Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979;

(c) 1979 Robert A. Freitas Jr. All Rights Reserved.




Chapter 13. Sensations

"It was the face that made Otis stare. The mouth was toothless and probably constructed more for sucking than chewing. But the eyes! They projected like ends of a dumbbell from each side of the skull where the ears should have been, and focused with obvious mobility. Peering closer, Otis saw tiny ears below the eyes, almost hidden in the curling fur of the neck...
-- attr. to H.B. Fyfe (1951)70

"Those races that had the electric sense gave us some difficulty; for, in order to understand their thought, we had to learn a whole new gamut of sense qualities and a vast system of unfamiliar symbolism. The electric organs detected only very slight differences of electric charge in relation to the subject’s own body. Originally this sense had been used for revealing enemies equipped with electric organs of offense. But in man its significance was chiefly social. It gave information about the emotional state of one’s neighbors. Beyond this its function was meteorological...."
-- Olaf Stapledon, in Star Maker (1937)1946

"A being who hears me tapping
The five-sensed cane of mind
Amid such greater glories
That I am worse than blind."
-- unidentified American poet81


Of all the abilities possessed by creatures of other worlds, perhaps none is so important as information processing. According to negentropic definitions of life, the gathering and utilization of environmental data is somehow the point of living. On our own planet, neurons, nervous systems and brains have been instrumental at the highest level of organismic functioning: Intelligence.

If intelligence is data processing, senses must provide the data. It’s a good bet that any entity complex enough to have intellect will also have evolved a fairly complicated sensory network to keep its mental equipment well-supplied with information.

The sensoriurn of any extraterrestrial creature must, at the very least, be sufficient to ensure survival. There will be few if any monosensory (having only one sensory modality) races in the Galaxy. Even the lowest microscopic lifeforms on Earth are not so restricted. Aliens will have a multitude of complementary senses which "support, confirm, modify and duplicate the major one."1000

On the other hand, there’s no point in evolving more senses than the brain can handle. Those which are essential to survival will be developed, but there will be high efficiency and little surplusage. New senses will arise only if they clearly increase the organism's chances for survival, or if the basic environmental constraints -- the rules of the game of living -- change. So extraterrestrials should have just those senses which are optimal or optimized for survival, and none which are superfluous or which might overload the brain.

The sensorium will be based on natural phenomena commonly occurring in or around the immediate habitat. Of course, any form of energy that can be emitted, transmitted, modulated or received theoretically may serve as a basis for sensation. But there do seem to be a few normal limits on the diversity of biological sensors.

For small organisms tactile senses (touch) are usually sufficient to get by, since the immediate vicinity and the imminent future are all these creatures care about. Taste, touch, vibration, temperature, and all other qualities of the external world that can be communicated by direct physical contact alone are valid possibilities for tactile beings.

Among larger creatures, contact senses tend to be less important because the organism now must be apprised of distant events and more extensive time spans. Remote sensing is of greater significance -- perhaps chemical diffusion (smell), acoustic radiation (sound), electromagnetic radiation (seeing), particulate radiation (alphas, betas), and so forth.

Seldom do we pause to consider just how many separate worlds exist side by side on Earth. Every lifeform has its own way of knowing and its own unique brand of intelligence, because each operates with a different set of input data. The dog lives in a world of scent; the porpoise in a world of ultrasonic sound; the frog in a world of motion; the human in a world of color. Each of these organisms has a singular window on reality, a novel way of looking at the material universe. Indeed, each sentient creature occupies a different subjective universe altogether.

If we attempt to "see" through the senses of other beings, how different would be our view? Without sight, or sound, what would human culture and science and society be like? What would it mean to be able to sense the myriad electrical fields around us, or the variations in barometric pressure which herald the arrival of a thunderstorm? With other kinds of knowledge directly accessible to our brains, how much differently might we think and act?

The windows on reality of intelligent extraterrestrials may be wider than ours in some places, narrower in others, and occasionally absent altogether. Vistas of natural beauty and panoramic splendor may be available to them about which we can only dimly speculate. If they draw different conclusions about the cosmos, we shall not be surprised. Indeed, we should be delighted, for it is one of the highest aspirations of xenology to elevate humanity to a new awareness of itself and its limitations.



13.1 Tactile Senses

The contact or tactile senses are the most primitive of all. Virtually no terrestrial organism alive today fails to respond in some manner when physically touched. Pressure receptors seem useful in almost any environment imaginable, so it appears reasonable to presume that aliens will have at least some simple tactile sensitivity.

Vibration sensing is a direct extension of the sense of touch. The blow fly is an excellent example. This amazing creature has an elaborate network of vibration sensors along the leading edges of its wings. This serves two purposes. First, the insect is alerted to changes in speed of the prevailing winds. When local gusts greater than about 1 kph are detected, the animal drops down and quits flying against this dangerous headwind. Second, air passing over the wings sets up vibrations of particular kinds. In response to this tactile information, the blowfly carefully adjusts the shape of the airfoil and the frequency of flapping during each wingbeat cycle.82 It is a remarkable example of sophisticated biological avionics.

There are many other vibration sensitive creatures on Earth. Earthworms have no eyes or ears, yet they are so responsive to ground vibrations that they can actually feel the footfalls of an approaching shrew.79 Among honey bees, ants and other insects touch is both a method of sensing the environment and a means of social communication. Bees are known to be thoroughly distracted by vibrational frequencies between 200-6000 Hz. (One in particular, an octave or two above middle-A, produces virtual anesthesia in the insects.82)

Bees also seem to have an "absolute architectural sense." This enables them to construct perfectly hexagonal honeycomb structures to engineering tolerances of less than 0.1 millimeter. This extreme accuracy is achieved by the use of many tiny tactile hairs located on minute sensory bristles on the creatures’ legs.1000

Could touch be the sense modality of choice for some advanced alien species? It is admittedly difficult to conceive of a plausible environment which would favor touch over all other competing senses. Still, one can imagine dark, turbulent, noisy surroundings which render seeing, smelling and hearing virtually ineffectual. Organisms evolving in such a milieu might be able to glean reliable nonconflictory data only by slowly feeling their way along, sampling the taste, texture, and vibrations in the solid surface below. It would be a kind of two-dimensional existence -- "up," a direction both dangerous and without useful sensation, would have little meaning for these entities.

Such beings, relying almost exclusively on tactile data, could easily develop a most precise and detailed system of communication. Aside from such familiar passive systems as Braille books and typewriters, active techniques might easily be employed even be less intelligent alien lifeforms.

About two decades ago, Dr. W.C. Howell and his colleagues at, the University of Virginia attached a series of vibrators to the chests of male human volunteers. These devices could be triggered at any of three different frequencies, three intensities, and with any of three distinct types of signals (akin to "dot," "dash," and "long dash" in Morse code). A 27-letter "alphabet" was thus set up.

After about 75 hours of training, one student was able to master a language jokingly called "vibratese" by the experimenters. He had become proficient enough to understand sentences vibrated through his skin with better than 90% accuracy, and at a rate of up to 38 five-letter words per minute. (This is better than "proficient Morse" in radio telegraphy.) It was estimated that with further practice, rates approaching 67 words/minute might have been achieved.1694

The significance of "vibratese" to xenobiology is driven home when we realize that many animals on Earth have at least the latent capability for such tactile languages. Consider, for instance, the octopus. These clever cephalopods possess highly responsive sensory tentacles which allow them to chemically sample and feel their way around the ocean floor. In addition to these amazing organs of taste-touch, however, octopuses are also known to be extraordinarily sensitive over all parts of their bodies to tactile stimuli.

This could be an important factor in social communication among these animals, because the octopus can alter the texture of its skin at will. Like a shifting mosaic of brailed skin, the cephalopod’s integument can be altered to display many shades of roughness -- from perfect smoothness, to slightly corrugated, to a kind of "gooseflesh" or tiny raised dots, to larger pimples called "papillae," to even coarser irregularities which have been described as "arborescent projections." A kind of interactive tactile Braille-talk seems quite possible among octopoids, should they ever be of a mind to develop it.

What sort of technology could be mustered by predominantly tactile creatures? Dr. Frank A. Geldard and others at the University of Virginia have extended the earlier work of Howell in a way that gives some insight into the possibilities.

Geldard provided ten subjects with cutaneous vibrators similar to the ones used in Howell’s experiments. The chest devices were buzzed in sequence to give the illusion of direction and speed, and the volunteers were required to turn an automobile steering wheel in response to the perceived motion. The humans proved capable of keeping on target using chest-vibratory tracking just as well as they could using visual clues alone. Writes Geldard:

Although the visual conditions are not optimal for this sense -- the target was "traveling" at the rate of only 3.5 degrees per second, and the eye can handle speeds many times at great -- the tracking task imposed on the subjects was one that would keep all but the speediest vehicles comfortably on course, and the skin was handling the assignment fully as well as the eye.1694

The "Optacon" (OPtical-to-TActile CONverter) developed by NASA is another step in a similar direction. The device converts normal inkprint into a tactile format on an array of small vibrating rods and enables the blind to feel-read without Braille at speeds as high as 90 words per minute. Various attachments allow the unsighted to read typewriter and electronic calculator displays in a similar fashion.

Of course humans are visual, not tactile, beings. The fact that people can do so well is incredible, given that the skin is one of our least sensitive organs. How much more might ETs on other worlds be able to accomplish after their sense of touch has undergone millions of years of evolutionary honing and polishing?

Geldard’s experiments and the results of NASA research seem to suggest that much as radar converts radiation we cannot see into useful visual information on a glowing screen, perhaps tactile aliens, likewise aided by their machines, may be able to drive cars, fly airships, and communicate over long distances using a tactile telegraphy technology.

While a sensorium based mainly on touch seems horribly restrictive to sighted and auditory beings such as ourselves, there may be others in this Galaxy who think otherwise. With eons of natural evolution behind them, such tactile systems could be fantastically complicated, beautifully refined, and far more versatile than any comparable means found on Earth.



