Saturday 11 August 2012

Why sight is superior to echolocation

In the previous post some characteristics of echolocation were discussed, and the results were somewhat worrying as far as a comparison with vision is concerned: echolocation involves 'shouting to hear a whisper', meaning that its range is limited and the sender is loudly proclaiming its presence.

Optics of pinhole and lens eyes. Source here

In my opinion there are two other major difficulties with echolocation that favour vision. The first is the ability to locate objects: with eyes such as ours it is very easy to locate objects. Rays of light can be bent by lenses and can reflect from surfaces, but in between they follow nice straight lines. That is the reason why even a simple pinhole camera such as in the image above will produce a good image: any particular point on the retina can only be lit be rays coming from a direction specific to that point. Such a pinhole will not let much light in, and solving that by increasing the pupil will blur the image. If you put in a lens you can have a large pupil for lots of light with a sharp image. Problem solved. The point of all this is that seeing an object is almost the same as knowing where it is.

From: Animal Eyes (2nd Ed.), Oxford; copyright Land & Nilsson

Above you see an image of lineages of eye design, leading to pinhole eyes and eyes with lenses. Those who wish to read more about eye evolution should read the new edition of Land and Nilsson's 'Animal Eyes'. It also describes the very high number of eye designs (there are even eyes based on mirrors!). Another very nice book is Evolution's Witness, with hardly any physics but boasting numerous examples of wonderful eye designs.

From: Animal Eyes (2nd Ed.), Oxford; copyright Land & Nilsson

In 1994 Nilsson and Pelger calculated that a good camera eye with a retina and a lens could evolve from basic elements without any localizing ability in fewer than half a million generations. With one generation a year this amounts to a geological blink of an eye (sorry for that one), meaning just half a million years. Vision can apparently evolve so quickly and conveys such a large advantage that some say it explains the runaway evolution known as the Cambrian explosion. In a very short time things such as armour, speed and vision evolved. Giving animals an unobtrusive ability for precise long-range sensing may just have been the impetus to start this accelerated runaway evolution: claws, shells, teeth and brains co-evolved quickly. Perhaps vision was not the only factor jump-starting the process, but the idea is too powerful to ignore a role for vision altogether, I think. It is tempting to think that most planets with complex life would have their own 'Cambrian Explosion' in the early evolution of complex animals. Of course, the label 'Cambrian' would not apply on Furaha, Snaiad, Nereus, nor on real exoplanets. We need a more general name for the phenomenon; how about the 'Sight Spark'? (and if it sticks can I copyright it?).

Back to sound

Seeing how a pinhole eye is simple and works well, how about evolving a 'pinhole ear'? Suppose we place lots of microphones on the inside of a sphere and cut a hole in front of the sphere to let sound in. Wouldn't each microphone only pick up sound from the bit of the world it 'sees' through the opening? If so, we would have an ear with perfect localising ability. Alas, no. Sound does not travel in neat straight lines but travels around corners. You can hear people talking through an open door even when you cannot see them.

Sound 'bending' around a building. Source here.

How the size of an opening affects diffraction of sound. Source here.

When sound waves hit objects, those interfaces form new sound sources, a process called diffraction. In the misbegotten 'pinhole ear' idea, the 'pupil' would simply act as a new sound source, so all microphones on the 'retina' would receive sounds from all directions. As a location device this would be utterly useless.

The physical reason why sound bends around corners and light does is not that diffraction is limited to sound. But in fact diffraction affects light and hence vision too. The effects of diffraction depend on wave length, and the wave lengths of sound have a range of a few cm to 15 or more meters; those of light are measured in micrometers. The diffraction of light is seen at microscopic scales, but that of sound occurs at the scale we live in, that of doors and people.

So the physics of sound conspire against it providing an easy way to tell where a sound is coming from. Evolution solved that problem as it did others, but the solution requires combining the signals from two ears (I know that using two eyes improves distance detection, but you can do it with one eye, and locating the direction of an object needs just one eye). Tiny differences in arrival time of a sound at two ears allow a suitable brain to calculate the direction of the source of a sound in the plane of the ears, but not whether it is to the front or the back nor up or down. Finding out things like that call for ingenious trickery such as tilting heads or complexly shaped outer ears that subtly change the characteristics of a sound depending on where it is. Wikipedia has a nice article on the subject. Some animals (owls!) perfected the art of sound location, but theirs is a small niche compared to the ubiquity of good camera eyes.

