The Best American Science and Nature Writing 2012

by Dan Ariely

  Superorganisms and Supercolonies

  Ultimately the capacity for extreme forms of warfare in ants arises from a social unity that parallels the unity of cells in an organism. Cells recognize one another by means of chemical cues on their surface; a healthy immune system attacks any cell with different cues. In most healthy colonies, ants, too, recognize one another by means of chemical cues on their body surface, and they attack or avoid foreigners with a different scent. Ants wear this scent like a national flag tattooed on their bodies. The permanence of the scent means ant warfare can never end with one colony usurping another. Midstream switches in allegiance are impossible for adult ants. With perhaps a few rare exceptions, each worker is a part of its natal society until it dies. (Not that the interests of ant and colony always coincide. Workers of some species can attempt to reproduce and be thwarted—much as conflicts of interest between genes can occur within an organism.) This identification with their colony is all ants have, because they form anonymous societies: beyond distinguishing castes such as soldiers from queens, ant workers do not recognize one another as individuals. Their absolute social commitment is the fundamental feature of living as part of a superorganism, in which the death of a worker is of no more consequence than cutting a finger. The bigger the colony, the less a small cut is felt.

  The most breathtaking example of colony allegiance in the ant world is that of the Linepithema humile ant. Though native to Argentina, it has spread to many other parts of the world by hitching rides on human cargo. In California the biggest of these “supercolonies” ranges from San Francisco to the Mexican border and may contain a trillion individuals, united throughout by the same “national” identity. Each month millions of Argentine ants die along battlefronts that extend for miles around San Diego, where clashes occur with three other colonies in wars that may have been going on since the species arrived in the state a century ago. The Lanchester square law applies with a vengeance in these battles. Cheap, tiny, and constantly being replaced by an inexhaustible supply of reinforcements as they fall, Argentine workers reach densities of a few million in the average suburban yard. By vastly outnumbering whatever native species they encounter, the supercolonies control absolute territories, killing every competitor they contact.

  What gives these Argentines their relentless fighting ability? Many ant species, as well as some other creatures, including humans, exhibit a “dear enemy effect,” in which, after a period of conflict, death rates sharply decline as the two sides settle on a boundary—often with an unoccupied no man’s land between them. In the floodplains where Argentine ants originated, however, warring colonies must stop fighting each time the waters rise, forcing them to higher ground. The conflict is never settled; the battle never ends. Thus their wars continue unabated, decade after decade.

  The violent expansions of ant supercolonies bring to mind how human colonial superpowers once eradicated smaller groups, from Native Americans to Australian Aborigines. Luckily, humans do not form superorganisms in the sense I have described: our allegiances can shift over time to let immigrants in, to permit nations to fluidly define themselves. Although warfare may be inescapable among many ants, it is, for us, avoidable.

  PART THREE: Humans (the Good)

  DEBORAH BLUM

  The Scent of Your Thoughts

  FROM Scientific American

  THE MOMENT THAT STARTS Martha McClintock’s scientific career is a whim of youth. Even, she recalls, a ridiculous moment. It is summer 1968, and she is a Wellesley College student attending a workshop at the Jackson Laboratory in Maine. A lunch-table gathering of established researchers is talking about how mice appear to synchronize their ovary cycles. And twenty-year-old McClintock, sitting nearby, pipes up with something like, “Well, don’t you know? Women do that, too.”

  “I don’t remember the exact words,” she says now, sitting relaxed and half-amused in her well-equipped laboratory at the University of Chicago. “But everyone turned and stared.” It is easy to imagine her in that distant encounter—the same direct gaze, the same friendly face and flyaway hair. Still, the lunch-table group is not charmed; it informs her that she does not know what she is talking about.

  Undaunted, McClintock raises the question with some graduate students who are also attending the workshop. They bet that she will not be able to find data to support her assertion. She returns to Wellesley and talks this matter over with her undergraduate adviser, Patricia Sampson. And Sampson throws it back at her: take the bet, do the research, prove yourself right or wrong.

  Three years later, now a graduate student, McClintock publishes a two-page paper entitled “Menstrual Synchrony and Suppression” in the journal Nature. (Scientific American is part of Nature Publishing Group.) It details a rather fascinating effect seen in some 135 residents of Wellesley dormitories during an academic year. In that span, menstrual cycles apparently began to shift, especially among women who spent a lot of time together. Menstruation became more synchronized, with more overlap of when it started and finished.