13.2 Olfaction

What about the sense of smell? While the related sense of taste may be too restricted for use as a primary sensory modality,* could intelligent extraterrestrials on some distant planet find that olfactory cues are optimal for survival?

In man, the organ of smell is quite small -- a total of 5 cm2 of odor-sensitive tissues representing some five million olfactory sensory cells. With their sense of smell, humans are remarkably responsive to a wide variety of odors. Sherlock Holmes once stipulated that a good detective should be able to recognize at least 75 distinct scents, but anyone restricted to so few would be the olfactory equivalent of "deaf and dumb." Humans normally can distinguish literally thousands of different smells, and extreme sensitivity to certain key substances does exist. (For example, a man can detect ally mercaptan in concentrations as low as 60 million molecules/cm3.1695)

Yet humanity is a visual species, in the main. Our language for describing scents is virtually destitute. Other animals are vastly more smell-conscious than we, proving that the adoption of such a strategy may well be a viable alternative for intelligences on other worlds.

The nostrils of an unaspiring rabbit hide some 100 million olfactory cells. Dogs do even better. Dachshunds have 125 million cells, fox terriers nearly 150 million, and the German sheepdog 225 million smell cells -- enough for 45 human noses, all packed into a single snout. (This figure should be compared with the 125 million optical sensory cells in man’s eyeballs.) Experiments have suggested that the dog’s sense of scent is a million times more acute that of a human being.

An even better smeller is the silkworm moth Bornbyx mori. The female of this insect species secretes minute quantities of a fatty alcohol called "bombycid" which evaporates rapidly to permit wide dispersion. A male moth can become "drunk" on this sexy scent from as far away as 20 kilometers, catching the chemical messengers on some 17,000 sensory hairs located on each of two feathery directional antennas. Each hair responds to single quanta of odor, and the male takes to the air when the concentration of bombykol rises to about 14,000 molecules/cm3. Test subjects released several kilometers from their prospective mates have returned, like sexual guided missiles, in less than half an hour.565,2511

The idea of substituting smell for sight as the primary mode of perception for intelligent beings is hardly farfetched, despite its relative neglect by science fiction writers.2510,2536 According to the renowned Harvard University entomologist Edward O. Wilson:

It is conceivable that somewhere on other worlds civilizations exist that communicate entirely by the exchange of chemical substances that are smelled or tasted. Unlikely as this may seem, the theoretical possibility cannot be ruled out. It is not difficult to design, on paper at least, a chemical communication system that can transmit a large amount of information with rather good efficiency. The notion of such a communication system is of course strange because our outlook is shaved so strongly by our own peculiar auditory and visual conventions.2533

Chemical signals have many advantages over visual ones which might make them more competitive. For instance, pheromones (scent-messages) travel around obstacles rather easily, and pass through cracks, tubes, tiny holes, and around corners -- unlike light. Smells can be transmitted through total darkness or in extreme brightness, or through audibly impenetrable regions of high sonic noise.

From a strictly energetic standpoint, pheromone transmission is quite efficient. Less than one microgram of a compound (which is very cheap for the organism to manufacture) can produce a beacon covering many square kilometers for hours or even days.565 Odor broadcasting is also a very simple operation, requiring only the exposure of a chemical-soaked gland to the passing winds. Consequently, smell-talk probably has the greatest transmission range of any normal sensory or biological signaling mechanism -- including vision or sound.

Humans and other visual creatures depend to a large degree upon instantaneous line-of-sight communications. We see, we react; we don’t see, we don’t react. Our past is sharply distinguished from our present. But scents linger in the air. The odors that identify and describe events for the osmic intelligence tend to blur the passage of time. Olfactory cues have the peculiar ability to transmit data into the future. Indeed, the same creature who released the signal in the first place can return later and use the information again. Intelligent ETs relying heavily on smell would differ profoundly in their manner of thinking from sighted animals.

From the human point of view, the osmic alien lives in a world of echoes. Smell signals, buffeted by tiny breezes and trapped in every little nook and cranny, would seem to reverberate again and again as they are redetected long after emission. Were our noses sharp enough to pick out the subtle olfactory distinctions and patterns, we might regard them as a meaningless cacophony of insensible jabber.

From the alien point of view, human would be deaf and dumb -- anosmic illiterates incapable even of baby-talk. They would doubtless be perplexed by the brief human attention span, but might be equally astonished at our rapidity of movement and thought and at our emotional transparency.

Along similar lines, Doris and David Jonas have suggested that reliance on scent could lead to more serene, stable interpersonal relationships between intelligent ETs. "Apparently forewarned of mood changes by pheromone messages," the Jonases tell us, "the Olfaxes find it easier to adapt to each other’s emotions before they become extreme or frustrated."1000

Xenobiologists, by and large, remain skeptical of such simple conclusions. It may be that emotions are more easily sensed by olfaction -- although R.L. Birdwhistle believes that the human face alone can convey an enormous amount of emotional data for sighted beings, some 150-200 distinct paralinguistic signals.2523 But even granting the primacy of smell in this respect, it is anyone’s guess whether sensitivity to the emotions of one’s fellows would result in greater or lesser provocation. Surely the argument may be advanced that osmic aliens would tend to be creatures of the heart and the instinct, rather than of the intellect, since the constant bombardment by emotional cues from perhaps an entire city-full of beings would provide a most compelling distraction.

As regards interspecies contact, there is no guarantee that all pheromone messages will have universal meanings. For all we know, humans may normally (and quite innocently) emit odors which to the scent-conscious aliens are sexy, rapacious, or obscene. Such unintentioned misunderstandings could have fearsome consequences.

The slowness of olfactory transmission and message fade-out have long been viewed as tremendous disadvantages for osmic aliens. It is difficult to convey signals over a long distance and to swiftly change from one signal to another, because odors must diffuse slowly through the air. Of course, ants react quickly to "fear" and "attack" pheromones laid down along trails, and rats in the midst of combat will suddenly cut off the attack when the opponent releases the odor of submission.2546 But there is little evidence among Earth's creatures that pheromones are used to transmit rapid-fire messages -- representing changes in aggressiveness, status, or attitude -- which are so routine in audiovisual biocommunications systems. In other words, the odor vocabularies of the inhabitants of Earth appear to be extremely limited.

But there are other ways for intelligent osmic ETs to get their meaning across. When people talk, the sound emerging from their voice boxes consists of pressure waves in air of variable frequency and amplitude. Why couldn’t aliens, by analogy, transmit modulated waves of scent?

No case of information transfer by such means has yet been reported in any animal species on Earth. However, it is also true that this possibility has scarcely begun to be considered by zoological researchers. Dr. William H. Bossert at Harvard University has calculated that the theoretical information transmission rate using olfactory communication is surprisingly high.2509 As has been pointed out by others,565,1693 transmissions over large distances in a steady moderate wind are both practical and highly efficient. Under favorable conditions, an optimal system could transmit roughly 104 bits**/second of information by modulating the emission of a single pheromone. Using more realistic assumptions -- say, sending messages 10 meters in a steady 14 kph wind -- the maximum potential information flow is still encouragingly high. About 100 bits/second could be transmitted, the equivalent of four 5-letter English words each second. This is much faster than most humans can speak.

Of course, this is only the value for each modulated pheromone channel used. The capacity of the system increases by 100 bits/second for each additional chemical substance which the osmic alien is able to generate and properly modulate. Ants (typical insects) have a pheromone vocabulary of at least ten distinct odor messages. If olfactory ETs have as few as ten channels at their command, they could "speak" at the truly astounding rate of 40 words/second.

Another apparently serious disadvantage of smell as the primary sensory modality is its relative inability to fix direction accurately and the corresponding lack of spatial resolution. It is probably impossible to thread a needle, play a fast game of darts, read a newspaper, or soldier electronic components onto a circuit board using a sense of smell alone.

However, there are two very good reasons why this need not be an insurmountable problem. First of all, the language of the osmic aliens may consist of a very large number (say, a thousand) of alphabetic characters much like modern Chinese or the hieroglyphs of the Mayans and ancient Egyptians.*** Interpretation of written or spoken symbols would then depend far less upon positional cues than on the character of the symbols themselves. This would make possible such technological marvels as osmophones and smellprint, as well as the ability to publish books and newspapers readable by predominantly olfactory ETs.

In addition, the olfactory bulb in vertebrate brains may permit a spatial patterning analogous to the three-dimensional quality of vision.1000,1701 If this is the case, animals receiving scent messages from their environment can actually "see" in a kind of 3-D "smell-space" -- a mentally reconstructed depth-perceptive odor-hologram of sorts. Most flora and fauna vary in constitution (and thus in scent) over the surfaces of their forms, and also smell differently if they are hot or cold, wet or dry, dead or alive, etc. An osmic creature could see another animal three-dimensionally by perceiving separately the odors emitted from its tail, legs, the fur on its back, its mouth, and so forth. By adding together the bits and pieces of data entering through the nostrils, an extremely accurate composite model of the object under observation might be built up by the brain.

What about the technology of these beings? Could a sophisticated scientific dialogue take place in a language of odor? The Jonases believe so:

The Olfaxes {an hypothetical race of ETs whose primary sense is scent} would not see the sun, moon(s), or stars of their planetary system. However, they feel the warmth of the hidden sun, or sense cosmic electromagnetic emanations. Using this information, they will be able to construct instruments that can penetrate their atmosphere and receive signals that they can translate into olfactory terms, just as the instruments of our astronomers can receive electromagnetic and other energy pulses and register these in visual terms.1000

Finally, where might we expect to find a race of "Olfaxes"? One oft-cited argument against the possibility of osmic aliens claims that it is comparatively difficult to imagine an environment which would favor smell over sight or hearing. But on a perpetually foggy or extremely hot planet, visual images would be wavering, dimly perceptible, and highly distorted. In rarefied atmospheres, sound waves would be weaker and less audible. Since pheromone molecules travel the farthest the fastest in hot, thin atmospheres, such an environment might favor olfactory modalities.

