Friday, 27 July 2012

Echolocation: a sound choice?

In January I wrote a post on whether detecting heat could supplant vision, and concluded that it was, in fact, just a form of sight. I wished to tackle echolocation next, but wondered where to start: with echolocating animals in fictional biology? Other possible questions would be which atmosphere would be best, which frequencies to use, how it can be compared with vision, etc. In the end I decided to start -there's more!- with a post on the nature of echolocation; so here we go...

The basic principle is simple: you send out a sound and if an echo returns, there is something out there. As everyone knows, dolphins and bats are expert echolocators., but it is less well known that some blind people are quite good at it, and that they in fact use their occipital cortex to process echoes, a brain region normally busy with analysing visual signals. That direct link between vision and echolocation is perhaps not that surprising, as both senses help build a spatial representation of the world outside: what is where?

A major difference between vision and echolocation is how distances are judged. In vision, judging distances depends on complex image analysis, but in echolocation the time between emitting a sound and the arrival of the echo directly tells you how far an object is away. The big problem here is that echoes are much fainter than the emitted sound. The reason for that is the 'inverse square law', something that works for light as well as for sound.

Click to enlarge; copyright Gert van Dijk

The image above explains the principle. Sound waves emanate from a source near the man in the middle and spread as widening spheres (A, B and C). As the spheres get bigger, the intensity of the sound diminishes per 'unit area'. A 'unit area' can be a square meter, but can also be the size of your ear. When you are close to the source your ear corresponds to some specific part of the sphere, and when you move away your ear will correspond to a smaller part of the sphere: the sound will be less loud. Now, the area of the sphere increases with the square of the distance. If you double the distance from the source, the area of the sphere increases fourfold, and the part your ear catches will decrease fourfold. To continue; increase the distance threefold and the volume decreases ninefold. Move away ten times the original distance from the source, and the sound volume becomes 100 times smaller!
In the image above, only a tiny fraction of the original sound will hit the 'object', a man, at the left. Not all of that will bounce back, and the part that is reflected forms a new sound: the echo. The echo in tun decreases immensely before arriving at the sender, and that is the essence of echolocation: to hear a whisper you have to shout.


Click to enlarge; copyright Gert van Dijk

As if the 'inverse square law' is not bad enough, there is another nasty characteristic of echolocation. At the left (A) you see a random predator using echolocation. Oh, all right, it's not random, but Dougal Dixon's 'nightstalker' (brilliant at the time!). It sends out sound waves (black circles) of which a tiny part will hit a suitable prey; there's that man again. As said, the echoes travel back while decreasing in strength (red circles).
There will be some distance at which a prey of this size can just be detected. Any further away and the returning echoes will be too faint to detect. Suppose that this is the case here, meaning 10m is the limit at which a nightstalker can detect a man (as mankind is extinct in the nightstalker's universe no-one will be hurt).
Here's the catch: most of the sound emitted by the nightstalker travels on beyond the prey. These sound waves can be picked up easily by other animals further away than 10 meters (I assume you recognise the creature listening there; it's pretty frightening). For animals out there the sound only has to travel in one direction and none of it gets lost in bouncing back from the prey. The unfortunate consequence of all this 'shouting to hear a whisper' is that the nightstalker is announcing its presence loudly to animals that it cannot detect itself!
This suggests that echolocation could be a dangerous luxury. One way to use it safely would be if other predators cannot get to you anyway. Is that why bats, up there in the air, can afford echolocation? Another solution would be to be big and bad, so you can afford to be noisy? If so, echolocation is not a suitable tool to find a yummy carrot if you are an inoffensive rabbit-analogue. The carrot does not care, but the wolf-analogue will.

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Getting back on topic, we now know that echolocation tells you how far away an object is. To make sense of the world you will also need to know where the object spatially: left and right and up and down. With hearing this is more difficult than with vision, but it can be done. The spatial resolution of bats is one or two degrees (see here for that), which is impressive but still 60 to 120 times less good than human vision. For now, let's take it for granted that an echolocating animal can locate echo sources. Next, let's try to visualise what it may be like.


Click to enlarge; copyright Gert van Dijk

Here is a scene with a variety of objects on a featureless plain. The objects have transparency, colours, shadows, etc. At one glance we see them all, as well as the horizon, the clouds, etc., without restrictions regarding distance, all in high resolution. The glory of vision, for all to see.


Click to enlarge; copyright Gert van Dijk

Colour is purely visual, so to mimic echolocation it has to go. All the objects are now just white. They are also all featureless, but that is for simplicity's sake only: vision and echolocation can both carry information about things like wrinkles and bumps, so I left texture out.


