Try this experiment. Make a loud constant “shhh” sound as if telling someone to be quiet. Now close your eyes and move your hand back and forth in front of your face as if waving. Hear that? What you are now doing is very simple version of biosonar, or echolocation.

Bats do it way better. Most cavers know that bats use echolocation– a kind of sonar. But perhaps it’s demeaning to call what bats are doing “sonar” (an acronym for the human technology of SOund Navigation And Ranging), because comparing a bat’s echolocation to our military sonar is like comparing human vision to an earthworm avoiding light. Human-engineered sonar can detect submarines, but echolocating bats can detect miniscule flying insects, minnow fins protruding from a water surface, the tiny water ripples made by a calling frog, even the gossamer strands of spider webs. How do they do it?

Most echolocating bats produce a very loud chirp that sweeps across a precise range of frequencies. And when I say loud, I mean loud— louder than a smoke alarm or a circular power saw. Some bats actually disengage their ear bones to avoid deafening themselves. But the pulse is so brief and so high-pitch, your ears cannot perceive it. When the echoes reflect off an object and come back to the bat, she can form an image of what’s in front of her, whether it’s a cave wall or a moth. Note that a cave wall, especially a large flat surface, is much easier to detect than a moth, which is one reason why I think bat echolocation first evolved in caves.
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Right: A bat flies through the dark of a cave with its mouth open. Photo by Brock Fenton.

In many ways, a cave is the perfect environment for a primitive form of echolocation to get started. Many animals take refuge in caves to avoid predation. If you’re a bat from the Eocene, caves are pretty safe and environmentally stable places to, literally, hang out. The problem is that deep inside where it’s safest, it’s also absolutely dark. But solve that problem and you can make a living deeper and deeper in a cave. The hard solid walls of some caves can produce crisp clear echoes compared to a cluttered leafy forest or open air. Actually, the easiest environment for echolocating would be inside a building with flat smooth solid walls. But alas, caves are the closest thing that Mother Nature gave bats. There are only a few natural environments where vision is absolutely useless, and the two that most easily come to mind are underground and under deep or murky water. And echolocation evolved independently in both these places. Bats and toothed whales both “invented” sonar in their own way.

Cave-dwelling animals have also independently evolved the ability to echolocate several times. This happened at least twice in two different groups of birds, oilbirds and swiflets, which both live in caves (Brinklov et al. 2013 Frontiers in Physiology). And as you’ll read below, it happened at least three times in cave-dwelling bats.

Do bats see with their ears?

The story of how people discovered bat echolocation is actually quite interesting. In 1793, an Italian scientist named Spallanzani found that flying owls would crash into obstacles in absolute darkness and they needed a bit of candlelight to avoid them. Bats on the other hand could avoid the same obstacles even in pitch-black darkness. He then did a rather unsavory experiment. He poked out the bats’ eyes and found that they could still somehow magically “see”. Further experiments showed that by plugging their ears, the bats could no longer orient and would crash into objects in total darkness. Spallanzani’s conclusion was simply that bats… somehow… see with their ears. Of course, this seemed ridiculous.

The puzzle was finally solved more than a hundred years later in 1939-1941 by a young biologist named Donald Griffin who realized, using fancy new microphones, that flying bats were making high-frequency ultrasound beyond the range of what a human was capable of hearing. So the story goes that when Griffin first presented his results at a scientific conference, his fellow scientists thought he was crazy (just like Spallanzani). In Griffin’s words,

“One distinguished scientist was so indignantly incredulous that he seized [my colleague] by the shoulders and shook him while complaining that we could not possibly mean such an outrageous suggestion. Radar and sonar were still highly classified developments in military technology, and the notion that bats might do anything even remotely analogous to the latest triumphs of electronic engineering struck most people as not only implausible but emotionally repugnant.” (quote from p. 35, Dawkins 1986 The Blind Watchmaker)

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We now know that there are more than 1,300 bat species, and most of them echolocate. Different bats echolocate in very different ways. Most echolocating bats produce the echolocation sounds with their larynx, just like when you speak or sing. The local bats of the Eastern USA all scream loudly through their tiny mouths, but some bats in other parts of the world also send the signals out through their nose– which is one reason why their faces are often so weird-looking. To see what I mean, just do a Google image search for “bat noseleaf”.

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Left: A bat that echolocates through its nose leaf

Natural selection creates two kinds of bat echolocation

This laryngeal echolocation later evolved into two distinct forms. The original “design” was to produce a series of brief pulses (each about 1/500th of second) separated by much longer pauses. For the bat, this is a bit like a strobe light, except the bat uses the timing of echoes to determine distances. So imagine a strobe light where you see near surfaces in first, then farther surfaces second, than even farther surfaces third and so on. (Now, for all I know, the bats might compile all these images into a single movie image, just like your brain puts together the illusion of a large visual field from a mosaic of your many focused eye movements.)

Some bats evolved a completely different kind of laryngeal echolocation, where they emit a long signal and then use Doppler shift to detect objects. That is, they use changes in frequency, not time, to detect objects and how they are moving. This is similar to how an ambulance that is approaching you sounds different than one that is driving farther away. This kind of bat echolocation is better for longer distances and hunting insects in open environments. Amazingly, it evolved twice independently in two completely unrelated groups of bats. (This is, by the way, not that strange: two different groups of bats also independently “invented” suction cups on their thumbs to cling to smooth surfaces. Google it.).