Furthermore, both audition and vision were fairly late developments in the evolution of life on this world. Osmosensory mechanisms, on the other hand, were among the first to evolve. It is entirely conceivable that else where in the Galaxy the early development of a highly refined sense of smell reduced the need for more elaborate alternatives. Evolution might never have had to go to the trouble of bringing forth complicated ears and eyes if complicated noses were already available and were adequate to ensure the survival of the species.

There is no reason why alien lifeforms who rely on smell as their primary source of information about the surroundings should not be common among the sentient extraterrestrial races of our Galaxy.


* A few science fiction writers, especially Stapledon,1946 might dispute this. The dolphin is known to have a fairly well-developed system of taste-navigation, as do the salmon and the snake.217,2537 It has recently been discovered that the tiny laboratory bacterium E. coli has a highly sensitive gustatory response capability: It can taste a chemical concentration gradient of only 0.01% over its two-micrometer length.2452

** "Information" may best be thought of as a choice of one message from a set of possible messages. The simplest type of choice is one that is made between two equally likely alternatives. The "bit" (binary digit) is the single unit of information, the answer to a simple yes/no question.

*** Chemically, this is very easy to do. Countless molecular species may act as "alphabetic" information carriers. The typical pheromone has a molecular weight from 80-300, with 5-20 carbon atoms.2547 Even assuming the fixity of these rather restrictive ranges, there are still literally thousands of different substances to play with.



13.3 Acoustical Senses

Hearing is extremely important in many animal species on Earth, so we may expect it to be widespread among the lifeforms of other worlds as well. Modulated waves of pressure, impressed on the gaseous or liquid medium in which the creature dwells, can convey much valuable information relevant to survival.


13.3.1 Two-Dimensional Sound

One fascinating but little-discussed acoustical sense is the surface wave communication found among a few specialized terrestrial species. There are many insects that have utilized the peculiar two-dimensional quality of their environment to develop a rather exotic mechanism for transmitting and receiving data.

For instance, water striders (Gerris buenoi) are small, stilt-legged insects that skim over quiet ponds, supported by the force of surface tension. Much like the kinesthetic sensors in human bodies which provide continuous positional and velocity data for each limb, water striders can detect the slightest disturbance traveling across the surface of the water. This is highly useful survival-oriented information, because it alerts these organisms to the presence of various dangers such as predators, competitors, and obstacles.

A somewhat more sophisticated surface dweller is the whirligig beetle (Gyrinus), which has devised a kind of "sonar" or echolocation system for use in its peculiar two-dimensional world. This creature senses the vibrations of its own ripples over the water’s surface as they are reflected from the shore or from objects moving within a certain range. Depending upon the exact nature of the return, the clever beetle can determine size, distance, velocity, and even texture of all nearby targets.

Not a few small animals on this planet use surface waves directly for communication. One species of water striders (Rhagadotarsus) is known to conduct its entire courtship display using complex patterns of modulated surface waves:

The sequence begins when a male grasps a floating or fixed object on the water surface and vibrates it in a way that sends out waves at the rate of 17-29 per second. Females nearby respond by moving toward the source. When one approaches to within 5-10 centimeters of the male, he switches to "courtship calling" and finally to pure courtship signals. At 2-3 centimeters the female responds with courtship signals of her own, followed by a series of tactile signals that finally lead to copulation.565

Several species of spiders are known to use a form of surface wave communication which involves strumming the webs they weave in specific rhythms and patterns. (This is usually used to pass data between mother and offspring.) Desert scorpions can also detect compressional and surface waves in sand to locate prey.2573

There is no reason why surface-dwelling aliens could not respond to and utilize this exotic 2-D "way of knowing." Because of the peculiar nature of the medium, the universe inhabited by such creatures would be strange indeed. This is due, in part, to the fact that two-dimensional waves are fundamentally and qualitatively different from three-dimensional ones we’re used to hearing.

One striking feature would be the amazing persistence of messages. We know that 3-D acoustical waves pass an observer located at a fixed point in space only one time, never to return again. Except for the single wavefront, the medium is relatively undisturbed. In contrast, oscillations in 2-D media die away only very slowly from frictional forces. The entire surface space is set in motion by such stimuli, and damping is often very weak. The media continues to "wave" for a long time after the emission of the original signal.

ETs speaking by means of surface waves would sound like they were in an echo chamber. Words would have a peculiar drawn out quality, persisting long after they have been spoken. And since the higher frequencies always travel faster than the lower ones, each repetition of the echo will sound distinctly different. The word will stretch itself thin, the higher pitched treble notes bunching together at the beginning of the sound and the progressively lower bass tones trailing behind.



13.3.2 Three-Dimensional Sound

The most common acoustical sense on Earth is 3-D sound perception. Such sound waves normally don’t carry much energy. For example, the pressure variations in a room due to people talking are only about 10-6 atm. Yet our ears can detect waves with so little energy (10-16 watts/cm2) that our eardrums only vibrate 10-9 cm in response.79 It is a fact that the primary human acoustical organ is extremely well-developed, able to distinguish at least 1600 discrete frequencies and to hear the quantum hiss of molecular motion of the air -- the theoretical lower limit of sonic sensitivity.

A good sense of hearing is highly advantageous for a number of reasons. Much like scent signals, sound waves diffract around obstacles in total darkness -- at night, in dark caves, deep underwater -- or in blinding light to reach the recipient. And while pheromones are a less expensive way to transmit data over large distances, auditory signals are considerably more efficient than optical ones in biocommunication.

Unlike visual displays which require whole body motion or complex lighting patterns, or scent-talk which requires a separate glandular "voicebox" for each chemical letter in the odor alphabet, sound may be adequately generated by a single organ. A relatively simple system can produce wide variations in pitch, tone, intensity, wave shape, and timing. This means higher data flow between brain and terrain, significantly increasing the chances for survival.

Perhaps the only real disadvantage to audition is that the sonic alien must provide its own source, since natural sounds in the environment are rarely sufficient to permit a thorough surveillance of the surroundings. But sonic senses may be highly directional with fine resolution: Even humans, who have no echolocation system at all, can accurately separate distinct sound sources located only 10-20° apart.

Animals with built-in sonar fare much better.* Many can virtually "see" with sound. For instance, bats are small mammalian avians able to navigate at high speeds using ultrasonic echolocation. Although visually blind, these creatures easily avoid millimeter-wide wires strung across their path by investigators. Even when 0.3 mm wires were substituted the animals managed to avoid them more often than not. Only when tiny threads the thickness of human hair -- about 0.07 mm -- were used in the obstacle course were the bats unable to dodge them.219,2514

The upper limit for spatial resolution of targets is a function of the wavelength of sound. The tiniest object that can just be discerned has a size roughly equal to the wavelength. Smaller objects are too small to give an appreciable reflection and so remain invisible.

Typically, the sonic beings of planet Earth use frequencies from 20-100 KHz or higher for echolocation (Table 13.1). This corresponds to a wavelength on the order of millimeters in normal dry air at a range of several meters. Much higher frequencies, say, 500-1000 KHz, would permit the resolution of targets 0.1 mm in size. This is quite enough to thread needles and soldier circuit boards at distances of about 10 centimeters from the creature’s face.


Table 13.1 Range of Hearing for Terrestrial Animals48,1698


Range (Hz)


Frequency Range (Hz)

Axolotl (salamander)

up to 240

 HUMAN (adult male)

15 -- 20,000


up to 340


up to 33,000


up to 1,200


up to 34,000


up to 2,700


up to 40,000


up to 7,000


up to 40,000


up to 10,000

Guinea Pig

up to 40,000

Characinidae fish

up to 10,000


300 -- 45,000


50 -- 10,000


430 -- 45,000

Cricket (Gryllus)

250 -- 10,000


30 -- 50,000


100 -- 12,000

Bat (Plecotus)

20 -- 80,000


up to 13,000

Deer mouse

50 -- 95,000

Harvest mouse

up to 17,500

Bat (Myotis)

20 -- 120,000


100 -- 19,000


100 -- 200,000

Cricket (Acridium)

up to 20,000


up to 240,000


Of course, aliens using sound waves with that kind of resolution would take a great deal of energy to produce. They would be "decidedly dangerous to human explorers," according to science fiction writer Hal Clement:

A story could be built on the unfortunate consequences of the men who were mowed down by what they thought must be a death ray, when the welcoming committee was merely trying to take a good look.

But there is no fundamental reason why sentient extraterrestrials couldn’t use sound as their primary sense, and to build and create as well as any sighted being.

Audition has been developed as the primary sense modality among many aerial and land based animals on Earth. Creatures evolving on planets with unusually thick atmospheres with heavy refractive effects might tend to rely far more on hearing than on seeing. Also, there is much evidence that the sea is also a most auspicious environment for the development of sonic senses. It’s quite possible that hearing may even be the sense of choice for intelligent organisms residing in the murky oceanic media of other worlds. Why is this so?

Sound travels about four times faster in water than in normal air. This allows faster response times and greater ranges of communication and data collection. For example, at sonic frequencies of 500-1000 KHz, spatial resolution in ordinary seawater is equivalent to that in air at 100-200 KHz -- but the range is about a hundred times greater.

Sound fares well against competing senses in the watery environment. Pheromones are relatively ineffectual in water, since diffusion in liquid media generally proceeds 103-104 times slower than in air. Smell signals are emitted, but creep slowly away from the source. Ocean currents are tamer than atmospheric ones, so pheromones would have to be about a million times more concentrated in water than in air to achieve comparable results.

Vision doesn’t compare much better. Light and other electromagnetic radiation cannot penetrate most liquids to any appreciable degree and are subject to countless distorting effects. In a tropical sea at high noon under very clear water, useful visual information can be gained only out to about 30 meters. Using acoustic channels instead, this range is extended to many kilometers -- comparable to vision in air on a hazeless day.

These facts are reflected in the physiological differences between humans and dolphins. The eyeballs of a man receive an estimated 50 million bits of information each second, while his ears manage only 2 million bits/second. In porpoises the emphasis is exactly reversed: Dolphin sonar handles perhaps 40 million bits/second, while the eyes manage only 5 million bits/second of information.**201 We can easily imagine that sound may be the preferred sensory modality among many, if not most, intelligent aquatic races scattered throughout the universe.