Click to enlarge; copyright Gert van Dijk

In sight the main source of light is the sun shining from above, but in echolocation you have to provide your own energy. To mimic that, the only light source left is at the camera. The resulting image looks like that of a flash photograph, for good reasons: the light follows the inverse square law, as does sound. Nearby objects reflect a lot of light (sound!) for two reasons: they are close by, and part of their surfaces face the camera squarely, turning light directly back at the camera. This is an 'intensity image'.


Click to enlarge; copyright Gert van Dijk

However, you can see nearby and far objects at the same time, but that is not true for sound. Sound travels in air at about 333 m/s, so sound takes about 3 ms to travel one meter. An object one meter away will produce an echo in 6 ms: 3 ms going to the object and 3 ms travelling back. The image above shows the same scene, but now the grey levels indicate the distance from the camera. Light areas in the image are close by, dark areas are further away. This is a 'depth image', formed courtesy of the ray tracing algorithms in Vue Infinite.

video
Copyright Gert van Dijk

Now the scene is set to mimic echolocation. Let's send out an imaginary 'ping'; each interval in time determines how far away an echo-producing object is. For instance, the interval from 6 to 12 ms after the 'ping' corresponds to objects 1 to 2 meters away. While the depth image tells us how far away objects are, the intensity image tells us how much of an echo is produced there. To make things easier for the human eye a visual clue was added: echoes returning early are shown in red, while those returning later are blue. Above is a video showing three successive 'pings'. As the echoes bounce back, areas close by will light up in red, and objects furet away will produce an echo in blue, later on. I blurred the images a bit to mimic the relatively poor spatial resolution of echolocation.
I personally found it difficult to reconstruct a three-dimensional image of the world using such images, but my visual system is not used to getting its cues in such fashion.

video
Copyright Gert van Dijk

One easy processing trick to improve the image is to remember the location of early echoes. The video above does that, by adding new echoes without erasing the old ones. The image is wiped as a new ping starts. More advanced neuronal analyses could take care of additional clues such as the Doppler effect, to read your own or an object's movement. By the way, the above is in slow-motion. In real life echoes from an object 10 m away would only take 60 msec to get back. Even without any overlap you could afford 16 pings a second for that range. That is not bad: after all, 20-25 frames a second is enough to trick our visual system into thinking that there is continuous movement.

So there we are. Is this simple metaphor a valid indication of what echolocation is like? Probably not, but it does point out a few basic characteristics of echolocation. Echolocation must be a claustrophobic: no clouds, no horizon, just your immediate surroundings. It would seem the meek cannot afford it, as it may be the most abrasive and abusive of senses.
Is it therefore completely inferior to vision? Well, yes and no...

11 comments:

Luciano N. Ribeiro said...

Personally, I always thought the greatest problem with echolocation was the energy expense. After all, you have to keep screaming all the time just to look around. However, I know next to nothing about physics and I might be overestimating this issue. It works fine for bats after all.

For a very sophisticated (extraterrestrial?) species, perhaps the sound could be produced automatically during exhalation. While we have to put strenght on the throat to scream, they would simply let the air pass through a whistle-like organ, or something like that.

Also, think about how we need energy to keep our hearts pumping, our lungs moving and our glucose-based brains working all the time, and perhaps energy is not an issue after all!

Nonetheless, I always think about Barlowe´s "Expedition" and how all creatures there have to spend energy on both echolocation and bioluminescence, these two being sort of contradictory. But Barlowe´s creatures are so awesome I can´t pretend to care much about their scientific feasibility anyway :D

PS - Hm, sorry for the extremely long comment. I do that a lot...

mithril said...

it's hard to imagine a purely echolocation base ecosystem, in part because nearly all the echolocation using animals on earth also still have eyes that work. while our language has sayings like "blind as a bat", bat eyes are actually quite functional. Dolphins and whales also have quite functional eyes, it seems likely the properties of sound in water is the main reason for echolocation to be so important to that species.

since we know that the eye has evolved multiple times across the animal kingdom, it seems likely that even if echolocation were to be somehow become a main sense in an alien ecosystem, that eyes would evolve eventually as well.. and open up the possibility for eye using species to gain an advantage over echolocation based animals, and thus out compete them in some niches.

Anonymous said...

I hadn't thought about the disadvantages of echolocation- an organism sending out pings is announcing its presence to creatures it can't detect yet!! This is suicidal for an inoffensive herbivore, and a big scary predator would probably rather not scare all its prey away with bone-vibrating ultrasonic pings before it can even "see" them. Maybe a big, bad herbivore could echolocate without worrying too much, if it can fight off or scare away predators. After all, the tasty carrot can't run away when the big, nasty rhino equivalent pings it. : ) Then again, it might run the risk of attracting Lord Cockswain on yet another of his off-world hunting expeditions...

