Part 2 (2/2)
These data tell us three very important things about olfaction. First, fetuses have a significant capacity for olfactory learning, since they remember a scent a.s.sociated with the womb and seek it out after birth. Second, certain odors are innately excitable or pleasurable to animals in that they can function as primary reinforcers of behavior and have an impact on behavior and physiological responses the very first time they are experienced. Finally, the capacity for olfactory learning and memory can offset innate odor preferences, making a scent that is normally attractive something to avoid after birth.
Humans show remarkably similar forms of olfactory learning, and prenatal exposure to odors seems to play an important role in parental bonding and kin recognition. Newborns have an innate fondness for the smell of amniotic fluid, particularly their own. Experiments performed in culturally diverse populations have shown that babies as young as one day old prefer the smell of their own amniotic fluid to that of age-matched controls. The most commonly used test of preference in newborns is, of course, sucking behavior. When given the choice between nursing on their mother's breast scented with their amniotic fluid or that of an age-matched control, they almost always choose the former. Newborns also cry less and show a diminished stress response when they smell their own amniotic fluid. Since the many odors that emanate from a mother-such as the smell of milk, colostrum, saliva, and perspiration-stem from the same genetic and dietary sources as the amniotic fluid, they will all have some shared chemical groups. Hence, a preference for the smell of maternal amniotic fluid may evolve functionally into a preference for the smells of Mom in general; an adaptation such as this would have obvious utility in keeping the newborn close to its primary caregiver. These behaviors are evidence that olfactory labeling occurs in humans as it does in other animals.
Olfactory labeling while still in the womb has a profound influence on our postnatal ability to identify a person as kith and kin. Within hours after birth, a breast-fed infant can readily identify and will orient toward a breast pad worn by their lactating mother over a breast pad worn by an unrelated lactating woman. Newborns show abrupt changes in behavior-such as decreasing arm and leg movements, initiation of the sucking reflex, and are generally calmer-when exposed to odors that originate from their mother's body, including those that emanate from her breast, underarms, and neck.Almost any natural smell that can be used as a reliable indicator of Mom's presence has these effects on behavior.
Olfactory labeling also has an impact on the development of odor preferences and aversions after birth. In the Alsatian region of France, there is widespread use of anise flavoring in the local cuisine.Taking advantage of this custom, a group of scientists compared the olfactory responsiveness of neonates who were born to mothers who had or had not consumed anise flavor during pregnancy. Infants born to anise-consuming mothers showed a stable preference for the smell of anise when tested immediately after birth and four days later. Contrasting this pattern, infants born to mothers who did not consume anise tended to display either an aversion or no response at all to the odor. This study indicates that olfactory labeling also occurs in response to dietary influences that may alter the in utero chemical environment. One can imagine the profound implications of this process for the newborn in that it most likely influences a host of functions that range from the emergence of odor and food preferences to the development of early mother-infant attachment.
Although not all scientists agree, it appears that newborns may also have innate preferences for certain odors, such as floral and fruity smells. These odorants are not necessarily present in the amnion of most mothers, yet the preference for these smells emerges in most newborns across different cultures and persists into childhood. Researchers have found that newborns can discriminate among a number of qualitatively different odorants, evidenced by changes in body movements, facial responses, and heart and respiratory rates. It is much easier, however, to test verbal children who can simply tell you whether they find a smell pleasing.The few published studies that have focused on the olfactory preferences of verbal children have not always found consistent effects, and one reason may be that differences in experimental design influence the results. For instance, it is well known that young children tend to answer a positively phrased question in the affirmative. When these and similar methodological issues are controlled for, however, some universal tendencies do indeed emerge.
It is generally accepted that children as young as three years old exhibit stable hedonic preferences for specific odors independent of the culture in which they were raised. Some of the most popular smells include strawberry, floral, spearmint, and wintergreen, while odors such as butyric acid (strong cheese/vomit) and pyridine (spoiled milk) are universally disliked. That most children find fruit and floral odors pleasing should come as no surprise, since they often signal the presence of a nearby nutritionally rich food source-an important adaptation, to be sure, within an evolutionary context.