Mind you, I have no idea why there are no animals with more than two ears. Having four, placed at the corners of a tetrahedron, would be nice. Then again, perhaps there are arthropods with more than two functional ears. I have never heard of any but have not looked either. Are there any?

An unfair advantage of sight over echolocation

I wrote above that I thought there were two more difficulties with echolocation. The second one is based on the fact that echolocating animals have to produce their own signal, limiting the range at which they can detect anything. How about vision? There was an omission in the discussion, and it is a glaring one: the sun! (sorry about that one too). Sunlight, free for all and there regardless of whether anything of anyone is using it, is what allows vision to work as a long range sense. Compared to echolocation this free gift to sight is not really fair.

Copyright Gert van Dijk

The images above were made for the previous post: the right one showed what echolocation might be like, with some energy coming from the 'camera', only illuminating objects close by. Compare that to the left image, a visual scene lit by the much more powerful sun.

But vision is not always available, and echolocation has a chance when there is no light. On rotating planets like ours, sunlight is only available for half the time, so the night would seem a good time for animals to start echolocating. But that is not the case; most animals prefer to more or less shut down at night. In previous discussions on when echolocation would be better than vision some dieas came up: caves, planets with permanent fog, planets without suns and seas underneath ice caps. One region seemed to have been forgotten though: the deep dark seas, where the sun does not reach. Shouldn't they be filled with echolocating animals, squeaking and pinging away? For Earth whales, the ocean floor may be too deep to reach, but fish were there a long time before the first whale ancestor took its first dive. Why are there no echolocating fish? I asked experts, but they did not know either. Fish have suitable ears and brains, and nothing seems to stand in the way of them evolving echolocation. But they have not. Or is echolocation simply too much like a burglar who enters a silent dark house and then starts shouting 'Hellooo!'? I do not know.

However, I do know of one final twist in the comparison of echolocation and vision; but I will keep that for the last post on this subject...


Petr said...

Awesome! I can't wait for the final part! =)

Anonymous said...

"Some animals (owls!) perfected the art of sound location, but theirs is a small niche compared to the ubiquity of good camera eyes."

So have some raptors. Skulls of a small species of troodontid dinosaur show that these animals had asymmetrical ears like owls.

"Then again, perhaps there are arthropods with more than two functional ears. I have never heard of any but have not looked either. Are there any?"

Many insects have only one. In addition, it seems like in many insects the only reason why hearing is developed is in order to hear other flying predators (bats, birds). Most insects were deaf until pterosaurs entered the scene, and many secondarily flightless lineages of insects have lost the ability to hear.

The reason why there are numerous echolocating mammals (whales, tenrecs, bats, shrews, possibly some extinct "condylarths") and no echolocating fish may be due to the fact that mammals have a very complex ear, capable of hearing better than many other groups of vertebrates. This fact, and the fact that marine mammals tend to emphasize their sense of hearing and thicken parts of the basicranium, suggest that mammals are "pre-adapted" in some ways to develop echolocation.

Anonymous said...

By the way, have you guys seen this?

Spugpow said...

I wonder if echolocation and sound detection become more attractive for very large animals. We find light to be useful because its wavelength is very small compared to us. Not so much for microscopic arthropods and the like, which have very simple eyes. Sound is just barely useful to us, since its wavelength is far larger, and to insects it's practically useless, so most insects are deaf.

Would this pattern continue such that very very large animals would find sound similarly useful to light (ignoring light's unfair advantage of the sun)?

j. w. bjerk said...

"Or is echolocation simply too much like a burglar who enters a silent dark house and then starts shouting 'Hellooo!'?"

I don't think that's the reason, since many deep sea creatures are bioluminecent.

Echolocation is just one of the means of sensing the world using vibrations. Fish use hearing of course, and the very sensitive lateral line system to detect slight changes in the pressure, and movement of the water around them. This passive system probably serves much the same purpose that echolocation would.

Sigmund Nastrazzurro said...

Petr: thank you.

Anonymous 1: as far as I know hearing / mechanoreception is quite common in arthopods, and in insects some have tympanic membranes but others have vibration-sensitive hairs. I had not heard the theory that hearing in insects only developed in the Mesozoic; it is interesting and will try to find more on this.

Anonymous 2: strandbeesten in motion!

Spugpow: that is an interesting idea. Wave length certainly affects our hearing: for instance, our heads hardly cause a 'sound shadow' with low-frequency sounds, but they do with high frequencies, and our hearing uses the high-frequency information to tell directions.
However, the frequency difference between sound and light is so enormous that I do not think body size an make up for it.

j.w.bjerk: I'll keep bioluminescense for later. You are right that fish have a side line system as well as normal ears; both are mechanoreceptors and can be included under a similar heading. Both are for passive listening though, and is you are silent, no one can detect you with that. Echolocation can detect silent animals; that should convey an advantage...

