I ended by assuming that the darkness of the deep sea would make it a perfect habitat for echolocators. Of course whales do exactly that, and they fit the job description of being Big and Bad. But they have not been around all that long, and the seas have been full of fish and squid for much longer, so you would think that they would have had the time to evolve echolocation. So where are the marine echolocators? Nothing. Silence.
So I asked a biologist, Steve Haddock, who was kind enough to enlist a colleague, Sonke Johnsen. Here is their conversation, Steve Haddock first: "I don't know of any examples. Lots of fish make sound (the midshipman), but it takes a lot of energy and seems to be largely for mating. Maybe the distance between their 'ears' is too small to be effective? Even humans underwater can't tell what direction sound is coming from. That doesn't explain bats, but different speeds of sound in air vs. water? Not sure, but it is an interesting question!"
I had not thought of that, but sound certainly travels faster through seawater than through air. At a depth of 2 km, sound travels at a speed of over 1500 m/s. Compared to about 333 m/s in air at sea level, the speed of sound in the deep sea is about 4.5 times faster. That matters, because you can tell the direction of a sound by measuring the differences in arrival time between two ears. Immersing those ears in water immediately makes the difference in arrival time 4.5 times smaller and therefore more difficult to detect. Could it still work? To find out, I first assumed a distance between the two ears of 20 cm. With that, a sound coming in from the side will arrive 0.6 ms later at the farthest ear in air, and 0.13 ms later in the deep sea. That does not seem like a lot, but Wikipedia informs us that humans can detect differences in arrival times of sound of 0.01 ms. So, given some good neural software, it should be possible to use this trick in the deep sea.
Anyway, Sonke Johnson added the following to Steve's reply: "There seems to be no good reason why fish don't echolocate. There are certainly fish and sharks whose heads are wider than echolocating dolphins. It's also not a marine mammal thing, since seals don't echolocate. Many fish and mammals eat the same things, so it's not that either. Cetaceans have great hearing, but that's sort of a chicken-egg thing and there's nothing preventing fish from having better hearing. You can't even say it's a warm-blooded-only club, because certain large fish (e.g. swordfish, tuna) actually heat up their brains and eyes so that the work faster. It's probably just one of those things. One possibility is that early cetaceans may have started in muddy rivers. Muddy river animals sometimes evolve interesting sensory systems (e.g. electroreception) because it's impossible to see. Even today, some cetaceans inhabit murky rivers and lagoons. Of course, many fish do too...."
I thanked both through email, but would like to repeat my gratitude to them here.
Back to the light
So far, we have to conclude that we do not know why the deep seas are not filled with Big Bad Echolocating Fish (BBEF) or Squid (BBES).
Click to enlarge; from this source
The sea is full of bioluminescent animals, and the image above shows the ways it can be used for offensive purposes, something we will focus on. An amazing array of life forms, from bacteria to many diverse major groups, have bioluminescence. They use it for a wide variety of purposes, that can be basically divided into defensive and offensive ones. Steve Haddock has written a very comprehensive review, that can be obtained free of charge, and which is very readable for non-biologists. There is also an excellent website. I will focus on just one of the many uses of bioluminescence: to illuminate prey using photophores.
Click to enlarge; source here
First, what are photophores? Well, the word simply means 'light bearers', so they are organs producing light. Without ever having studied them, I thought they would be just sacs with bioluminescent chemicals in them. But as the image above proves, showing a squid photophore, they turn out to be much more complicated than that. Perhaps you recall the reasoning that the physics of light quickly led to the evolution of a camera-type eye, with a retina, lens and diaphragm? Well, there are lenses and shutters in photophores as well. There must have been a process very similar to that of evolution of the the eye, but here the question must have been how to produce the best biological flashlight possible. The image above shows a photophore from a squid. At the centre there is a light producing mass, surrounded by a mirror, reflecting light until it exits the photophore through a lens. I have unfortunately not found a review paper comparing the optical design of photophores, but this should be enough to prove how complex they can be.