  Today the concept of human menstrual synchronization is generally known as the McClintock effect. But the idea that has continued to shape both her research and her reputation, the one that drives a still flourishing field of research, is that this mysterious synchrony, this reproductive networking, is caused by chemical messaging between women—the notion that humans, like so many other creatures, reach out to one another with chemical signals.

  It has been harder than expected to single out specific signaling chemicals and trace their effects on our bodies and minds as precisely as entomologists have done for countless insect pheromones. But in the four decades since McClintock’s discovery, scientists have charted the influence of chemical signaling across a spectrum of human behaviors. Not only do we synchronize our reproductive cycles, we can also recognize our kin, respond to others’ stress, and react to their moods—such as fear or sadness or “not tonight, honey”—all by detecting chemicals they quietly secrete. As researchers learn more about this web of human interaction, they are helping to bridge an arbitrary dividing line between humans and the natural world.

  Animal Kingdom Chemistry

  The very intriguing idea of animals sharing invisible chemical cues has a long and illustrious history, at least as far as other species are concerned. The ancient Greeks talked enthusiastically of the possibility that female dogs in heat might produce some mysterious secretion capable of driving male dogs into a panting frenzy. Charles Darwin, pointing to several famously smelly species, proposed that chemical signals were part of the sexual selection process. Throughout the late nineteenth century the great French naturalist Jean-Henri Fabre puzzled over evidence that the siren call of chemistry could stir winged insects into determined flight.

  Still, it was not until 1959 that the science really began to gain traction. In that year Adolf Butenandt, a Nobel laureate in chemistry, isolated and analyzed a compound that female silk moths release to attract males. Butenandt dissected the insects and painstakingly extracted the chemical from their microscopic secretion glands. He collected enough to crystallize it so that he could discern its molecular structure by x-ray crystallography. He called the compound bombykol, after the Latin name for the silk moth.

  It was the first known pheromone, although the term did not yet exist. Shortly after, two of Butenandt’s colleagues, the German biochemist Peter Karlson and the Swiss entomologist Martin Lüscher, coined that name from two Greek words: pherein (to transport) and horman (to stimulate). They defined a pheromone as a type of small molecule that carries chemical messages between individuals of the same species. The compounds must be active in very tiny amounts, potent below a conscious scent threshold. When released by one individual in a species and received by another, the two researchers wrote, they produce a measurable effect, “a specific reaction, for instance, a definite behavior or a developmental process.”

  Since then, an astonishing array of pheromones—the best-known and establi
shed class of chemical-signaling molecules exchanged by animals—have been found in insects, not just in silk moths but in bark beetles, cabbage looper moths, termites, leaf-cutter ants, aphids, and honeybees. According to a 2003 report from the National Academy of Sciences, entomologists “have now broken the code for the pheromone communication of more than 1,600 insects.” And pheromones serve many more purposes than simply attracting mates: they elicit alarm, identify kin, alter mood, tweak relationships.

  By the late 1980s pheromones had also been found to influence a wide spectrum of noninsect species, including lobsters, fish, algae, yeast, ciliates, bacteria, and more. As this new science of chemical communication grew—acquiring the more formal name of semiochemistry, from the Greek semion (meaning “signal”)—scientists extended the search to mammals. Almost immediately they ran into resistance from their colleagues.

  “In the 1970s and 1980s people would jump at you if you said ‘mammalian pheromone,’” recalls Milos Novotny, director of the Institute for Pheromone Research at Indiana University. “They’d say, ‘There’s no such thing: mammals are not like insects. They’re too evolved and complex to be spontaneously responding to something like a pheromone.’”

  But by the mid-1980s Novotny had not only identified a pheromone in mice that regulated intermale aggression, he had synthesized it. Such compounds were also verified in rats, hamsters, rabbits, and squirrels. And as the list lengthened, it also became apparent that mammals’ pheromones were very like—if not identical to—those found in insects. As an example, most researchers cite the stunning work of the late Oregon Health and Science University biochemist L.E.L. “Bets” Rasmussen, who showed in 1996 that a sex pheromone secreted by female Asian elephants is chemically identical to one used by more than one hundred species of moths for similar purposes of attraction.

  McClintock had proposed a similar idea in 1971 in her pioneering paper on menstrual synchrony. “Perhaps,” she wrote then, “at least one female pheromone affects the timing of other female menstrual cycles.”