The emergence of this evolutionary adaptation in our species is echoed in the development of each individual with the growth and maturity of brain pathways that connect olfactory cortical areas with midbrain and orbitofrontal regions that mediate natural reward. While the midbrain reward centers develop at a fairly early embryonic stage in humans, the pathways that connect these regions to the areas responsible for perceiving odors do not mature until rather late in gestation and are known to depend on experience. Because this is the case, many scientists believe that although these olfactory preferences are very similar across cultures, their development probably results from learning to a.s.sociate these smells (in the womb) with flavorful food and the onset of an intrinsically rewarding behavior-eating.
Consummatory behaviors such as eating and s.e.xual activity are known to increase levels of circulating neuropeptides called endorphins, those lovely chemicals that provide a feeling of relaxation and calm and that are chemically similar to morphine.The development of these pathways-which depend on exposure to odors that signal the presence of potential sources of nutrition-may occur during the last trimester, while the fetus is exposed to the coappearance of certain smells and tastes with an increase in amniotic endorphin levels that have a calming effect on mother and fetus alike. Hence, the emergence of ”universal” olfactory preferences is likely to result from the same learning mechanisms that mediate olfactory labeling in all mammals.
The Smell of Attraction Like it or not, we smell, and the subtle odorous messages we send and receive-often unknowingly-have a profound influence on our social ident.i.ties and a wide range of behaviors, including mate selection, courts.h.i.+p, and the timing of ovulation.The word pheromone pheromone calls up a variety of images to mind: mammals communicating using a hidden language of scents; trendy socialites paying $300 per ounce for a vial of boar effluvia that promises to allure the opposite s.e.x; and sorority sisters who menstruate in synchrony month after month. calls up a variety of images to mind: mammals communicating using a hidden language of scents; trendy socialites paying $300 per ounce for a vial of boar effluvia that promises to allure the opposite s.e.x; and sorority sisters who menstruate in synchrony month after month.
Although it has proven rather difficult to isolate and identify a human pheromone, there is a growing body of evidence that we use them to communicate chemically much like other mammals.The first convincing evidence came from an unexpected place-an undergraduate dormitory room at Wellesley College. In 1967, an undergraduate student named Martha McClintock noticed that many of the girls in her dorm menstruated on the same days and wondered if such coordination might have survival value. She asked two simple questions in her research project: ”When did you last menstruate?” and ”Who are your two best friends?” The results surprised everyone.Women who spent the most time together tended to menstruate at the same time.
It wasn't until ten years later that the mechanism that causes this synchrony (now known as the McClintock Effect) was discovered. In a simple experiment, psychologist Michael Russell and his colleagues at Sonoma State Hospital in California rubbed an extract from the underarm of a woman with a very regular twenty-eight-day cycle under the noses of sixteen other women three times a week. Within four months, all of the women were menstruating within three days of one another.The odors of a single person, it turns out, can influence the menstrual cycles of many others. It was still unclear, however, how such an effect might have survival or adaptive value.
The answer came the following year, when it was discovered that men have cycles as well, and that their regular rise and fall in core body temperature and the production of essential steroids such as testosterone can be modulated by the presence of other males. The final link came when additional experiments showed that the production of testosterone and other androgens in men often becomes synchronized to the menstrual cycles of their wives and lovers.Taken together, an impressive display of synchrony emerges between men and women in close contact with each other on a regular basis, and this patterning might facilitate the timing needed for effective s.e.xual reproduction.
In addition to regulating menstrual and physiological cycles, pheromones have a say in other very personal affairs, such as distinguis.h.i.+ng those we find s.e.xually attractive from those who remind us of a sibling. But what is the physical basis for this hidden conversation of scents? Humans don't seem very interested in smelling the urine or underarm odor of potential mates, so where do human pheromones come from and what do they smell like?
The Desana Indians of the Amazon rain forest have a cosmological worldview built around olfaction. For them, the essence of a person is revealed by their smell, which emanates directly from their bones. Mores that guide courts.h.i.+p and social relations.h.i.+ps are intertwined with the relations.h.i.+ps among different smells. The Desana believe that people of the same tribe share a common smell, and strict rules exist forbidding marriage between those who have similar scents-an olfactory-inspired incest taboo. Certain smells should never be mixed, yet some naturally go together. An answer to the puzzle of how human pheromones influence behaviors such as mate selection came out of left field and was inspired by studies of the Desana.