From: Kenaley CP. J Morphol 2010; 271: 418-437
Now, finally, we are ready for the final twist in the comparison of vision and echolocation. There are fish, shown above, using well-developed photophores as searchlights to find their prey. This use of light is very similar to echolocation: the animals have to provide their own signal, resulting both in a limited range and in becoming rather conspicuous.
Click to enlarge. Photophores from the dragonfish Malacosteus. In the two images, 'c' is the light-emitting core, 'r' is the reflector surrounding it, and 'f' is a filter to give the emitted light a red colour. The light bounces around until it exits the photophore through the aperture 'ap'. Form: Herring and Cope, Marine Biology 2005; 148: 383-394
The fish best known for this behaviour are so-called dragonfish, and their use of photophores involves the kind of wonderful bizarre features that only real evolution produces. These fish send out red light, which is unusual because red light doesn't carry very far in water. Most bioluminescent signals therefore use blue light, and accordingly most animals in the deep sea cannot see red light. They also cannot see the red light emitted by the dragonfish, which is rather cunning and makes the searchlight invisible. The snag is that some dragonfish species do not have a pigment in their retinas to see red light either...
Instead, they use a trick: there is an antenna protein in their eyes that is sensitive to red light, and this transferred the energy to the pigments sensitive to blue and green light that the fish does have. That transfer pigment works like chlorophyll, not a protein you expect in an animal at all. That's because the fish obtain it from their food and somehow transfer it to their retina. All this can be found on the website I mentioned. I certainly would not dare to use such outrageous traits in my fictional animals!
The oceans may not be filled with predatory BBEF, but there aren't many Big Bad Flashy Fish (BBFF) either. Neither option seems to have gained evolutionary prominence. Perhaps their characteristic conspicuousness makes these options too risky. I must say I like the option of equipping animals with flash lights.
Click to enlarge; copyright Gert van Dijk
So here is a quick and rough sketch of a possible Furahan animal with searchlights, which has just spotted a tetropter. Would the edge of better prey detection outweigh the increase in its own predation risk? I do not know. There are other worrying thoughts: why is bioluminescence on Earth so rare outside the oceans? It is hardly found on land, and does not even seem to occur in fresh water either. Are there reasons for that? Is the poor animal shown above doomed already?
25 comments:
personally, i think the reason fish didn't evolve echolocation was because they evolved the vibration end electrical field sensitive Lateral Line early on. Lateral lines are hyper sensitive to the changes in the environment that other animals make just by moving, so they have little evolutionary pressure to evolve an active sense like echolocation.
for fully aquatic mammals like Cetaceans, which don't have such a sense, evolution would have put pressure on sight, smell, and hearing. from paleontological evidence, early whales didn't have an active component to echolocation, and seem to have had a stronger reliance of sight and smell. presumably however their sense of hearing was acute though. somewhere along the line they developed the ability to enhance their own vocalizations and began to rely on those more. while the fossil record is fairly complete for understanding the gross physical evolution between land dwelling animal to fully aquatic animal, i'm afraid it is still very spotty regarding the evolution of their senses.
most other aquatic mammals seem to be (from what has been found so far), far more recent than cetaceans. most other aquatic mammals also are under different pressures. Pinnipeds (seals) aare hunters, but most don't go deeper than the twilight regions.. and we have examples of a form of 'proto-echolocation' for the ones that do.
Sea cows and dugongs are herbivores, and don't need echolocation to find food.
and Otters don't hunt deep enough to need anything other than vision.
Reading through the paragraphs on bioluminescence, an idea struck me: could it be possible these echolocators have developed some kind of system that constantly produces sound? It could be tied to their respiratory system, like vocal cords that vibrate as the animal breathes in and out.
As I read about the dragonfish, I had another thought. These animals could produce sound at different frequencies that may be inaudible to predators or prey.
