THE UNSEEN WORLD OF Taste & Smell
We’re not so smart. Animals can hear with their noses, insects sniff with their feet and salmon taste their way home from the seas
N. J. BERRILL
Smell, and the thought of the taste associated with it, plays a much stronger part in our reactions than we generally recognize. Billboards, magazine advertisements, television and radio may assault our eyes and ears but they rarely make us drool. There i.s nothing especially appealing to the eye about a hot-dog stand, but one whifT of the aromas from it is enough to start the saliva flowing. The smell of good food, or of perfumes, will do more to make you sit up and take notice than all the sights and sounds human ingenuity can devise. And why not? Taste and smell are as old as life, far older than eyes and ears, and the olfactory nerves that service your sense of smell are virtually an extension of the brain itself. The message of smell is direct and your whole person becomes alerted to whatever comes in. whether it is to whet your appetite, make your blood tingle, ring an alarm (as in the case of smoke), or make you pull back in disgust. Your responses are primarily emotional or downright physiological, rarely a matter of mental consideration.
High-pressure merchandisers are beginning to catch on. Some chain stores are using devices that produce the smell of frying bacon to lure customers to the bacon counter, a cheese smell to give them a desire for cheese, and apple and celery smells to draw them to the fruits and vegetables. Secondhand-car dealers can even buy bottles of new-car smell, which may not really fool the customer but may influence him more than he knows. The shape of things to come is perhaps embodied in a new air-conditioning unit that will fill your house with the scent of pine forest, tall grass or seashore, according to which button you press.
Much, if not most, of w'hat you taste is also
due to smell. You have only to hold your nose and bite into an apple to find that the ordinarily tasty fruit becomes almost as tasteless as a slice of potato. With a stuffy head cold nothing tastes good. We are, in fact, all mixed up between what we taste and what we smell, for flavor depends on both.
You taste mainly with taste buds scattered over the surface of your tongue, although w'hat you taste does not amount to much, in variety. Taste can distinguish merely between bitter, sw'eet, salt and sour. Yet these four qualities, which by themselves leave food flat-tasting, are of great value to man and other living creatures including fish and insects. These qualities say yes or no to what enters the mouth. The sweet and salty regions respond to the sweet juices of plants and the saltiness of flesh and blood. If they are stimulated they flash a come-on signal to the brain to say this is good food. The bittersensitive areas at the back of the mouth on the other hand warn against swallowing anything bitter and possibly poisonous. At least, this is how the system works in the case of natural foods.
Some chemicals however are confusing. Dulcin. a chemical unrelated to sugars, tastes sweet to most people but is tasteless to three or four out of ten. This “taste blindness” apparently is a hereditary deficiency, which started w'ithin the Caucasian race and spread to others. Many other examples have been discovered. The common food preservative, sodium benzoate, is tasteless to most people but bitter or sweet to about one in four. There are thousands of distinctive odors, in comparison with four kinds of taste, and many are detectable when present in the merest trace.
You can easily test your relative sensitivities
with a little ethyl alcohol. If you take as a standard the concentration of the alcohol just strong enough to produce a burning sensation on the skin, about one third the strength is needed to stimulate taste but no more than 1/60,000 is necessary for you to smell it. Mercaptan, which is essence of skunk, is apparent to most persons when only one molecule is present per fifty trillion molecules of air. Yet some otherwise normal people cannot smell skunk no matter how strong, and a few even find the smell pleasing.
Investigators have attempted to classify odors, although no really satisfactory scheme has yet heen worked out. A typical one divides them into about six types: flowery, fruity, spicy, resinous, putrid and burning, although any particular odor of the thousands that exist may be any combination of these. The scent of thyme, for example, is both flowery and spicy.
The smell situation is complicated both by the vast variety of substances that have odors and by the different sensitivities of people. Women have a more acute sense of smell than men. Children have greater acuity than grownups. If we smell a particular odor for any length of time we become more and more insensitive to it. Coal miners long ago recognized that they could no longer smell coal gas after they had been exposed to it for a while, and they carry canaries or mice with them to give warning. Not that these creatures have a better sense of smell, for they have not, but they show signs of collapse when there is still time for men to get out.
In other circumstances olfactory fatigue or adaptation may be a blessing, as in the case of tannery workers, who must work amid odors outsiders find overpowering. It is surprising what you can get used
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For animals a smell may stake a claim, warn a foe or woo a mate
to, although if you go away or outside and breathe fresh air for awhile, the impact of city smog, for instance, or cat smell in the house, may be overpowering when you return. Then you become adapted once again and put up with what you should be doing something about. For similar reasons we shouldn’t eat raw onions unless our companions-to-be are similarly contaminated. As long as we smell together, even to high heaven, no one knows the difference and no one cares.
To a human, as we have seen, an odor may be more emotionally disturbing than anything he can see or hear, and it may stir long-forgotten memories. But what do odors mean to the animals and plants that produce them? In the case of mammals they mean almost everything of importance—sex, friend or foe. this property is mine, keep away. When a dog lifts a leg against a tree it is to add a little scent of his own to what has gone before, as if to say, “I’m one of the gang and I’m here too—hope you'll know me when next we meet!” It keeps the tribe together in a loose and friendly way.
In the wild, however, many creatures use their scented secretions for staking claims. Certain antelopes have scent glands placed on each side of the head between the eye and nose, and each pair of animals marks out its feeding territory by putting drops of the gland’s secretion on twigs of saplings or bushes along the boundary, though always depositing some urine and dung close by to clinch the matter. Several kinds of deer mark their boundaries in much the same way. Beavers deposit their musk-gland secretions on hillocks of mud or projecting stumps.
A skunk doesn’t have to fight
Each animal presumably knows its own family smell and recognizes that of others as different, so that under normally good circumstances there is a sort of gentleman’s agreement to respect one another’s property.