There are many properties of sound that make it a totally unique window on reality. For instance, the ultrasonic world is very quiet in comparison to the normal range of human hearing. This is because of the relatively short range of ultrasound. There is little noise because sources remain localized.

In a dense fog, both sighted and sonic organisms are ill at ease: The former, because light is scattered away causing everything to appear visually white; the latter, because the droplets of moisture or particles are excellent ultrasound absorbers and cause the entire field of view to appear acoustically black.2514

Human vision is limited to the surfaces of objects, but sound penetrates and exposes the insides of targets to view. Objects may be scanned for composition and internal structure using almost distortion- and reflection-free sound waves. A pelagic sonic alien (perhaps modeled after the dolphin) thus views its fellows, not as a sharp contour of lines and edges and distinct boundaries, but rather more like an x-ray snapshot. Skin, muscles, and fatty tissues are virtually transparent to ultrasound. Bones and teeth, internally-trapped gas bubbles, and cartilaginous structures give good reflections. Hard parts, as well as the digestive and respiratory tracts, stand out in clear relief.

Might aliens with such sonic sight be more honest and less deceitful than humans generally? Physiological reactions to other individuals might be instantly perceived by those others, a kind of body-telepathy. At least one writer has remarked: "If your visceral reactions are obvious to everybody, you don’t waste much time trying to lie."2855

Dolphin echolocation may properly be compared to human vision. Just as people are able to see by the reflected white light of the sun, porpoises emit ultrasonic clicks and trills that illuminate the surroundings with white noise. Differences in texture and composition are as obvious to their sonic senses as to our visual ones. As Dr. Winthrop N. Kellogg points out; "Wood simply 'sounds different' from metal to a porpoise in the same way that it looks different to the human eye. It is the sound spectrum of the returning vibrations which gives the clue to the nature of the reflecting surfaces."1698 Even more sophisticated, the dolphin has sound generators on either side of its head, which allows sonic depth perception176 using binaural hearing -- what Lilly has called "stereophonation."201

Acoustically-oriented aliens will also have a rather clever use for color. The color of an object is just the frequency of radiation it emits, and we may expect that objects may take on various sonic colors. But there is more. Objects moving rapidly towards a sonic sensor will reflect sound at a higher frequency because of the Doppler effect. Likewise, objects in the field of view which are moving away will reflect lower frequencies.

If sonic beings divide their audible frequency spectrum into colors, then the sonic Doppler effect will cause approaching targets to appear "bluer" and retreating targets to appear "redder." As objects pass in front, their color alters noticeably depending on relative speed, angle of approach, and distance. If the medium is in motion, as with gusts of wind or surging water currents, the apparent color will flicker in frequency -- a phenomenon quite alien to normal human visual experience.

The linguistic significance could be enormous. Much as the Eskimos have more than fourteen different ways to say "snow" (depending on its firmness, wetness, age, etc.), intelligent marine ETs will have words without analogue in our language. There might be terms describing approaching-red, receding-red, stationary-red, stereophonic-red, circulating-red, gulfstream-red, and dozens of other subtle distinctions we cannot begin to imagine. Or perhaps these sea-folk communicate in a manner which is possible only among creatures who use the same sense to see as to talk. Many researchers believe that the language of dolphins consists not of words as we understand them, but rather of a series of sonic images transferred into speech:

In this view a dolphin does not "say" a single word for shark, but rather transmits a set of clicks corresponding to the audio reflection spectrum it would obtain on irradiating a shark with sound waves in the dolphin’s sonar mode. The basic form of dolphin/dolphin communication in this view would be a sort of aural onomatopoeia, a drawing of audio frequency pictures -- in this case, caricatures of a shark. We could well imagine the extension of such a language from concrete to abstract ideas, and by the use of a kind of audio rebus -- both analogous to the development in Mesopotamia and Egypt of human written languages. It would also be possible, then, for dolphins to create extraordinary audio images out of their imaginations rather than their experience.2552

It is difficult to emphasize too strongly how unfamiliar the world may appear when viewed through other senses. Extraterrestrials will have experiences and capabilities that are difficult for humans to fully appreciate. A case in point in the perception of a peculiar sea phenomenon known to oceanographers as the sofar channel.

The speed of sound in water increases with pressure and decreases with temperature. Moving downward from the surface the temperature plummets, and about 100 meters down the speed of sound reaches a minimum value of 1480 m/sec. At greater depths the temperature remains fairly constant (close to freezing) but the pressure begins to build, causing the speed of sound to go back up. This region of minimum sound speed is called the sofar channel.

Sonic radiation, due to the physics of refraction, is actually attracted towards the channel. As with astronomical black holes (from which light waves cannot escape), acoustical signals sent from within the sofar channel likewise can’t get out. Transverse waves cannot cross it easily, which must cause it to appear sonically dark and foreboding when "viewed" from above or below.

If sufficient courage can be mustered, the channel has one very interesting property as regards long range marine communications. Sounds generated at the proper depth remain trapped in the zone, much like light in a strand of optical fiber. Messages may travel virtually unattenuated for literally thousands of kilometers in all directions. Sentient oceanic creatures of other worlds may regularly broadcast long-distance calls to other "universes" in the sea.


* Part of the difficulty is ear design -- most of the energy in sound is reflected away at the surface of the structure. There are various solutions to this problem. Star Trek’s Mr. Spock’s ears are adaptations to the thin air of his home planet Vulcan. They permit greater directionality of sonic reception by utilizing a back-curving pinna ("pointed" ears). Another solution commonly found on Earth is the independently targetable ears of goats, cats, dogs, and others.

** Lilly’s figures are probably overoptimistic. Homer Jacobson and others have done a more careful analysis, and have concluded that the human eye is capable of transmitting only 4.3 million bits/second and the human ear only about 50,000 bits/second maximum.955,979,980



13.4 Electrical and Magnetic Senses

Nearly all organisms, including humans, emit direct-current electricity when swimming in ordinary seawater. This is due to the gradient in electrical potential between normal body fluids and the saline electrolytic ocean. Voltages are actually developed across different parts of the body.

Sharks -- rather large animals of low intelligence -- are sensitive to incredibly minute electric fields. They can detect the equivalent of a single flash light battery at a distance of 1500 kilometers (about 10-8 volt/cm). A small wound can double a person’s voltage gradient in water, and sharks have no trouble sensing this.574

Intelligent extraterrestrials may have developed an electric sense, given the proper evolutionary environment. By maintaining a carefully shaped field around their bodies, such creatures could detect the entry of foreign objects and other lifeforms into its personal space. Electrosensitive aliens could clearly identify the exact nature of the intruder by noting the location, magnitude, and shape of the distortion in the static field. Also detectable are such nonbiological sources as motions of and compositional changes in seawater, subsurface ore concentrations, earthquakes, thunderstorms and other meteorological disturbances.

Many terrestrial animals use a kind of electropulse reflection system analogous to sonar. Such mechanisms have evolved independently on Earth at least six different times among fishes. The most familiar include the electric eel (Electrophorus electricus), the electric ray (Torpedo), the skate, the knifefishes, the electric stargazers (Astroscopus), and the electric catfish (Malapterurus electricus) of the Nile.

The electrosonar fishes generate pulses at regular intervals. Amplitudes rarely exceed a third of a volt, and the frequencies range from 55 Hz for the Gymnotid species Sternopygus macrurus up to 300 Hz for Eigenmannia.474,2516 (Peak surge rates as high as 1600 Hz have been recorded, though.)

Sense organs located in the head of Gymnarchus niloticus respond to currents as small as 0.003 picoamperes and voltages as low as 0.15 microvolt/cm. Dr. Hans W. Lissmann of Cambridge University believes that this typical electric fish uses its 300 Hz electrosonar for navigation. Its natural habitat is the muddy waters of Ghanese rivers in Africa. Since light cannot penetrate, the organism is forced to rely heavily on its inboard electronics. The fish uses its electric sense both to avoid obstacles and to detect and capture prey.2516

The knifefish Eigenmannia has been closely studied by marine biologists because it uses its electrical sensitivity for social communication as well as navigation. When two of these animals meet, each adjusts his frequency up or down to avoid jamming the signals sent out by the other. And these high-speed discharges aren’t simple sine waves; rather, they are rich in harmonic variability and might easily serve as highly sophisticated communication systems in more cerebral alien creatures.2545

Dr. Carl Hopkins of Rockefeller University in New York has studied the sex habits of the electrical fish Sternopygus. When the male of this species passes a female, his normally polite and mild-mannered electronic emissions suddenly become an impassioned tangle of impulses -- the chaotic mixture of harmonics and timing that are his love song to potential mates. Literally, the female "turns on" the male.2541

The Mormoryd or "elephant trunk" fishes provide our last example of social electrics. These organisms send pulsed electrical messages to each other as a form of territorial display to warn off competitors. When a stranger intrudes on the home turf, the defender raises his frequency in angry protest. Mormoryds also show a distinct fondness for playful activity, and at least one researcher has documented what might be called a "listen mode" of behavior. In this mode, one fish attentively ceases all electrical emanations when certain of its fellows pass by.2540 Similar patterns of electrocommunication have been observed in Gymnotus carapo, a knifefish.2550

One major question remains: What kind of planetary environment might give rise to intelligent aquatic beings who relied primarily on an electric sense? As first Lissmann and later the Jonases have suggested, there would appear to be two basic requirements.

First, to evolve electrosensitivity as the preferred sensory modality, the medium in which the creature dwells should be quite dark. This must be a permanent (not diurnal, seasonal, or sporadic) condition. Otherwise, eyes are so useful that it would be very difficult to rule them out as the primary modality. Even Gymnarchus, living out its entire existence in darkened and perpetually muddy waters, has a set of weak, poorly-developed eyes which it uses to tell day from night.

Second, virtually all electrosensitive creatures on Earth live either in the ocean depths or in a turbid, fast-flowing watery habitat. If evolution is to favor these beings, the environment should probably be both dark and turbulent. Sonic senses would then be scrambled, vision virtually useless, and olfactory messages rapidly mixed, diluted, and swirled away. Electric field sensing could then become the sensation of choice.

