Intelliscents

Vibes

Reading time: 6m 2s

We leave traces of ourselves wherever we go, on whatever we touch. One of the odd discoveries made by small boys is that when two pebbles are struck sharply against each other, they emit, briefly, a curious smoky odor. The phenomenon fades when the stones are immaculately cleaned, vanishes when they are heated to furnace temperature, and reappears when they are simply touched by the hand again before being struck.

An intelligent dog with a good nose can track a man across open ground by his smell and distinguish that man's tracks from those of others. More than this, the dog can detect the odor of a light human fingerprint on a glass slide, and he will remember that slide and smell it out from others for as long as two weeks, when the scent fades away. Moreover, this animal can smell the identity of identical twins, and will follow the tracks of one or the other as though they were made by the same man.

We are marked as self by the chemicals we leave beneath the soles of our shoes, as unmistakably and individually as by the membrane surface antigens detectable in homografts of our tissues.

Other animals are similarly endowed with signaling mechanisms. Columns of ants can smell out the differences between themselves and other ants on their trails. The ants of one species, proceeding jerkily across a path, leave trails that can be followed by their own relatives but not by others. Certain ants, predators, have taken unfair advantage of the system; they are born with an ability to sense the trails of the species they habitually take for slaves, follow their victims to their nests, and release special odorants that throw them into disorganized panic.

Minnows and catfish can recognize each member of their own species by his particular, person-specific odor. It is hard to imagine a solitary, independent, existentialist minnow, recognizable for himself alone; minnows in a school behave like interchangeable, identical parts of an organism. But there it is.

The problem of olfactory sensing shares some of the current puzzles and confusions of immunology, apart from the business of telling self from non-self. A rabbit, it has been calculated, has something like 100 million olfactory receptors. There is a constant and surprisingly rapid turnover of the receptor cells, with new ones emerging from basal cells within a few days. The theories to explain olfaction are as numerous and complex as those for immunologic sensing. It seems likely that the shape of the smelled molecule is what matters most. By and large, odorants are chemically small, Spartan compounds. In a rose garden, a rose is a rose because of geraniol, a 10-carbon compound, and it is the geometric conformation of atoms and their bond angles that determine the unique fragrance. The special vibrations of atoms or groups of atoms witrhin the molecules of odorants, or the vibratory song of the entire molecule, have been made the basis for several theories, with postulated "osmic frequencies" as the source of odor. The geometry of the molecule seems to be more important than the names of the atroms themselves; any set of atoms if arranged in precisely the same configuration, by whatever chemical name, might smell as sweet. It is not known how the olfactory cells are fired by an odorant. According to one view, a hole is poked in the receptor membrane, launching depolarization, but other workers believe that the substance may become bound to the cells posessing specific receptors for it, and then may just sit there, somehow displaying its signal from a distance, after the fashion of antigens on immune cells. Specific receptor proteins have been proposed, with different olfactory cells carrying specific receptors for different "primary" odors, but no one has yet succeeded in identifying the receptors or naming the "primary" odors.

Training of cells for olfactory sensing appears to be an everyday phenomenon. Repeated exposure of an animal to the same odorant, in small doses, leads to great enhancement of acuity, suggesting the possibility that new receptor sites are added to the cells. It is conceivable that new clones of cells with a particular receptor are stimulated to emerge in the process of training. The guinea pig, that immunologically famous animal, can be trained to perceive fantastically small amounts of nitrobenzene by his nose, without the help of Freund's adjuvents or haptene carriers. Minnows have been trained to recognize phenol, and distinguish it from p-chlorophenol, in concentrations of five parts per billion. Eels have been taught to smell two or three molecules of phenylethyl alcohol. And, of course, eels and salmon must be able to remember by nature, as the phrase goes, the odor of the waters in which they were hatched, so as to sniff their way back from the open sea for spawning. Electrodes in the olfactory bulbs of salmon will fire when the olfactory epithelium is exposed to water from their spawning grounds, whereas water from other streams causes no response.

We feel somehow inferior and left out of things by all the marvelous sensory technology in the creatures around us. We sometimes try to diminish our sense of loss (or loss of sense) by claiming to ourselves that we have put such mechanisms behind us in our evolution. We like to regard the olfactory bulb as sort of an archeological find, and we speak of the ancient olfactory parts of the brain as though they were elderly, dotty relatives in need of hobbies.

But we may be better at it than we think. An average man can detect just a few molecules of butyl mercaptan, and most of us can sense the presence of musk in a vanishingly small amount. Steroids are marvelously odorus, emitting varieties of musky, sexy smells. Women are acutely aware of the odor of a synthetic steroid named exaltolide, which most men are unable to detect. All of us are able to smell ants, for which the great word pismire was originally coined.

There may even be odorants that fire off receptors in our olfactory epithelia without our being conscious of smell, including signals exchanged involuntarily between human beings. Weiner has proposed, on intuitive grounds, that defects and misinterpretations in such a communication system may be an unexplored territory for psychiatry. The schizophrenic, he suggests, may have his problems with identity and reality because of flawed perceptions of his own or others' signals. And, indeed, there amy be something wrong with the apparatus in schizophrenics; they have, it is said, an unfamiliar odor, recently attributed to trans-3-methylhexenoic acid, in their sweat.

Olfactory receptors for communication between different creatures are crucial for the establishment of symbiotic relations. The crab and anemone recognize each other as partners by molecular configurations, as do the anemones and their symbiotic damselfish. Similar devices are employed for defense, as with the limpet, which defends itself against starfish predators by everting its mantle and thus precluding a starfish foothold; the limpet senses a special starfish protein, which is, perhaps in the name of fairness, elaborated by all starfish into their environment. The system is evidently an ancient one, long antedating the immunologic sensing of familiar or foreign forms of life by the antibodies on which we now depend so heavily for our separateness. It has been recently been learned that the genes for the marking offf self by cellular antigens and those for making immunologic resposnes by antibody formation are closely linked. It is possible that the invention of antibodies evolved from the earlier sensing mechanisms needed for symbiosis, perhaps designed, in part, to keep the latter from getting out of hand.

A very general system of chemical communication between living things of all kinds, plant and animal, has been termed "allelochemics" by Whitaker. Using one signal or another, each form of life announces its proximity to the others around it, setting limits on encroachment or spreading welcome to potential symbionts. The net effect is a coordinated mechanism for the regulation of rates of growth and occupations of territory. It is evidently designed for the homeostasis of the earth.

Jorge Borges, in his recent bestiary of mythical creatures, notes the idea of round beasts was imagined by many speculative minds, and Johannes Kepler once argued that the earth itself is such a being. In this immense organism, chemical signals might serve the function of global hormones, keeping balance and symmetry in the operation of various interrelated working parts, informing tissues in the vegetation of the Alps about the state of eels in the Sargasso Sea, by long, interminable relays of interconnected messages between all kinds of other creatures.

This is an interesting kind of problem, made to order for computers if they came in sizes big enough to store in nearby galaxies. It is nice to think that there are so many unsolved puzzles ahead for biology, although I wonder if we will ever find enough graduate students.

-- Lewis Thomas (1913-1993), The Lives of a Cell, Notes from a Biology Watcher, Vibes pp 37 - 41, Published in Penguin Books 1974.