The same smell in greater strength may, however, be used for actual offense. Not only by skunks but civets, mongooses and others possess an armament of smell. Few animals desire to fight for the sake of fighting and an effective threat is better than force. And to make such warnings unmistakable, civets and skunks have striking black-and-white coats that announce clearly who they are. Yet musk glands are often discharged in fright by less pugnacious creatures, such as the peccary, a wild relative of the pig, and the smell of fear is a very real thing in the animal kingdom. I he mammalian animal undoubtedly changes its smell according to its emotional state, whether of fear, rage or sexual phase.
What odors are important to an animal can be determined in the laboratory as well as by observing nature. Using delicate wire electrodes such as neurosurgeons use in exploring the surface of the human brain during operations, Dr. Adrian, of Cambridge University, has recently explored the olfactory part of the brain of a number of creatures, while at the same time presenting various aromatic substances to the nose. Whenever a particular odor registered, the nerve stimulus was picked up, amplified and recorded.
In general, the substances effective for human noses produced reactions in cats, rabbits and hedgehogs, while pure air, carbon dioxide and carbon monoxide had no effect. Rabbits responded best to fruity and flowery smells, indicating their sensi-
tivity to and interest in vegetation. Cats were unresponsive to these, but were especially sensitive to decaying animal material and to fishy smells in particular. The most effective smell stimulus for a catfish was a kind of thick and repulsive
soup of decayed alligator head!
Flavors can be a matter of life or death to some fish, particularly to bottom feeders such as cod, hake and catfish—fish that feed where there is little or no light. Taste and touch assume great importance and it is no coincidence that these are for all practical purposes nakedskinned fish—so much so that it is little exaggeration to say that they have become almost all face and tongue. Taste buds are scattered on the lips, the barbel, the pectoral and pelvic fins and the whole body of cod. Catfish arc like the cod
and the barblets hanging down from the chin are as sensitive to taste as is the top of the tongue. Food held at the flank or near the base of the tail will cause the fish to turn and snap at it. Internally, most fish also have large taste organs on their palates and in their gill passages, in order to check the quality of the water as well as the volume of food.
Their sense of smell, too, is well developed, although unlike that of taste, it remains where it properly belongs, in the nasal pits. Water streams in and out of them without entering the mouth and
throat, so that the kind of confusion between taste and smell that we have to contend with is hardly possible.
Yet the two senses do act together and many fish rely upon their combined chemical senses for finding their way during their long migrations. The eels that leave the rivers on either side of the Atlantic for their distant destiny in the ocean depth manage to find their way by the taste of the ocean currents, so far as we know, for there appear to be no other signposts at the depth at which they journey.
Pacific salmon return to spawn in the stream in which they themselves were spawned, after several years of feeding far out in the ocean, showing that they have both the capacity and memory for selecting the right route. What used to seem such an uncanny mystery, perhaps even a sixth sense, now appears to be a memory of taste and smell—in other words, a flavor they run to its source like a dog trailing a scent. The smell and taste of rivers flow far out from the coast. There the salmon pick up their home flavor and head in the direction
where the flavor grows stronger. And sc they proceed upstream, at every junction choosing the branch of the river that offers the familiar taste, until finally they arc back where they were born. Experiments conducted at the University of Washington, in Seattle, demonstrate that fish can even find their way back to artificial ponds where they were raised.
Insects exploit the senses of smell and taste even more than fish or mammals do. Caterpillars, for instance, make spitting movements in response to salt, acid or bitter substances, and bees reject honey treated with quinine or salt. But insects are more generously equipped with taste organs than are humans. In addition to having a sense of taste in the mouth, a great many insects, such as butterflies, fruit flies, and honeybees, also possess taste organs at the lower ends of their forelegs. When their feet touch sweetened water their tongues extend to a considerable distance, unrolling like the coiled paper horns you see at children’s parties.
This is particularly well seen in the butterfly and you can test the matter for yourself by placing a butterfly on the back of your hand moistened with a little sugar water. The greater the concentration of sugar the farther the tongue unrolls.
All sweetness however is not necessarily sweet to all creatures. Out of thirty-four different kinds of sugars, for instance, thirty taste sweet to humans but only nine are sweet to bees.
The sense of smell in insects is for the most part located in the antennae or feelers. Cockroaches can locate cheese from a distance by moving their antenr.ae about to get the direction. Bees can be trained to distinguish essence of orange from forty-three other scented oils, and both bees and ants distinguish members of their own community by their smell. Ants follow trails of scent to and from the nest and their whole sense of whereabouts is based on smell rather than sight.
Male insects moreover are generally better equipped to detect odors than the females. The reason is that in the search for mates the males as usual must do most of the searching, although also as usual the female does her gentle best to show the male the way. Moths, being night fliers, rely especially on their remarkable sense of smell for bringing the sexes together.
In one test made with Chinese silk moths, a female was placed in a gauze cage and a number of males were released at different points. Forty percent found their way to her from a distance of two and a half miles and more than twenty-five percent from seven miles. The scent of the female reached that far and could be detected by the males.
Over shorter distances males can even find their way to a place on which a female has recently settled, although she is no longer there. Even a dog, and most ^certainly a man, smells in a crude and clumsy way compared with this.
However, having attracted her suitor from a long way off, should the lady no longer be in the mood, what can a male insect do? One butterfly, at least, has found the answer. Both before and during courtship he sweeps his own scent glands, which are carried on the wings, and shakes a perfumed dust over the female in a shower of exciting masculinity.
So throughout the living kingdom the senses of smell and taste unite us in a common feeling, making distinctions between good food and bad, between friend and foe, and above all, perhaps, the presence and purpose of the opposite sex. it