So we might expect to find electric intelligences in the deep oceans of fast-spinning pelagic worlds. Another possibility could be a terrestrial planet with some oceans but which orbits a bright star at a great distance or a feeble star at a moderate distance. (Subjovian worlds might work well too.) A third alternative might be the dark side of a tidally-locked one-face planet located near the inside edge of the stellar habitable zone, a set of conditions likely to give rise to violent oceanic currents and turbulent winds.

How would electrosensitive ETs view their world? We can imagine that it would be an utterly alien environment compared to anything in normal human experience.

Sensations could be detected only at relatively close range due to the high electrical resistivity of water.82,565 The creature’s immediate sensory universe might extend out to about 100 meters for small objects, perhaps to a kilometer for larger ones. It should be a simple matter for these creatures to locate and distinguish various objects. Electric field lines diverge from a poor conductor -- such as rubber, plastic, glass, and other insulators -- and converge toward good conductors -- such as metals. Tests with electrosensitive fishes have proven that they are capable of detecting tiny glass rods 2 millimeters in diameter which are optically invisible in water. Two objects of the same size and shape, but constructed of different materials, are also easily distinguished.2516 Depending on the texture, conductivity, and chemical composition of the target, electrosensitive extraterrestrials might have an equivalent to our "color" which we could scarcely understand or appreciate.

The natural magnetic field of the planet, or submerged ferromagnetic ore lodes, would produce minute variations and distortions in the electrosensory field. A warmer or cooler layer in the watery medium could give rise to similar effects. And electrical impulses can play over the skin of an object or penetrate it to varying depths, permitting both interior and exterior views.

A related but more restricted sensory modality is the perception of magnetic fields. Zoologists have not yet found any creature on Earth capable of generating its own magnetism. Such ability cannot be ruled out elsewhere, since terrestrial fishes can generate electric fields and circulating electric fields give rise to magnetic ones. Nevertheless, the uniform absence of biomagnetism on this world seems to suggest that it may be restricted to a somewhat passive role elsewhere -- as on Earth.

The magnetic sense has been documented in the common mud snail (Nassarius)82 and in planarian worms.765 Magnetotaxis -- attraction to magnetic sources -- has also been clearly demonstrated in marsh bacteria and a few other microscopic species. These diminutive organisms have tiny chains of iron-rich beads incorporated within their bodies which allow them to orient themselves along Earth’s north-south axis and swim towards Magnetic North.2518

Insects, too, make use of the planetary field. Bees are known to build their hives in perfect alignment parallel to Magnetic North. If a strong magnet is placed nearby, the insects obligingly construct their new home in accordance with the new distorted direction of the magnetic field vector. (In one experiment a deflection of more than 40° from true north was achieved in this manner.438) Some species of termites are similarly affected, building their nests either parallel or perpendicular to the geomagnetic field at that location.219 These insects are believed to be sensitive to a mere 10-5 gauss, yet to date no organs or receptors have been isolated that could serve in this capacity.

Finally, one of the leading theories of bird navigation holds that these avians (besides sighting on familiar landmarks and the stars2532) take some of their directional cues from the position of the planetary magnetic field. There is much recent experimental evidence that birds can respond to fields as weak as 10-3 gauss (Earth’s field is about 0.5 gauss at the surface).2517,2519 There is also some unconfirmed evidence of a tiny organ in the corner of the eyes of the homing pigeon which is sensitive to magnetic flux.79

Magnetosensitivity may well serve in at least an auxiliary role for many extraterrestrial animal species.



13.5 Vision

Virtually all higher lifeforms on planet Earth have some optical sensing capability, testimony to the tremendous advantage in being able to see. Light is most familiar to us, but there are also many other forms of electromagnetic radiation. Likewise subsumed under "vision" must be eyes that see by gamma rays, x-rays or ultraviolet rays in the higher frequencies, and by infrared (heat), microwave or radio waves at the lower end of the spectrum.

There are many advantages to sight. While all but radio frequencies cannot diffract around obstacles or turn corners, vision provides the greatest accuracy, highest directionality, and the finest resolution of any sense available. Since photons travel faster than any material pulse -- as with olfactory or auditory signals -- vision permits virtually instantaneous response times. And light can carry far more bits/second than any other stimulus.980

One possible disadvantage of vision is that it requires elaborate whole body motions,2534 body color changes,2506 or complex biochemical mechanisms such as are found in bioluminescent insects and "flashlight" fishes2522,2526,2544 for social interaction to take place. So visual messages may be an inferior mode of communication between individuals unless there are overriding environ mental factors at work -- such as an unusually thin atmosphere which transmits sound poorly.

But this difficulty is more than offset when we consider vision as a means of sensing the surroundings generally. The sighted animal has a tremendous advantage enjoyed by no other: An external source of illumination.

It has been suggested in earlier chapters that most lifeforms will probably evolve on planets circling other stars. If this is a valid assumption for the majority of extraterrestrial races in the Galaxy, it follows that most of these environments will also be reasonably well-lighted. While osmic alien must emit their own pheromones and auditory beasts must radiate homemade sonar pulses if they desire high resolution, light falls from the sky like sensory manna from heaven. Photons bounce off everything and thus collect information which is free for the taking by any organism equipped with eyeballs.

Naturally, there are a few restrictions. Despite the fact that stars emit radiation at all frequencies in varying intensities, planetary atmospheres tend to absorb a great deal of it. Depending on the composition of the air, its pressure and a hundred other factors, there will exist one or more "windows" through which environmental illumination may pour.

As a rule, gamma rays and x-rays are absorbed by energy-level jumping in the atoms of air. It’s true that snails are known to be especially sensitive to x-ray’s,551 and Arthur C. Clarke has speculated on the possibility of an x-ray sense,81 the fact is that such high energy photons will probably be unable to penetrate any planetary atmosphere of reasonable density.

Ultraviolet (UV) is absorbed too, although to a lesser extent. In a thin or unshielded (e.g., ozone-free) atmosphere like that of Mars, ultraviolet rays might reach the surface and become useful for vision. It is a surprising fact that the human retina is quite sensitive to UV down to at least 3300 Angstrom.2528 This soft-ultraviolet radiation normally never reaches the retina because it is filtered out by the lens. A few persons have undergone lens replacement operations to remove cataracts, and since the artificial lens passes UV the full potential of the retina is finally realized. Such people can see a "color" that the rest of us cannot!* Unfortunately, on most planetary surfaces any scene viewed in ultraviolet light would probably be quite dark.

Near-infrared (IR) radiation is partly removed by the vibrations in molecules of water, carbon dioxide, methane, ammonia and a variety of other atmospheric constituents, and far-IR is absorbed by molecular rotational transitions common in the air of all planets. Still, some infrared does get through and may become useful for seeing.

We are left with "visible" light, some near-infrared, and some radio frequencies to which normal atmospheres are transparent. Thus there are three main bands of electromagnetic radiation which may profitably be exploited for vision on the surface of any world: The visible, the infrared, and the radio.


* During World War II, senior citizens who had undergone lens replacement operations were used by the Office of Strategic Services to pinpoint the flashing UV signals from agents stationed on enemy coasts. Secrecy was maintained be cause these messages were completely invisible to anyone else.2529


13.5.1 Visible Vision

The "visible" range of light is actually slightly broader than the visual spectrum for human eyes. Our sight is normally limited to 4000-7000 Angstrom. Bees, however, are fully sensitive to ultraviolet radiation down to about 2500 Angstrom but they cannot see red. All animals have somewhat different reactions to visible stimuli, as shown in Table 13.2 below.

Table 13.2 Wavelength of Maximum Sensitivity to Visible Light Among Terrestrial Fauna


Wavelength (A)


Wavelength (A)



 Blowfly larvae


Fruit fly






Freshwater fishes






(Green plants)
















Guinea pig








Saltwater fishes





Many have advanced the interesting notion that ETs evolving under the light of other suns will necessarily have eyesight which is most sensitive to the frequency of peak output of their home star. Hotter stars, under this scheme, would spawn creatures with bright-light accommodation and heightened sensitivity to blue light. Cooler stars would give rise to organisms more attuned to the red. (See especially Anderson,63 Clarke,81 Clement,292 Macvey,61 and Shklovskii and Sagan.20)

There are strong reasons to doubt the above hypothesis. Consider Table 13.3 below, which shows the visible radiation from various stars incident on planets located near the center of the Earth-normal habitable zone. Note that among all stellar classes of xenobiological importance the shift in the visible power spectrum is not dramatic. While some small variation exists in the relative intensities of blue, green and red from star to star, the differences are decidedly underwhelming -- hardly sufficient to represent a decisive evolutionary selective factor.


Table 13.3 Electromagnetic Power Delivered to Planet, Orbiting in Earth-Normal
Ecosphere, by Various Stars in Visible Wavelengths of Light

l (A)

Power Incident on Planet from Star (watts/m2-Ang)


























Furthermore, the peak sensitivity of terrestrial lifeforms is highly variable, ranging from 3600 Angstrom for bees (the equivalent emission peak of an A7 sun) up to 6500 Angstrom for seagulls (the equivalent emission peak of a K3 star).

And yet all of these creatures evolved under the same sun. It seems clear that eye sensitivity probably relates more closely to other environmental factors than the stellar class of the home sun. For example, it is often suggested that the wavelengths of light to which humans are most sensitive are virtually identical to the color of sunlight filtering down through a dense forest canopy of green vegetation and foliage, thus reflecting our arboreal origins. Aliens will answer in similar fashion to their own peculiar evolutionary heritages.

If sight is so important to living beings, then what kinds of photoreceptors might we expect to find on other worlds? Will ETs have eyes like ours, and if so, how many?