Anyhow, that’s all laryngeal echolocation. Laryngeal echolocation has become astonishingly sophisticated due to an evolutionary arms race with bat-detecting moths. But that’s a whole other story. Our story continues in a different direction. One group of bats, the Old World fruit bats, stopped living in caves, started hanging in tree branches, developed great vision, and started eating fruit. No one knows in which order those things happened. But they eventually developed big cute eyes and they are often called “flying foxes” because that’s what they look like. In fact, they look and act so different from other bats that biologists once classified them as something else entirely. (It’s possible that the ancestors of flying foxes never had echolocation and that their close relatives evolved it independently, but I think the more simple explanation is that the ancestors of flying foxes simply lost the ability to echolocate. That’s a common theme in evolution: use it or lose it.)

Echolocation by tongue

Ok, now comes a plot twist: one group of these non-echolocating flying foxes eventually moved from trees back into caves and re-evolved the ability to echolocate in a completely different fashion!

Instead of using their larynx, they click with their tongues (like those cool clicking languages used by some human tribes in southern Africa). Researchers use to think that tongue-clicking echolocation in bats was pretty unsophisticated, but work by the biologist Yossi Yovel showed that the bats actually steer and adjust their sonar beams in complex ways much like the more advanced laryngeal echolocators (Yovel et al 2010 Science, 2011 PLOS Biology).

There are some small flying foxes, like the flower-feeding Dawn bat, that don’t seem to echolocate at all, even though they live in caves. Or so we thought.

Echolocation by wing

In 1988, a biologist named Edwin Gould, who was mentored by Donald Griffin (the guy who first discovered echolocation), observed Dawn bats leaving caves in Malaysia and noticed that they made clicking sounds when in total darkness deep inside the caves but not near the lighted entrances. He did some experiments and concluded it was not tongue-clicking but wing-clicking. He then put paint on one wrist of some bats and found that it was transferred to the other wrist, so he hypothesized that the bats were actually clapping their wings together (Gould, 1988, Journal of Mammalogy). Nobody followed up on this work for 25 years, and at least some bat biologists thought this idea was a little crazy (sound familiar?).

temp4Left: A Dawn Bat. Photo by Alyssa Stewart.

Arjan Boonman, a postdoctoral researcher working with Yossi Yovel, now a professor at Tel Aviv University, followed up on Gould’s work and found that yes indeed—the Dawn bat does echolocate with its wings, along with two other supposedly non-echolocating fruit bats they tested (Boonman et al 2014 Current Biology and comment). The team showed that these bats were not actually clapping their wings to produce the sound, but they failed to figure out exactly how the clicks were created. So that’s one puzzle for the future.

But the team did clearly show that this rudimentary form of echolocation is pretty primitive. That is, it’s not great. The bats could detect and land on a flat solid surface that was one meter squared, but they struggled to detect smaller objects and they helplessly crashed into thin wires.

So, in summary, to navigate the pitch-black total darkness of caves, bats have evolved at least three different kinds of echolocation: wing-clicking (bad), tongue-clicking (good), and vocalizing (great).

There is one fact, however, that is a bit inconsistent with my simple story that caves created the necessity for the evolution of bat echolocation. Only the Dawn bat, one of the three wing-clicking bats that Boonman and his team discovered, was a cave dweller. It was the most frequent clicker and the best performer. But the other two species roost in foliage. What does that mean? I’m not sure. It’s still unclear how many kinds of Old Fruit bats can echolocate with their wings. One possibility is that all Old World fruit bats can echolocate with their wings, and that some of them later developed this ability further to varying degrees. Once a bat could do this well, it might be much easier to start echolocating using its mouth, because it’s already able to process the echoes. Boonman and his coauthors point out that primitive echolocation is more easily “evolve-able” than we once thought; it only requires that animals make some kind of sound (like clicks or squeaks) and then can make some sense of the echoes. So even though not all bats have sophisticated echolocation, perhaps many or all have rudimentary echolocation? After Boonman’s recent work, we may have to re-evaluate the whole evolutionary history of echolocation.

Echolocation for all (even humans)

Boonman and Yovel also pointed out that many mammals can probably be trained to echolocate. Humans are a prime example. Indeed, many blind people have learned the ability to echolocate to get around and do all sorts of useful things. Ben Underwood was a blind kid without eyes who could echolocate well enough to ride a bike and shoot a basketball into a hoop. Look it up on Youtube and have your mind blown. Daniel Kish is a blind echolocator who teaches echolocation to other blind people. He started a non-profit called World Access for the Blind and has teamed with researchers studying human echolocation who claim that in just 2 hours a day for 2 weeks a student can learn to detect large objects using tongue clicks, and in another two weeks, they can discriminate trees from walls.

So there you have it. If you ever want to have the ultimate backup to your headlamp, just remember that you can always train yourself to echolocate so you can tongue-click, hand-clap, or yelp your way through a cave without a light. But it might still take a few million years of living and evolving in the darkness before humans can reach bat-level and hear the soft echoes reflecting off a moth or a spider web.

Gerald Carter is a PhD Candidate at The University of Maryland a postdoc working with John Ratcliffe at University of Toronto and Rachel Page the Smithsonian Tropical Research Institute. He is not an expert on echolocation.

Feb 4, 2015. Written for The Speleograph, the publication of District of Columbia Grotto, a local chapter of the National Speleological Society. When I lived in Maryland, I went caving with the DC Grotto of the National Speleogical Society. I wrote an essay about caves and echolocation for their newsletter, the Speleograph, but I don’t think it was ever published. This essay came from a ‘what-if’ conversation I had with someone in a cave about humans using echolocation to navigate caves without a headlamp. I argued that it’s easier for us to hear echoes inside a building with hard flat walls, rather than say in a forest. And since caves were the only ‘indoors’ before humans existed, echolocation probably first evolved as a way for animals to live deeper in caves. So I thought I would write an article about that idea using some neat stories from bats. Now, a year later, I just found it sitting on my hard drive. 

 

About Gerry Carter

I study the behavioral, sensory, and social ecology of vampire bats. http://socialbat.org.
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