I agree with mithril that the most likely reason that fish haven't developed echolocation is because they have other lines of sensory input in place that solve the same problems. The lateral line sense and their occasional modification into electroceptors do just fine in helping fish detect their surroundings. And there is plenty of evidence pointing to cetacean echolocation, especially in toothed whales, so there must be some pressure for it.
Bioluminescence is a subject that could (and probably should) get treatment all its own, especially given the complexity of the photophore. As far as its treatment in this post, one of the first things I wonder about is the brightness of the light, and therefore the range of benefit from these bioluminescent 'flashlights'.
When I look at pictures of bioluminescent fish the bright spots are certainly apparent, but they don't seem to radiate much light. As far as I understand it, that light is only useful in helping a fish catch its prey with more precision at the last moment, rather than spotting prey at a distance. So I call power and range into question. Could a creature actually make bright beams of light shine outward on prey with enough intensity to spot prey at a distance? Is this physically possible?
My second thought is motivation. Why not use other senses that will help you find prey without broadcasting your presence? What if it doesn't care about being seen? If it has poisons or other predatory deterrents, then it has little to fear and the only question is the physical possibility of its 'flashlights'.
Arachnus, the idea of producing sound at frequencies outside of detection can be problematic. Even if it's outside the ear's sensitivity it creates a wave of pressure (that's what sound is) that could eventually be detected by others.
I have a feeling that a big part of why bioluminesence is rare on land (on Earth), is because the clade who went from water to land didn't have it.
{neither the lungfishes nor coelocanths have anything that would function as a storage place for bioluminesent chemicals or bacteria - though coelocanths' eyes seem made for *detecting* such light)
maybe if something like loosejaws or viperfish were to start making (mudskipperish) invasions of the land - at night, to dodge some predators and dehydration - we might end up with Flashlight Snakes and Glow-Spot Eels.
Or-- dare I say it-- land squid? ;)
Mithril: that seems certainly possible, and might explain differences between marine mammals. I wonder what the largest range is at which the lateral line system can work. You would expect system to provide more benefit as the range increases, so it would be nice to compare ranges under water of various senses.
Arachnus: Producing sound is probably not very costly, so I would expect it can be generated in many ways. If I were a predator I would prefer a system that I could also turn off.
As for producing sound at frequencies inaudible to others, that would work. But the range of hearing is already very wised in many animals, so it might not be easy to find a high frequency range that your prey cannot (yet) hear. If it can hear it just a bit, it will evolve quickly to improve that sense.
Evan (1): Bioluminescence is definitely worth much more consideration. I thought about writing on it, but at present I am still amazed at the many paradoxes it seems to offer (such as that glows appear to attract and flashes appear to repel; can't predators adapt?)
I picked out only one use of bioluminescence, the searchlight, because it forms a nice parallel to echolocation. Read the review I liked to and be surprised. But the 'offensive searchlight mode' seems to be fairly rare. It exists though, and in speculative biology that is a pretty argument, I should think... But I have my doubts about its applicability, as the last lines of my post show.
Rodlox: that is a good point as far as vertebrates are concerned. I do not know how many other groups of fish have bioluminescence; there appear to be many. As for invertebrates, bioluminescence seems almost part of their basic kit in the sea.
Evan (2). Luminescent land squid (or, the film 'Monsters'). May I say that I still like the concept, even though it's been done to death.
There may be various reasons why 'flashlighting' is less advantageous than echolocation;
- Echolocation involves producing vibration, which seems to be something that animals do rather easily. Bioluminescence requires the transformation of chemical energy into light (the efficiency of this process is not simply relevant to this discussion, but relevant to discussion of bioluminescence as a whole).
It may be that echolocation is more efficient than 'flashlighting' in terms of sensor return for energy expended.
- Echlocation tells the
echolocator the range to an object simply as part of the mechanism itself. Flashlighting does not, due to the much higher speed of light.