Anonymous said...

Both this post and the one before it are quite impressive, and it is quite satisfying to see your usual scientific deconstruction applied to the issue of eyes.

In regard to the availability of light, it should be pointed out that animal vision is possible (and common) in levels of light far lower than daylight. Many animals on Earth are nocturnal and still possess eyes. Our large moon can provide a lot of light when it is at a low phase angle, and night vision is also possible in starlight (at least for technological night vision devices).

The lack of a large moon has been theorised to have other, far more serious consequences, but one could also imagine what ramifications the lack of a prominent moon could have on the lack of light at night.

In the place of moons, some planets, part of binary systems may have 'second suns'- very prominent stars in their skies. Achird is a double star, so depending on time of year or position on the surface (depends on the inclination of the planet's orbit to that of the other stars in the system) Achird B would shine upon Nereus with a strength from 1.3 to 11 times that of our Moon (currently, it'd be about magnitude -13.5, around twice the brightness of our moon).

Habitable moons would have absolutely magnificent skies, with the parent planet potentially exhibiting an angular diameter of several degrees (!). Coupled with the high albedo of gas giant planets (theorised to be as high as 0.81 for giant planets in the habitable zone; see Sudarsky classifications). On the nearside of such a moon one could say that night in the familiar sense wouldn't occur at all.

On the other end of the scale, there would be planets with less impressive skies (with moons, but smaller or more distant ones than our own). Mars is an example; the moon Phobos is minute compared to our moon, yet has an angular diameter roughly third the size, owing to its very low orbit around Mars. Phobos has a maximum brightness only around 5% that of our Moon, but provides light nontheless.

There are also various scenarios that could 'starve' life of light- a planet with thick clouds, like Venus, may render starlight or moonlight useless at night. A planet with a high axial tilt would have a low arctic circle, and present large areas of its surface with months-long periods of darkness (such a high obliquity would presumably also cause severe seasonal conditions). A tidally locked planet would have a permanently dark nightside; if circulation of heat from the dayside were efficient enough, temperatures here may still be above freezing, and could provide life with a light-deficient environment to evolve within.

When considering light-deficient environments such as these, it is probably worth noting that many deep-sea creatures still possess eyes despite their low-light environment.

Friendly rant over,

Sigmund Nastrazzurro said...

T. Neo: Thank you. I agree that a lot more can be said about the amount of light falling on planetary surfaces. Besides the many you mentioned, you can also think of elliptical orbits and rings ( Imagine a ringed habitable moon around a gas giant on an elliptical orbit: interesting opportunities for daily and seasonal variations...

On reading about eyes, I am more and more impressed with how useful even a little bit of light apparently is. I understand the wish to do away with eyes to make creatures more alien, but doing so seems less and less probable.

Anonymous said...

excellent post.
(will reply better later, sorry)

> Of course, the label 'Cambrian' would not apply on Furaha, Snaiad, Nereus, nor on real exoplanets

It would be more a parallel or a "here it happened" expression, and not a specific geological epoch.

Anonymous said...

>and no echolocating fish may be due to the fact that mammals have a very complex ear,

that may be putting the cart before the horse, Anonymous -- fish skulls have no lack of tiny bones they oculd co-opt to help with hearing.

Spugpow said...

Fish seem to do most of their hearing with the lateral line system (which is homologous to the mammalian ear). A large group including catfish, however, do have a sophisticated hearing set up that incorporates the swim bladder:

M said...

Echolocation WOULD be favoured in dark and aquatic environments.

If it is possible for life to evolve on a world without light. (Like under a thick ice-shell.) Echolocation would make sense, whereas sight would not.

If very dim stars can host complex life (Something I doubt, but who knows!) then having functioning eyes might be to biologically expensive compared to echolocation as well, and thus not favoured either.

Biology is generally a cost game. Anything an organism doesn't need will phase away (like legs on snakes, or forearms on one-fingered dinosaurs) And if echolocation is more useful then huge expensive and vulnerable eyes in a dim light environment, then echolocation will be favourable.

Infrared vision COULD be viable too. Although dim-environments are most likely to be aquatic (Even the dim star ones, because of the radiation making the land less then attractive), where the echolocation would be more useful.

In biology form ultimately follows function. Lifeforms are never perfect, but they ARE adapted to their environment, and generally cost-efficient if they are to survive.