It has been known for ages that foreign tissue implanted into a host is often rejected by the host's immune system. Each of our own cells bears proteins that our immune system recognizes as ”self.” When a foreign cell enters the body, the immune system attempts to cla.s.sify the nonself intruder by attaching a labeling protein to it and generating antibodies designed to destroy it. The immune system has a memory for intruder cells, and the next time the same foreigner is encountered, the antibodies can be launched even faster, since they do not need to be generated from scratch.
A segment of our DNA called the major histocompatability complex (MHC) codes for the immune cells that identify intruding disease organisms, essentially functioning as our immune system's first line of defense. Unlike many genes that have only a few alternative versions (called alleles), MHC genes have upward of a hundred or so, with each providing immunity against different sets of potential disease strains.
When we think of heredity, we typically have in mind the cla.s.sical pattern of single allele combinations-the dominant-recessive pairings that play a winner-take-all game with traits such as eye and hair color. If one parent has blue eyes and the other brown, one gene will dominate, meaning the gene from the other parent that controls this trait is not expressed.
MHC genes work differently in that they are codominant. Say you inherit one version of an MHC gene from your father that improves resistance to disease A and another version of the gene from your mother that happens to help fight disease B. Since MHC genes are codominant, you will be able to resist both diseases. Thus, parental combinations that have the greatest degree of MHC genetic heterozygosity 3 3 will produce offspring with the most robust immune functioning.These offspring would have a distinct survival advantage over offspring from parents who have considerable overlap in their MHC genes, providing resistance to a smaller spectrum of disease strains. Variation in this dimension, then, can serve as an important selection factor in the evolution of our species. The question is, what serves to attract us toward mates who have MHC genes different from our own? will produce offspring with the most robust immune functioning.These offspring would have a distinct survival advantage over offspring from parents who have considerable overlap in their MHC genes, providing resistance to a smaller spectrum of disease strains. Variation in this dimension, then, can serve as an important selection factor in the evolution of our species. The question is, what serves to attract us toward mates who have MHC genes different from our own?
The initial clues emerged from animal experiments. If a female mouse is offered two suitors, she inevitably chooses the mate whose MHC genes have the least overlap with her own, and it is now known that they do this through smell. Scientists have found that each version of the MHC gene codes protein by-products that are excreted from the body, and they have a unique odor. Mice that have damage to their olfactory nerve or olfactory epithelia cells perform this test at chance levels-deciding on the heterozygous mate only about half of the time.
So if you're a smart rodent, a big part of your mate selection process is in deciding if a suitor has the right smell. Mice that are most attracted to the smells of potential mates with dissimilar versions of MHC genes will be less likely to inbreed and will maximize the genetic fitness of their offspring. Can such a process be important for human mate selection? The fact that perfume and cologne sales account for approximately 12 to 15 percent of annual consumer luxury item spending suggests that we believe smell is a key factor in shaping our own attractiveness.
Intrigued by the idea that humans may use a very similar process to select mates, evolutionary psychologist Chris Wedekind and his colleagues conducted an experiment in which they asked more than one hundred men and women to score the odor of T-s.h.i.+rts worn for two consecutive days by male and female subjects. Each person tested was brought into a room with six odorous T-s.h.i.+rts stored in separate plastic containers and asked to rank them in terms of their ”s.e.xiness” and ”pleasantness.” The results were surprising.
Scores of the pleasantness and s.e.xiness were indeed found to relate to the degree of MHC similarity between the smeller and the T-s.h.i.+rt wearer. For most subjects, the most pleasant and s.e.xy smells were a.s.sociated with members of the opposite s.e.x whose MHC genes had the least overlap with their own. When asked why they liked a specific smell, many subjects offered that it reminded them of their present lover or an ex-mate. Interestingly, lower-ranking odors were said to remind the smeller of a sibling or other relative. This is one case where opposites definitely attract, and for a good reason-the observed mating preferences stemming from these choices would naturally increase immune system heterozygosity of the offspring.