- Light can carry extremely far; a dark environment may be just as noisy as a light one, but a point of light at night will be very easy to detect; likely not disrupted easily by environental sources or wind, etc. This compounds the problem of making oneself conspicuous by using an active sense. While being Big and Bad, as you have discussed, does help, I do not feel that it removes the problem entirely, especially in regard to detection by prey items.
A torch beam can only illuminate things usefully out to a certain distance, but can be detectable from a much greater distance. If your prey detect you long before you have even entered their vicinity, you are at a huge disadvantage.
- Vision is an extremely prolific sense, and thus such detection of your torch beams should be rather easy. While the prey items of echolocators may not by any means be deaf, vision is generally more prolific than hearing and more useful even at a simpler stage of development. You can, of course, use a wavelength of light that none of your prey items can see (efficiency of producing it permitting), but there are limitations to this as well. It should be considered that none of the prey items of the stoplight loosejaw can see the wavelengths it emits at, because the environment in which they live is extremely deficient in those wavelengths.
These issues can also be solved somewhat by having a dimmer beam, and more acute eyes to detect the reflections... but this brings the discussion down the pesky path of why you would use a flashlight at all if you can just enhance the acuity of your eyes to enable passive sight in dark environments- which would eliminate any issues with 'flashlighting'.
Even if the energy budget of throwing such a beam is acceptable to the animal, and the use of energy is efficient enough, it is still a use of energy, and I would bet that a flashlight will use more energy than an eye adapted for night vision would.
If the reason for the absence of echolocation in fish is the presence of decent passive senses, than the higher energy requirements and increased evolutionary complexity of 'flashlighting' may be what precludes its abundance.
The loosejaw fish is an organism in a quite light deficient environment- the deep sea, that has stumbled through a number of lucky evolutionary events that allow it to use 'invisible' bioluminescence to spot prey.
If I may go a bit off-topic here, in a manner that relates to some of the rambly stuff I have posted here previously; I know you have described the other planets in Furaha's system, and some astronomical traits of Furaha itself (axial tilt and soforth), but I cannot remember whether you have stated whether Furaha has any moons and what their attributes are. I can certainly understand if this information is intended only for publication in book form; I am asking about this issue because the presence of a moon (and thus of moonlight) is quite relevant to nocturnal animals.
Regards,
T.Neo
T. Neo: thank you for these well-thought out comments. This topic certainly seems to bring out the essayist in all respondents!
Mind you, I haven't said that all oceans should be filled with 'flashlighting animals'. I too have my problems with the concept as an overall winner. The very fact that it exists is wonderful in itself, though, and it deserves consideration. The real problem may be that it is very hard to weigh al the pros and cons. Just for the sake of getting to the heart of the matter, let me see if I can counter some of your arguments.
- flashlighting does not tell you range by itself. True, but as it makes use of vision, the resolution of the information bounced back is much higher. Animals using it will have eyes already capable of combining images to infer range anyway.
- both flashlighting and echolocation share the disadvantage of being very noticeable from a distance. I share the feeling that this works better for light, but the reason for that might be that we as humans simply have very good eyesight but relatively poor hearing. Is there an objective scale to compare the two.
- You raise he point that you can use dimmer light if you enhance the sensitivity of sight. True again, but there are physical and biological limits to increasing eye sensitivity. Eye sensitivity to a large extent comes down to eye size, and beyond a certain point that development stops. You cannot keep on increasing sensitivity, so there is a point at which adding a bit of light will provide an additional benefit.
I am not making all this up to be difficult; your points are good, but I think that many of the costs and benefits are hard to calculate. Without that, it becomes speculative biology... ;-)
Bioluminescence is fascinating topic for speculative biologists. Have you known that there are "firefly tourism" in southeast Asia? http://www.youtube.com/watch?v=4dg8HEn1CVc
I can think of no reason why bioluminescence on land must be limited only to small creatures. With bigger animals, there could be also more complex behaviour. Predators hunting in packs can use bioluminescence to disorient their prey instead of revealing their position. Social animals can use it to a complex communication (there are some similar predators on Nereus I think).