Many artists like to paint pictures of alien animals who rely exclusively on echolocation, heat detection, and other non-visual senses- even if they live in a open environment with plenty of light- but this does not seem to fit with the actual evolution of non-visual senses on Earth. Most Earth animals who use echolocation, heat detection, or other non-visual senses also posses functional eyes. Bats use echolocation, but they still have eyes. Rattlesnakes can sense their prey's body heat, but they still use their eyes. Sharks and other fish possess lateral lines to sense vibrations in the water- but they still use their eyes.

I think you should mention possible prey countermeasures to echolocation. Just like how a jet airplane can try to avoid detection by jamming enemy radar stations, certain moths click at a rate of 4,500 clicks per second to confuse hungry bats, at least according to this article. Could alien prey try to escape hungry echolocating predators by frantically pinging the predator?

Christopher Phoenix

Sigmund Nastrazzurro said...

Luciano: I have not looked at the energetic costs of producing sound, but doubt that they are high. Small songbirds can produce very loud sounds and do so for hours on end. Make something vibrate and you have a sound right there.
As for Barlowe's Expedition, I feel that he tried a bit too hard to obtain 'otherworldiness', but he paints so well that you prefer to forget about other matters (except for his animals not having toes; that irritates me each and every time).

Mithril: I agree. I felt that vision was very superior to echolocation, and the tried to identify the reasons for that superiority. That was harder than I thought; the next post on the subject will be about just than matter.

Christopher: having multiple senses must indeed be preferable to having just one. Earth life seems to prove that with abundant examples.
I knew that moths dropped like a stone on hearing a bat's ping, but had not heard of actual jamming the signal. Odd: you would think that the bat would simply switch to 'passive listening mode' turning the moth's defence against it.
But you could have acoustic camouflage; I read something about that once and will look it up.

Petr said...

once again, a really interesting article! I've always thought that "just" echolocation eould be "short-sighted" but having echolocation in addition to eyes can apparently be advantageous, given bats, whales and shrews.
And just a thought on the "passive mode" mntionned in your comment. owls have "super hearing" and hace asymetricaly arranged ears so they can perfectly judge distance between them and prey. I guess this is better than echolocation in a way, because you don't reveal yourself, you're just recieving. ;)
And it must work very well, i've seen it in a documentary on TV where a snow owl could detect and catch a mouse under a quite thick layer of snow, just by listening to the chirping. =D

Oceaniis said...

Interesting, did you took a look to Didson acustic cameras?

Anonymous said...

Hi, Sigmund- from the article I read on this, it seems to be that the moths "jam" the bat's echolocation at the crucial moments just before the bat snaps up the moth. The bat needs to know exactly where the moth is to catch it, and when it suddenly confused by the moth's chittering, it loses track of its prey. Once the danger has passed, the moth probably stops clicking. Otherwise, as you said, the bat could just follow the moth by its own clicks!!

Christopher Phoenix

Sigmund Nastrazzurro said...

Petr: you are quite right in that owls have a superior ability to localise sound sources through hearing. I just learned that that system is in fact liked with vision, s apparently owls hone this skill with the help of sight. But passive sound location only works if something actually produces sounds. An silent mouse will remain undetected.

Oceaniis: Thank you for mentioning this. I had not heard of Didson acoustic cameras. I looked them up, and they produce amazing images. I had thought my simulations would be too 'visual', but these images show that they such images can in fact be produced in reality (whether animals do so is another question).

Christopher: the bats vs. moths game: who is currently ahead...
I loked up acoustic camouflage, and there is mention of animals altering their sounds on hearing an echolocating predator, but I found nothing on topics such as changing the way a body reflects sounds (which would make its echoes look like coral or weeds etc.)

Petr said...

Sigmund - You're right, silence can defeat even the keenest sense of hearing. =D
P.S. sorry for the typos in my previous comment. ;)

Anonymous said...

Though, one advantage of echolocation is that it's omnidirectional. Sure, you may only be able to a few meters in front of your face, but you can also see a few meters behind you as well, without having to spring for expensive and fragile eyes.

"(I assume you recognize the creature listening there; it's pretty frightening"

And then the future predator realizes that by killing the nightstalker, it has in effect killed its own grandfather, and disappears in a puff of ontological paradox.

Interestingly, spin-off information in Primeval indicates that the Future Predator has eyes, but they're just not prominent. You can actually see them in some of the artwork (they look like little dots).

Still, it seems like a combination of sight and echolocation would work better than just pure echolocation. Perhaps in an echolocation-heavy world echolocation evolved early on in sighted organisms, and those species that didn't have both ended up not being dominant in the present day.

Sigmund Nastrazzurro said...

Anonymous: I am not certain that echolocation is omnidirectional in many species. The emitted sound will spread in all directions but may well be stronger forwards, and picking up echoes is probably directional as well. The external ears of bats point forwards. The point is interesting though: can bats sense objects behind them?
I agree that multiple senses are advantageous most of the time, otherwise we would see mor animals with a secondary loss of one or more senses.