One of the most striking examples of parallel or "convergent" evolution in the terrestrial animal kingdom is the incredible similarity between the eyes of creatures with vastly different evolutionary histories. Animals in many separate phyla -- Chordata (amphibians, fishes, reptiles and birds), Mollusca (octopus, squid), Annelida (the alciopid worms Torrea and Vanadis2482), Coelenterata (the cubomedusan jellyfish2816), and even Protista (the elaborate lensed eyes of dinoflagellates2856) -- have independently evolved photoreceptors surprisingly similar in structure to the mammalian eye. There are some discrepancies; for instance, the photoreceptor cells in molluscs point towards the light, the opposite of vertebrates.1697 Nevertheless, close comparison reveals that the adjustable lens, retina, pigments, focusing muscles, iris diaphragm, transparent cornea, and eyelids all are immediately recognizable. The ubiquity of the eyeball is perhaps the clearest indication that it’s simply the best design for the job. Similar evolutionary forces and physical laws should lead intelligent ETs down much the same path.

Of course, the "camera lens" eye used by mammals isn’t the only imaging system lifeforms can use (though it’s probably the best for larger organisms). The next most common -- indeed more common if you just count species -- is the compound eye of insects and crustaceans.2535 Each organ looks like a small multifaceted jewel, but is actually a bundle of optical tubes. Light is directed down each of these tubes onto a large matrix of individual photosensitive spots on the retina. The image appears as a composite mosaic of tiny light-bits. Dragonfly eyes, to take an example, have more than 28,000 facets each.79 Motion may be discerned as far away as 12 meters.

Unfortunately, the compound eye has only very poor resolving power. (See Table 13.4.) It has been pointed out that an insect, poring over this page of print, would be quite unable to make out the individual letters.81 This is why larger creatures who need large amounts of accurate, well-resolved visual data will probably find the compound eye an unattractive alternative on any planet. Yet for smaller organisms it is ideal. Part of the reason for this is the laws of physical optics. If a flea had a spherical lens eye like that of humans, the pupil would be so minute that diffraction effects would make clear imaging impossible.958 Once again we see that the worlds of size are truly worlds apart.


Table 13.4 Resolving Power of Some Common Animal Eyes


Minimum Resolvable Angle


Minimum Resolvable Angle


(minutes of arc)


(minutes of arc)

Camera Lens Eyes

Compound Eyes











   Fiddler crab




   Fruit Fly







   Pigmented rat




   Albino rat





Other image-forming techniques are of limited importance on Earth, but this is no guarantee that the emphasis on other planets will be the same. For instance, alien species may utilize the pinhole camera concept. Such a system uses a open optical pit without lenses that is exceptionally useful in water. The beauty of the pinhole eye lies in its simplicity, and it has been adopted by at least one group of animals on Earth: the chambered nautilus.584

Another curious mechanism is the scanning eye of the snail. The image formed by a simple crystalline lens is scanned by moving a single retinal nerve sensor over the field of vision. The entire picture is slowly reconstructed from a series of these scans. Scanning eyes aren’t particularly useful for spotting rapid movements, but may be of value in highly viscous environments.

The principle of the optical reflector telescope has never been directly employed to form images by terrestrial lifeforms. But it is clearly possible to do so. And while pinhole and compound eyes cannot gather light but merely serve to redirect it, both lenses and reflectors can.439

Some animals have developed biological reflectors for other purposes. The luminous squid has retractable reflectors which, when coupled with its bioluminescence, produce a kind of searchlight. This elaborate apparatus, constructed of lenses, concave mirrors, diaphragms and shutters, emits a beam of illumination with which it sweeps the vicinity in search of prey.1000

The common house cat also makes use of curved reflectors. When light enters the feline eyeball not all of it is absorbed by the visual pigment in the retina. In most mammals this light is simply wasted. But the cat has evolved a mirrorlike coating on the inward-facing front side of its eye, called the tapetum. This specialized device reflects unused photons back into the retina for another try at absorption. Efficiency is increased and sensitivity heightened, although the image is blurred very slightly.

Another specialized adaptation is the split-pupil eyeball. This allows an organism swimming along the surface of a liquid body to enjoy bifocal vision.61 The Four-eyed Fish, one of the largest tropical tooth carps (Anableps), actually has this remarkable feature. It is an elongate fish with eyes projecting upwards, each member of the pair divided into an upper part adapted for vision in air and a lower part adapted for vision in water. Any creature so equipped -- alien or Earthly -- can keep tabs on events above and below at the same time!

Another kind of visual perception is the sensation of polarized light. Many animals on Earth have this sense -- insects, crustaceans, birds -- although most mammals do not.*

Radiation coming from the sun is unpolarized. But when these rays are intercepted by a planetary atmosphere the plane of vibration is altered depending on the position of the star in the sky during the day. Honeybees, to take one example of many, monitor these shifting patterns of light and use this information to map out the shortest direct air route home. Sky polarization thus serves as a highly accurate navigational aid for these creatures.2530

Since sunlight reflected off a lake or ocean also is polarized, sentient avians of other worlds who must seek their prey by diving through a shiny water surface may be equipped with internal Polaroid filters. (Herons, kingfishers and other terrestrial birds have these.961) Species inhabiting a "glassy" desert planet (with large expanses of volcanically or electrically fused surface sand) might also find this sense quite useful as sunlight bouncing off glass is polarized. And in constantly swirling irregular media, the detection of polarized light could help to maintain a proper orientation of "up" and "down."

We can imagine still more exotic schemes. Nature may have use for an eye with the ability to integrate light from a scene over a long period of time, much like photographic film. Such a system might be useful on a torpid, dimly-lit world (such as Venus) or in an environment populated by creatures with very slow response times. Since human rod cells are able to soak up dim light like a sponge for seconds at a time,82 there is no reason why aliens could not go us an order of magnitude or so better. This same ability could be used to design "wraparound eyes" for panoramic viewing.

What about eyes on stalks? Despite the attractiveness of this idea among devotees of the outré, both omnidirectional viewing (as found in some insects and terrestrial fishes) and independently swiveling eyes (like the chameleon) are generally seen by most xenobiologists as rather unlikely adaptations for thinking animals. It is said that eyes on mobile stalks are a feature more of animals likely to be preyed upon, and that "no superior intelligence could evolve in circumstances in which it lived in constant fear of being struck down and eaten."50

Bonnie Dalzell has produced three additional arguments against eye stalks for intelligent ETs that are very persuasive:

1. Eye stalks require an hydraulic support mechanism, which is very inefficient except in small animals.

2. Eye stalks are too dangerous. Eyes are often the most vital sense, yet a predator could just clip them off with the stroke of a claw or pincer. And such organs are more prone to normal injury -- in an accident, stalks could be bumped, slammed, or squashed rather easily.

3. Stalks, in fact all independently-targetable eyes, present severe problems for the brain in making the correct parallactic computations necessary for binocular vision.400 (However, the chameleon seems to triangulate on his food quite easily.474)

How many eyes are best? It’s clear that nature usually chooses the cheapest way to do a given job. Certainly for senses that are not highly directional, it would seem that a single receptor organ should suffice. This is perhaps why most larger organisms have but one organ of smell and one of taste.

On the other hand, senses that are highly directional can make good use of the benefits of stereo. The ability to accurately triangulate a source depends on the simultaneous operation of at least two physically separated receptors. One organ of sight or sound gives 2-D resolution; a second organ, by making use of binocular or binaural sensing, gives three-dimensional coverage.

But there appears to be little to gain from using more than two sensors.

A single pair is sufficient to ensure spatial resolution with depth perception -- a quantum leap over single organs but not much poorer than three or more. Notes one xenologist: "Three eyes represent not nearly the same improvement over two that two represent over one."20

Since the Principle of Economy tells us that nature invariably chooses the cheaper of several ways to do the same thing, this may partly explain why we have exactly two eyes and two ears. The advantages of having more than this number are slight or nonexistent, a conclusion bolstered by the apparent convergent evolution of stereoscopic vision among mammals, birds, and other animal groups on this planet.2520 Still, there is plenty of biological precedent for alternatives.

Cyclopean organisms are common in the microscopic world, but these are mere eyespots, useless for imaging or any discrimination between all but fuzzy patches of light. Scorpions and water fleas have a pair of compound eyes set so close together that they appear almost as one -- but these are not truly monocular.

It is possible that monocular vision might suffice even for large alien creatures. For example, a single eyeball that vibrated slowly from side to side could provide limited depth perception. Nearer objects would seem to move faster across the field of view than more distant ones, giving clues to their relative positions in space. (The human eyeball constantly quivers to prevent "visual accommodation," but the effect is too small to permit stereoscopy.)**

Very few animals have more than two imaging eyes, because third eyes don’t tell the being anything it didn’t already know. Reptiles (and man) have an ancient third eye called the pineal gland. In today’s reptiles it is only a day/night sensor and has no imaging capability whatsoever,2521 and in humans it is fully degenerate. But such may not always have been the case. The skull of Cynognathus, an extinct Triassic Period theriodont reptile, definitely shows three eyes. This animal was mammal-like, possibly warm-blooded and probably hairy.600

Nereis, the common sandworm or clamworm, has four eyes. The Horseshoe Crab (Xiphosura) is also tetraocular, though two of the four are degenerate. Most insects are pentaocular, with three small eyespots called ocelli located on the upper part of the head between the two compound eyes. Once again, how ever, there are only two image-forming eyes.965

In a few cases, several batteries of eye-pairs are used during the successive phases of the hunt for food. Dr. Norman J. Berrill describes the dinnertime antics of the spider, which has four pairs of eyes:

The rear pair serve to watch behind for either food or danger. The other three pairs work together but in succession. If something comes within the range of vision of one of the outermost pair, the head turns until the object is brought into the field of the two pairs of eyes in the middle, and the spider then advances. When the object is brought into focus of the forward pair, the spider jumps to attack. The whole business is much like a self-operated mechanism with seeing instruments, and the eight eyes together do not compare with the camera or the compound eye of a bird or a bee.89

The ultimate limit is probably reached by the scallop, whose literally hundreds of tiny, beautifully constructed "eyes" are spread around the circumference of its mantle like running lights on an ocean liner. These sense organs are very limited in function, however, as they cannot image. They serve only to initiate an automatic escape reaction at the approach of a hungry starfish (the scallop’s natural predator) heading in from any direction.