The energy expenditure of bioluminiscence could be in fact used for display of health and strength. Think about the abundance of energy spending and "unnatural" complexity in mating rituals of birds. So I think that bigger animals would not just use bigger "lamps", but also more elaborate shows.
Sigmund; I definitely understand where you are coming from, and indeed the difficulty of calculating the various costs and benefits was on my mind when writing my comment. The more I think of speculative biology topics, the more I feel that everything is either fairly well based on the laws of physics, geology, planetology, chemistry, etc... or its likelihood or evolutionary cost/benefit is difficult to pin down and is thus disputed...
Of course it is not impossible, or difficult, to perform range-finding with vision (after all, we do it constantly). My original point was that echolocation confers a specific sort of rangefinding ability and thus a specific sort of advantage. Of course, echolocation comes with a certain set of disadvantages over vision (and thus presumably, flashlighting). That they do work differently, of course, is relevant to how they evolve- but I would imagine that even a meagre understanding of exactly how those differences might affect the evolution of flashlighting would likely take a lot of thought and research.
Your point that other animals have better hearing than we do is a very good one; however, it should also be pointed out that there are also animals with better eyesight.
I was thinking more in terms of it simply becoming more physically difficult to detect sound at a certain distance than light; perhaps I am simply talking nonsense, but there must surely be a distance from an emitter at which point the signal from it diminishes to a level either intrinsically undetectable or undetectable against background
'noise'. At night, it's obviously relatively dark, but there's nothing preventing usual environmental sound
production- thus the level of visual 'noise' should be lower than the level of auditory noise, and it should be easier to detect a light source at a distance (as an example of this sort of principle, it's far more difficult to make out a torchlight at a distance in daylight, when it's generally brighter).
I would be willing to bet that the evolutionary and physical limits of eyes are the reason why the stoplight loosejaw uses flashlighting in the first place; it's a deepsea creature that would presumably already have rather well-developed eyes, and developing them further would incur heavier disadvantages than flashlighting.
Of course, a whole plethora of animals seem to get along fine with vision in low-light environments, such as at night. My gut feeling is that the costs of flashlighting become problematic before the costs of enhanced vision do, but I agree that from an actual quantitative point of view, the costs and benefits are difficult to ascertain.
Apologies for yet another long post,
T.Neo
>echolocation confers a specific sort of rangefinding ability
...when advanced enough, sure. but just making noise, is more likely to attract predators, than to tell you anything.
>but there must surely be a distance from an emitter at which point the signal from it diminishes to a level either intrinsically undetectable or undetectable against background
...I'm sure the US Navy wishes that were true, particularly where whales are concerned.
My knowledge of echolocation isn't all that good, but simply making a noise- of the right sort and with the right sort of 'hardware' to detect echos, [i]will[/i] give you information about your environment, even if it isn't very good information.
My knowledge of the US Navy isn't all that good either, but I'm not entirely sure how their operations invalidate what should be something based in fairly simple physical laws; signals diminish in intensity with distance, and at a certain distance it will become impractical or impossible to detect that signal. Obviously the specific signal, situation and sensor all change the dynamic of the scenario and thus its outcome, but that doesn't invalidate the basic principle.
~T.Neo
Anonymous: That distance could be extremely long in some cases
http://en.wikipedia.org/wiki/SOFAR_channel
Maybe it is obvious, but light and sound propagate very differently in different enviroments and even type of vegetation could completely change the advantages of respective sense.
if it were that easy to detect echoes, everyone could do it. (we humans can determine the direction a sound's coming from - "the tv on my left" - but only a handful of people are able to do any degree of echolocation)
I agree with T.Neo's observation about the gaps in speculative biology: often there is indeed too little information to state whether an idea is workable or not. From that point on we have to rely on an intuitive understanding of physics and biology.Of course, that can give rise to differences in opinion; personally, I enjoy the exchange of ideas.
