Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

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; http://www.xenology.info/Xeno.htm


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.


Last updated on 6 December 2008