In this mechanism we find that there is no single attractive smell that works for everyone-one person's sentir bon sentir bon may repulse another. To take advantage of this adaptation, lonely singles in search of a mate would have to get a genetic fingerprint of his or her intended before a scent could be custom-designed. But all hope is not lost. In 2001, Wedekind's group showed that we unwittingly use our own MHC geneotype information in choosing perfume and cologne for personal use. In this study 137 male and female students who had been typed for MHC were asked to rate 36 different scents for personal use-”Would you like to smell like that yourself ?” The researchers found a significant correlation between MHC genotype and scent rankings, indicating that people with similar MHC alleles preferred to wear similar-smelling perfumes. These results suggest ”that perfumes are selected 'for self ' to amplify in some way body odors that reveal a person's immunogenetics.” While it is commonly a.s.sumed that perfumes are worn primarily to mask a person's natural odors, Wedekind has argued that we actually prefer to wear scents that accentuate these olfactory cues, announcing our MHC genotype through a form of olfactory advertising. may repulse another. To take advantage of this adaptation, lonely singles in search of a mate would have to get a genetic fingerprint of his or her intended before a scent could be custom-designed. But all hope is not lost. In 2001, Wedekind's group showed that we unwittingly use our own MHC geneotype information in choosing perfume and cologne for personal use. In this study 137 male and female students who had been typed for MHC were asked to rate 36 different scents for personal use-”Would you like to smell like that yourself ?” The researchers found a significant correlation between MHC genotype and scent rankings, indicating that people with similar MHC alleles preferred to wear similar-smelling perfumes. These results suggest ”that perfumes are selected 'for self ' to amplify in some way body odors that reveal a person's immunogenetics.” While it is commonly a.s.sumed that perfumes are worn primarily to mask a person's natural odors, Wedekind has argued that we actually prefer to wear scents that accentuate these olfactory cues, announcing our MHC genotype through a form of olfactory advertising.
In this chapter we've seen that there are a few treasured scents that are universally appealing. Floral and fruity smells top the list in most countries, probably because they signify the presence of nutritious food sources. Newborns and children alike are attracted to these scents, independent of the culture in which they were raised.
Newborns and infants are also universally attracted to the smell of their own amniotic fluid and those that are breast-fed come to a.s.sociate maternal odors with the arrival of food. Attraction to maternal odors has obvious survival benefits by keeping the offspring in close proximity to its mother.
We have been wired by natural selection factors to find pleasure in these and similar smells because they have survival value. Pheromones, on the other hand, have a different kind of universal appeal. While no single odor is pleasurable to everyone, the rule is very simple (and universal): s.e.xy/pleasurable smells signify the presence of potential mates that can lead to viable offspring.The pleasure we find in these ”hidden” scents is driven not by natural selection factors, but rather through s.e.xual selection because these adaptations have clear reproductive value.
In the next chapter we turn our search for pleasure toward the epicurean in us all. We will discover that our l.u.s.t for certain tastes fosters normal immune system and brain development, but at a growing cost to public health in Western societies.
Chapter 6.
For the Love of Chocolate As life's pleasures go, food is second only to s.e.x.
Except for salami and eggs. Now that's better than s.e.x, but only if the salami is thickly sliced.
-Comedian Alan King
Music with dinner is an insult both to the cook and the violinist.
-G. K. Chesterton
If you ever make it to the Amazon, ask your guide to show you what is likely the most revered tree throughout all of South and Central America. The theobromo cacao theobromo cacao, or ”food of the G.o.ds,” was named by Linnaeus, the great eighteenth-century cataloger of nature. The designation makes clear his admiration for the almost indescribably savory taste of its fruit and seeds as well as the role of the tree in world history. Before taking a bite, few people stop to think-and who can blame them?-about the influence of chocolate on the social, political, and economic evolution of those cultures that came in contact with cacao beans as they spread from equatorial America to Europe, and then Asia.
After a short trek into the jungle, your guide will stop in front of an odd-looking tree, probably no taller than ten meters or so. If it is a mature tree-older than three years-it will have large patches of pink or blue cauliflorous growth on its bark, but your eyes will skip right past this feature and focus on the strange football-size pods that dangle expectantly from its trunk. The outer covering of the tree's fruit is a tough hide of corrugated green and yellow, which when broken reveals a soft, whitish pulp. The taste of the pulp will catch you off guard. Most people antic.i.p.ate the tangy sweet flavor of fruit with their first bite, only to be surprised by the subtle, bittersweet taste of chocolate. Enveloped inside the pulp are dark, purple-colored seeds-about thirty to forty per pod-that after being dried and processed can be recognized by epicureans around the world as ”chocolate beans.”
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