As for the decrease in intensity of a signal to uselessly low levels, that must be the case. The thing is whether the distance involved overlaps the range useful to animals. Sound and light decrease over distance in a similar manner, but sound may have an additional disadvantage: Being mechanical in nature you would think that there would be friction involved, diminishing the intensity in addition to the square-law effect. How much this affects the diminution of intensity I do not know (a quick search did not reveal much).
SN: Maybe this will help http://www.me.psu.edu/lamancusa/me458/10_osp.pdf
But I think that visibility is even more vulnerable to "additional disadvantages", especially in water
Ugh. I meant to comment on this earlier, but life intervened and I'm only getting around to it now. I think mithril might be right when it comes to the lateral line being a better alternative to echolocation, given it can provide a lot more sensory information without giving away an animal's position.
Actually, the fact that many marine fish have photoluminescent organs makes brings up the exact same problems as echolocators. In both cases you're emitting a beam of sensory information (light,sound) into the darkness essentially giving away your position to every predator out there. There has even been some suggestion that the large eyes of big, predatory squid are an adaptation to track down photoluminescent fish.
Interestingly, Cracked Magazine, of all people, chipped in a little bit of useful information on the whole issue. They noted that the kind of sonar you see in most movies, where a pulse of sound is let out into the surrounding water to identify nearby objects, is not the type of sonar you normally see submarines you. Instead they use passive sonar, which is essentially sitting there and listening really, really hard. They do this for some of the exact same reasons you used for echolocators above.
"It's also not a marine mammal thing, since seals don't echolocate."
There has been some suggestion in the past of echolocation in pinnipeds, but in every case the evidence has not stood up to scrutiny. It has been suggested that the reason echolocation never evolved in pinnipeds is because they spend at least some time on land, and its difficult to attune the auditory system to the degree of sensivity needed for echolocation AND maintain function in both environments.
"Instead, they use a trick: there is an antenna protein in their eyes that is sensitive to red light, and this transferred the energy to the pigments sensitive to blue and green light that the fish does have"
Something like this could actually make echolocation feasible in an alien environment. Bats today are able to echolocate at different frequencies, such as how some fruit bats are able to echolocate at frequencies audible to the human ear. All you would need to do is echolocate at a frequency that most predators cannot hear, and you can maneuver without attracting attention. Of course, some predator would eventually specialize in picking up your signals, and an evolutionary arms race would begin.
--Metalraptor
"If I may go a bit off-topic here, in a manner that relates to some of the rambly stuff I have posted here previously; I know you have described the other planets in Furaha's system, and some astronomical traits of Furaha itself (axial tilt and soforth), but I cannot remember whether you have stated whether Furaha has any moons and what their attributes are. I can certainly understand if this information is intended only for publication in book form; I am asking about this issue because the presence of a moon (and thus of moonlight) is quite relevant to nocturnal animals."
I agree. Neil Comins published a spec evo-related book, "What if Earth had Two Moons" in which he discusses the astronomic and biological effects of alternate arrangements of our solar system. According to this book, a second moon would have a huge impact on how bright a planet gets at night (assuming the moons are the same size). And studies on caenolestid and didelphoid marsupials have shown that a particularly bright moon can be lethal for such small, cryptic animals as these.
--Metalraptor
Rodlox, perhaps I'm a freak of nature, but I'm capable of detecting self-created echos in a suitable space, and gleaning some information about that space from the echo- whether it is a compact or volumnous space, covered in hard or soft surfaces, cluttered or empty. Of course it isn't echolocation, but is a means to gain information on one's environment.