Specific environments can be imagined in which more or less than a single pair of eyes might be selectively advantageous. But the cost in added neurological equipment will usually be prohibitive. From the many examples above we know that more or less than two eyes have seldom proven more adaptive than exactly two.

Large sentient aliens, if they see by visible light, will most likely be binocular.


* Humans can see polarized light, but just barely. In natural skylight "Haidinger’s Brushes" appear as a small yellow and blue Maltese cross in the center of the visual field, the blue segment oriented parallel to the plane of polarization. The phenomenon is so close to the borderline of perception, so faint and unreliable, that it cannot serve a navigation function for man.2531

** It should be pointed out that the possession of two eyes does not guarantee stereoscopic vision. The rabbit and the woodcock, for instance, have eyes located on opposite sides of the head -- which provides almost no binocular field.82



13.5.2 Infrared Vision

The second range of electromagnetic radiation generally considered useful for vision is the near-infrared. While about 45% of Sol’s energy arrives as visible light, and 10% as UV, most of the remaining 45% comes in as infrared of various wavelengths. But seeing in the IR has another great advantage which is independent of atmospheric conditions. Every material body in the universe, provided its temperature is above absolute zero, emits a continuous spectrum of "black body" radiation (Figure 13.1). Simply because they have some heat, all objects "shine" in the infrared. So to infrared eyes, all the world illuminates itself.


Figure 13.1 Peak of Blackbody Radiation at Various Temperatures2524

WAVELENGTH OF RADIATION emitted by heated bodies varies with the absolute temperature. The wavelength of the most intense emission is given by Wein's displacement law, which states that for a thermal radiator the product of the peak wavelength and the absolute temperature is a constant. In the units used here the constant is about 3000 micron-degrees. Thus the sun, whose temperature is about 5700 K, has a peak emission near 0.5 micron. The eye responds only to the narrow band of wavelength between 0.4 micron (violet) and 0.7 micron (red). The arrows along the bottom of the diagram indicate the center of narrow infrared "windows" where the atmosphere allows the passage of 50 percent or more of the incident radiation. There are poor windows near 35, 350 and 450 microns that can be exploited by observatories in dry locations. The atmosphere becomes essentially transparent to incoming radiation at about 800 microns. Astronomers use photographic emulsions and photomultipliers out to one micron in the near-infrared. Infrared techniques are used between one micron and a few thousand microns, where they overlap with radio techniques.


The rattlesnake is remarkably good at sensing heat waves. This creature has four eyes -- two imaging eyeballs operating in the visible, and two conical pits on either side of the head which house its binocular infrared cameras. Each one measures 2 cm2 in area and is packed with 150,000 receptors sensitive to the near-IR from 15,000-150,000 Angstrom.

To this reptile, the world of heat is as important as the visible world. The typical rattler can sense a mere 0.005 watt/cm2, equivalent to the infrared radiated by 1 cm2 of human skin.2525 Temperature differences of only 0.002°C can be seen by the snake. This allows it to hunt mammalian prey even in darkness, and to penetrate protective coloration ruses and camouflage ploys during the daytime.

What about visual acuity in the IR? Would the accuracy of sight be much impaired? Arthur C. Clarke has suggested that infrared eyeballs would give "coarse and fuzzy pictures, for the images they produced could not be sharply focused." Furthermore, since "a typical heat wave is about a hundred times longer than a wave of visible light, infrared eyes with vision as sharp as ours would have to be a hundred times larger." Clarke then conjures visions of monstrous two-meter-wide alien eyeballs which, he then concludes almost casually, "would certainly be inconvenient!"81

This result is extremely misleading. Extraterrestrial infrared sensors need not be grotesque at all. For example, let us consider an intelligent ET peering at a crowd of Homo sapiens through infrared eyeballs. The black body radiation emitted by a warm human body peaks at about 93,000 Angstrom. How large must the sensor be?

At the stated wavelength, the aperture* of the alien eyeball only needs to be 3.9 cm to enable it to resolve one minute of arc -- about as good as a man’s eye. This size is comparable to the diameter of the eye of the horse (5.0 cm) and the Indian elephant (4.1 cm), and is nowhere near as large as the eyeball of the largest cephalopods (up to 37 cm) or even the largest mammal, the blue whale (14.5 cm).1697 So infrared eyes have to be just a few times larger than our own to achieve comparable optical resolution.

An extraterrestrial stargazer with IR eyeballs would not see what we see, however. Most of the familiar constellations would be gone because most of the brightest stars radiate only faintly in the infrared. New constellations would appear, comprised of stars visible to humans only with telescopes in the visible range but which burn brightly in the IR.

Beings capable of seeing down into the infrared may have divided the spectrum into heat-colors. The coldest objects would appear "red," the hottest "blue." This is easy for us to conceptualize, even though we are incapable of making such fine distinctions of temperature and have no absolute sense of temperature. That is, the perception of a given temperature is variable, depending upon whether our skin was previously hotter or colder.

But there are creatures indigenous to Earth that do have an absolute temperature sense. Many insect, birds, fishes and rodents possess the ability in a limited way. Fish have been trained to respond to a specific temperature -- say, 14 °C -- regardless of whether they had previously been kept in warm or cold water.2543 Honeybees are known to have an exact thermoregulatory mechanism by which they maintain the hive at a constant temperature. And one large bird -- the Australian bush turkey or "incubator bird" -- can keep its nest within 0.1 °C of precisely 33°C.2542

With this kind of absolute heat sense, an intelligent alien brain might interpret an entire rainbow of color in the heat spectrum. Since electrical permittivity varies widely in the infrared from substance to substance, "seeing" the chemical composition of the surroundings should not be too difficult for these beings.1703 With both visible and IR vision, like the rattlesnake, complicated visual messages and intricate works of interactive thermal art would be possible.

Heat, of course, is a persistent phenomenon. The footprints of a barefoot person across a cold floor or the jet blast on an airport runway well after departure remain visible in the infrared long after the source of these traces has departed. Like olfaction, thermosensitivity produces a world of echoes. The past gradually melds into the present as the traces of hot objects that passed by earlier begin to dissipate. Not only would such a clear view of the immediate local past be extremely useful survival-oriented information, but ETs with this sense would perceive reality and the flow of time in ways we can scarcely imagine.


* Calculated using usual approximation to Rayleigh’s criterion for optical resolvability of two point sources in air (diffraction pattern overlap): qR = 70l/A, where qR is resolution in degrees, l is wavelength and A is eye aperture both in meters.



13.5.3 Radio Vision

In so many ways the visible portion of the electromagnetic spectrum seems ideal for use by living beings. The atmosphere conveniently allows these wavelengths to pass, and most photosensitive chemical substances respond well in this region. Photons of visible light are energetic enough to excite our senses, yet not so energetic as to damage our tissues. Wavelength is small enough to permit the resolution of extremely small objects without distortion.

As if this was not enough, we find that our sun emits its maximum power smack in the center of the visible range. Our sight spans just those frequencies in which the greatest power is available for illumination -- a most auspicious arrangement. Let’s take a closer look at this.

The peak wavelengths in the power spectra of all classes of stars are shown in Table 13.5. Note that all stars of xenobiological interest have peak emissions well within the visible range. So choosing a star other than Sol won’t alter our conclusions -- alien worlds will be impinged with quite similar wavelengths of bright visible light, though the mixture of colors may vary slightly.

The fact that F0 through K5 stars peak in the human-visible optical window argues strongly for the evolution of visual, rather than radio, eyesight. It has also often been asserted that radio eyes would be difficult if not impossible from a bioevolutionary point of view. To achieve the same resolution as the human eye, a radio eyeball using 1-meter radio waves (300 MHz) would have to be several kilometers in diameter.1338 If 1-centimeter waves (30 GHz) are used, the radio eye need only be about 40 meters wide, but Carl Sagan’s cryptic remark is still appropriate: "This seems awkward."20


Table 13.5 Peak Emission Wavelength of Radiation from Various Stars


Photosphere Temperature (K)

Amax (Ang)

Visual Color
of Maximum Wavelength






















































No organisms on Earth are known regularly to use radar as a functional part of their normal sensorium. Of course, we cannot exclude such a possibility elsewhere on this basis alone. Electrical senses are quite well-developed on this planet, and there’s no reason why alien creatures could not learn to manipulate kilocycle and megacycle signals with ease. Recordings have been made of electric fish sputtering along at 1600 Hz for brief periods,2516 and experiments conducted by Clyde E. Ingalls, Associate Professor of Electrical Engineering at Cornell University, have demonstrated the ability of some humans to actually "hear" radar waves beamed at the head.79

And there really is no need at all for a single, localized viewing organ. To demand such is to become an "eyeball chauvinist." For instance, the entire body of a moderate-sized macromorph could be used for this purpose, much as arrays of electric field sensors are embedded in the outer skin of sharks and the electric ray.

The central, most critical test is to find a good reason to evolve a radar sense. Certainly it is not an easy matter, and the rewards appear marginal because of the low radiative power of most natural sources of radio waves. In other words, if we hope to find radar beings elsewhere in the Galaxy, we must think up some excellent reasons why nature should go to so much trouble.

The power output of Sol in the visible is compared with its radio emissions in Table 13.6. The radio window, while far broader than the visible, has more than ten orders of magnitude less energy available for sight! With hotter class-F stars the disparity is even greater.


Table 13.6 Radiative Power Output at the Surface of Sol

----- Visible Window ----- 

----- Radio Window -----


Power Output


Power Output

l (A)






1 cm

2 10-12



10 cm

9 10-16



1 m

1 10-18


10 m

4 10-22


If radio is to be competitive with sight as a primary sensory modality, the levels of environmental illumination should at the very least be roughly equivalent in magnitude. This guesstimate is probably a trifle optimistic since the information-carrying capacity (bit rate) of radio is about a million times lower than for visible light -- but we’ll stick with it anyhow.

Where might we find the proper conditions? The planet Venus has a surface temperature of about 750 K. The hot rocks emit blackbody radiation in the radio at roughly the same intensity as in the visible. This would give radar eyes a fighting chance, except that we still have the enormous influx of visible light from Sol to contend with. It appears that if radio is to win, the visible window somehow must be closed.