In regard to SOFAR channels, the SOFAR channel does seem to have a very large effect on sound propagation in the ocean. The closest analogy in terms of light that I can think of is something like a fibre-optic cable; I doubt such a phenomenon could present itself on a large scale in a natural environment on land.
On the other hand, there is a phenomenon that could bolster flashlighting; eyeshine. The tapeta lucida employed by many animals for exactly the purpose of low-light vision act as naturally occuring retroreflectors, conveniently reflecting light back at a source. This gives a potential flashlighter the advantage of (a) being able to directly pinpoint the location of a prey item via eyeshine, and (b) potentially detect prey items at a greater range (or lesser illumination) than would be possible simply by illuminating the environment. Detection of eyeshine, specifically, is theorised to be the use of the Stoplight Loosejaw's flashlighting behaviour.
One could argue that detection of eyeshine is a far more advantageous purpose for eyeshine than simple illumination (for example, other factors aside, this use is just as advantageous for a creature adapted to dark environments). Of course, there are still disadvantages, such as those that come with an active sense, etc.
I did consider differences between sound and light propagation, but was unsure of the exact effects- I decided not to add another hazy point to an already hazy comment. I agree with Jan that light is not immune to such interference, but I would imagine that light will degrade much less in an Earthlike atmosphere than in water even in good visibility.
~T.Neo
all things being equal, any hearing person can tell if a basketball is being bounced in a small room, a large gymnasium, or an aircraft hangar.
...but I wasn't referring to that. I was referring to the point of echolocation: finding things your size or smaller in a room. (can your self-created echoes tell you if you're about to walk into a chair?)
I think that this link may provide some food for thought...
"Rodlox, perhaps I'm a freak of nature, but I'm capable of detecting self-created echos in a suitable space, and gleaning some information about that space from the echo- whether it is a compact or volumnous space, covered in hard or soft surfaces, cluttered or empty. Of course it isn't echolocation, but is a means to gain information on one's environment."
A lot of people can actually do that, given enough practice. It's just that the human body isn't as specialized for detecting the fine details in the reverberating sound (and hence, increase the quality of the echo) to the degree seen in bats and dolphins.
It seems to me that the discussion is beginning to resemble sounds echoing between walls a bit. Anyway, on the subject of human echolocation, here is something I found and linked to in my first post on the subject. It seems that people can learn to use echolocation to an amazing degree.
http://www.echolocaters.com/?page_id=111
Echolocation has a very specific potential advantage that hasn't been mentioned here. Waveform control.
Flashlighting is going to produce one or a few very specific wavelengths. If more than one wavelength is produced, you could vary the different wavelengths, but there's no obvious way (that I've thought of) to achieve stealth in this manner, other than the trick where the prey can't see the wavelength you're using.
On the other hand, sound production in animals can be controlled with a great deal of precision, as parrots amply demonstrate. This is because you don't need specific molecules and protein complexes for each wavelength. There are two stealth echolocation tricks that I've come up with by taking advantage of this difference, and I believe one leads easily into the other.
The first is simple mimicry. The echolocation pulse sounds like some ordinary noise in the environment. This relies on having an environment with a decent amount of noise, but that shouldn't be too hard. The big drawback with this is that it's probably going to be very suspicious if used continuously, so it would likely not be useful for navigation.
The next trick is based on noise radar technology. This uses random noise on a very wideband frequency spectrum to hide this signal among background noise. This has large signal processing requirements, but could potentially be used continuously without being noticed until the echolocator is pretty close to its target, and would allow fairly safe navigational use. This would be difficult for other animals to detect for the same reason that white noise machines work.
http://spie.org/x90215.xml
http://en.wikipedia.org/wiki/White_noise_machine
Andrew: thank you. You have posted several insightful comments lately, but on old posts, so I wasn't too sure what to do with them (I am none too certain that people still read these old posts). It does seem as if you know what you are talking about though; send me an email if you would like to converse in more detail.
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