It was once believed that the thick cloud cover over Venus would preclude the illumination of the planet’s surface by Sol’s rays. But on-site measurements made by the Russian space probes Venera 9 and Venera 10 have shown that such is not the case.2386 Sunlight penetrates the Cytherian gloom rather easily, providing light equivalent to Earth’s on a rainy, overcast day.

It’s doubtful whether cloud covers very much more opaque can feasibly be designed. Unless we resort to such planetological oddities as chlorine or sulfur atmospheres to blot out the light, vision in the visible range must still be preferred over radio in any reasonable solar system.

If we cannot easily close the visible window, only one alternative remains: We must turn out the lights!

Probably the best environment for the evolution of radio-sensitive creatures would be the surface of starless, self-heating planets. Ensconced in the cold, dark blanket of the interstellar void, these worlds might sustain life and intelligence. Radar beasts with 40-meter-wide sensors may well be found on subjovian, jovian, or superjovian planets in the dead of space. No sun shines; all illumination must percolate up from below, and most of that is in the radio.

What might extraterrestrial radio eyes see on such a world?

From orbit, the night sky would appear suffused with a faint radio glow. Normal stars -- as we know them -- would all be gone. All constellations would vanish. In their places would be found a few brightly flaring pulsars and quasars, visible to the naked radio eye because of its greater sensitivity. Tiny creases and splashes of multicolored light would mark the titanic explosions in distant radio galaxies. The Milky Way itself would cut a bold, wide circular swath around the field of view, a glorious aurora-like ribbon of brilliance punctuated by a giant "radio moon": The Core of the Galaxy. Probably visible even from the ground as a spot of light about the size of a dime held at arm’s length, it would inspire the awe and curiosity of the inhabitants of the starless world below.

On the surface of the planet most of the astronomical universe visible from the orbital platform would be blotted out by ground and sky glare. Since the ground is the source of all energy, it will appear the brightest. The atmosphere, in close equilibrium with the ground temperature but slightly cooler, will be brighter but appear as a "colder" (redder) color. A radio snapshot would look surprisingly like a photographic negative of a scene on Earth: A brightly gleaming world nestled beneath a soft hazy blanket of swirling, cloud-pocked, multicolored, dully-glowing sky.

On Earth there are an estimated 100 lightning flashes each second worldwide. On a radio world, the atmosphere would be constantly flickering with "fireflies" dancing in the distance. Radio waves generated by electrical discharges any where on the planet would spread over great distances. The horizon would literally sparkle with light all around.

Water and many other liquid surfaces would appear shiny and bright, but outcroppings of dry, solid rock must look dark and foreboding. If there are any clouds hanging over the planet, they probably won’t be seen because they are generally transparent to radio waves. Radar sensing can be used to peer deep inside non-metallic objects heated nonuniformly1338 (such as a living body). "Radio blue" might be defined as the radio color coming from the hottest parts of such objects, whereas "radio red" would be perceived from the coolest regions. How can we comprehend what it means to simultaneously view all parts of a solid body, heated from within or with layered surfaces, which appears "red" on the outside, "green" on the inside, and "blue" at the center? This is three-dimensional see-through color vision with a vengeance!

If radio-sensitive extraterrestrials have developed some form of space travel technology, they might tend to beware the blinding radio brilliance of stars. Should they happen to approach Sol, our sun would not appear to be the well-behaved, steadily-shining object we know it to be. The total radio flux can wander over as much as five orders of magnitude during periods of intense solar activity (during which the variations in optical brightness rarely exceed 1%).1339

Enormous storms lasting many days would be observed near sunspots, releasing powerful blasts of radio energy resembling bombs bursting. A typical outburst raging across the shimmering surface of Sol would last tens of minutes, but the star might suddenly flare up unexpectedly, climbing several orders of magnitude in brightness in a matter of seconds. To radio-sensitive aliens, all stars must appear to be quite dangerous places indeed -- variable, inhospitable, random, violent, and very uninviting in comparison to the tranquil, stately quiescence of the home planet.

But suppose for a moment that some foolhardy adventurer ventures close to our solar system in search of life. The terrestrial worlds would be only faintly visible, so the alien astronaut would first be drawn to the gas giants. The jovians emit vivid and colorful flashes "as if an enormous electrical storm were raging over its entire surface."49

With proper shielding and access to powerful telescopes, however, the alien might journey to Earth at last. On our world, there would be three sources of radio-light by which our strange visitor could see: (1) Radio emissions from our sun (a kind of flickering daylight to the ET); (2) black body emissions (given off by all warm bodies -- air, ground, human bystanders, etc.); and (3) artificial sources.

It is the last of these which is most likely to cause an early breakdown in interstellar relations. Earth-based radiotelescope transmitters, TV and radio broadcasting antennae, and BMEWS defense radars would appear as hot and bright as the unshielded surface of the sun to the alien’s eyes. When our military radar first scans the incoming spacecraft, it may be interpreted as an act of war. A beamed radio "welcome" signal would fare no better.

After all, would we take kindly to a megawatt optical laser beam trained on our vessel as we came in for a landing on another world?



13.6 Alien Senses

We are far from having exhausted all of the sensory possibilities. Man himself has many others -- the vestibular senses relating to body position and accelerations, pain, time (circadian rhythms), blood sugar level monitoring, thirst and hunger, internal temperature, and so forth. Most organisms, including jellyfish, shrimps, octopuses, and virtually all vertebrates are able to sense gravity to some degree. Even most of the higher plants have gee-sensors to keep them growing upright.*

The water scorpion Nepa uses a fathometer sensitive to hydrostatic pressure gradients to keep itself informed of the depth to which it has dived,2358 and the alciopid worm Torrea has an unusual double-retina eyeball which may serve as an accurate depth gauge.2482 Honeybees and a species of fire ant (Solenopsis) can detect changes in the concentration of carbon dioxide in the air. Other organisms have humidity sensors, brightness sensors, salinity sensors, and so forth. Any of these could be quite useful in combination with the more important modalities discussed earlier.

One possible extraterrestrial sense that is often overlooked is the ability to detect radioactivity. Such a sense could be much in demand on a world with highly concentrated radionuclide ores near the surface, or on a planet in the throes of recovering from a global nuclear holocaust. Biological Geiger counters would give warning to steer clear of large tracts of radioactive hazards.

The ability to respond to radioactivity has been artificially bestowed on a small group of experimental animals. Several cats were outfitted with portable Geiger counters which telemetered impulses directly to the "fear centers" of the feline brains. When confronted with a pile of radioactive materials in one corner of their cages, each cat shied away.92

We could also imagine a sophisticated meteorological sensorium, especially useful for ETs native to a world with highly volatile, perpetually inclement weather. Barometric (like the pigeon’s) and humidity sensors would be helpful, as would an anemometer to measure wind velocity (like the blowfly’s). It’s also well-known that atmospheric turbidity, which is closely related to developing weather patterns, greatly influences the degree of skylight polarization. A sensor responsive to the intensity and distribution of polarized light might permit its owner to seek shelter from the elements before disaster struck.

The ability of many animals to "sense" an earthquake or tornado before it strikes is documented fact. The phenomenon is thought by some to relate to the perception of very low frequency infrasonics or minute electrical field variations which immediately precede the event. And elephants are said to be able to sense water located a meter or so underground, as in a dry river bed. Although this allegation remains unproven, biological "dowsers" would be far more likely to survive on a drought-stricken planet.

Two unrelated points should be made in closing.

First, it is not necessary for an alien’s primary sensory modalities to be located near the brain or even on the head. There are many lifeforms on Earth which disobey this seemingly essential rule. Some animals carry their ears on their stomach (grasshoppers) or on the knee-joints of their front legs (crickets), or which hear through antennae (mosquitoes) or their whole body (cicadas). Still others smell and taste with the soles of their feet (flies), the tips of their tentacles (octopuses) or their antennae (bees). Then there is the scallop, a headless mollusc with hundreds of eyes distributed around its perimeter.

The argument is often heard that senses should be located close to the brain in order to minimize neural response times, facilitate navigation, and maximize safety. But it must always be borne in mind that these are only broad generalizations and not the holy grail. Nature may see fit to violate them if there are good enough reasons.

Second point: The role of the brain must not be neglected when we consider the kind of world an alien sees. The mechanisms and patterns of perception may depend on sociocultural factors as much as on biophysical ones.

An interesting example of this is the phenomenon in humans known as "color hearing." This is not just a matter of vaguely associating sounds with colors, which many people do, but rather of rigidly linking specific musical tones to specific colors. Sir Isaac Newton supposedly associated middle-C with red, D with orange, E with yellow, etc. The composer Alexander Nicholaevich Scriabin is said to have experienced the piano keyboard as a sequence of particular colors. Indeed, such is often interpreted as a symptom of mental disorder, in which a patient is literally incapable of distinguishing whether a given color stimulus is a sensation of sound or of light.79

The explanation seems to be that as infants we make little distinction between the various forms of sensory input -- the sounds, sights, and smells of the world around us. In our culture we are taught this demarcation at an early age. Yet anthropologists have reported the discovery of whole cultures in which the young are not taught to differentiate between audio and visual data. Their native languages reflect this fact.

Learning alters perception. Other worlds mean, not just other senses, but other ways of dealing with sensation. In the English language, smell and taste are closely linked; in other societies, seeing and hearing are allied instead. What peculiar patterns and combinations of information might alien cultures put together? If so many different senses are available, are not the permutations and synergistic blends almost uncountable?


* In the absence of nervous systems, sensors remain uncomplicated. A few "vegetable sense organs" have been discovered. The sensitive pea has scarlet beads at the base of its stalks. When this tropical plant is stimulated with heat, light, or various chemical substances, the beads can control the drooping of the leaves.90 Venus flytraps are pressure-sensitive, and plants are known to grow in the direction of an increasing gradient of moisture or light. But this, apparently, is the best that nonsentient leafy lifeforms can do.

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