  Odorous Landscape

  McClintock, now sixty-three, is sitting in a small, sunny room occupied by filing cabinets, computers, racks of stoppered vials and tubes, and scent sticks—all contributing to a faint, slightly sweet chemical aroma—and a dark-haired graduate student named David Kern. (“All the other graduate students would climb over my dead body to get in this room,” he says.) McClintock’s lab is at the University of Chicago’s Institute for Mind and Biology, of which she is a founding director. She wears a tweedy jacket over a bright patterned shirt, and she is thinking over a question: How far has the science of semiochemistry traveled since that day some four decades ago? The case for human chemical communication has been made, she says, and “our goal is to tackle identifying the chemical compounds. And then we can refine our understanding of what fundamental roles they play.”

  That task is anything but easy. Human body odor is estimated to derive from about 120 compounds. Most of these compounds occur in the water-rich solution produced by the sweat glands or are released from apocrine, or scent, glands in the oily shafts of hair follicles. The apocrine glands concentrate the most under the arms, around the nipples and in the genital regions.

  It is a complicated landscape, made even more complicated by our use of what researchers refer to as exogenous compounds, such as soap, deodorants, and perfumes, as Johan Lundström of the Monell Chemical Senses Center in Philadelphia points out. And yet Lundström marvels at how adeptly our brains sort through this chemical tangle. Neuroimaging work done at his lab finds a 20 percent faster response to known human chemical signals compared with chemically similar molecules found elsewhere in the environment. “The brain always knows when it smells a body odor,” Lundström says.

  This capacity is already present in infancy. Numerous studies in humans have shown that, as is true in animals, mothers and infants are acutely attuned to each other’s scent. This scent knowledge is so precise that babies even prefer the parts of clothes worn by their mother (and their mother only) touched by sweat compounds. The recognition, interestingly, is more acute in breast-fed infants than in those raised on baby formula.

  “We’re still just mapping the influential compounds from those that are not,” Lundström says. “I don’t think we’re dealing with one single compound but rather a range of different ones that may be important at different times.” Pheromones operate under the radar, he says, and they influence—but do not necessarily completely control—numerous behaviors. “If we compare these with social cues, they may be less important than the obvious ways we communicate,” Lundström says. But, he adds, the ability probably aided survival as we evolved, keeping us more closely attuned to one another.

  Psychologist Denise Chen of Rice University also argues that this kind of chemical alertness would have conferred an evolutionary advantage. In her research, she collects odor samples from individuals while they watch horror movies. Gauze pads are kept in viewers’ armpits to collect sweat released during moments of fear. Later the pads are placed under volunteers’ nostrils. For comparison, Chen has also collected sweat from people watching comedies or neutral films such as documentaries.

  One of her early experiments found that participants could tell whether the sweat donor was fearful or happy at the time the sweat was produced. The subjects’ guesses succeeded more often than they would by pure chance, especially for fear-induced sweat. Chen followed up with research showing that exposure to “fear sweat” seemed to intensify the alarm response—inclining participants to see fear in the faces of others. These exposures even enhanced cognitive performance: on word-association tests that included terms suggestive of danger, women smelling fear sweat outperformed those exposed to neutral sweat. “If you smell fear, you’re faster at detecting fearful words,” Chen explains.

  In a recent study, she and Wen Zhou of the Chinese Academy of Sciences compared the responses of long-time couples with those of people in shorter-term relationships, Those results indicated—perhaps not surprisingly—that the longer couples are together, the better the partners are at interpreting the fear or happiness information apparently encoded in sweat. “What I hope that people will see in this is that understanding olfaction is important for us to understand ourselves,” Chen says.

  And evidence continues to accumulate that unconscious perception of scents influences a range of human behaviors, from cognitive to sexual. In January, for instance, a team of scientists at Israel’s Weizmann Institute of Science in Rehovot, led by neurobiologist Noam Sobel, reported that men who sniffed drops of women’s emotional tears felt suddenly less sexually interested, in comparison to those who smelled a saline solution. Sobel found a direct physical response to this apparent chemosignal: a small but measurable drop in the men’s testosterone levels. The signal may have evolved to signify lower fertility, as during menstruation. More generally, the discovery may help explain the uniquely human behavior of crying.

  Hard Science

  A major goal now is to identify the key chemicals that convey signals surreptitiously and to learn much more about how the body detects and reacts to those signals. George Preti, a Monell chemist, has mapped out a research project that would include tracking these messengers by analyzing sweat and apocrine secretions and studies of hormone levels in those who sniff the chemicals. “We’ve yet to identify the precise signals that carry the information,” Lundström agrees. “And if we want a solid standing for this work, that’s what’s needed next.”

  McClintock also sees this as a priority. In recent years she has focused on building a detailed portrait of one of the more potent known chemosignals, a steroid compound called androstadienone. She believes that this particular small molecule is potent enough to meet the requirements of being called a human pheromone: it is a small molecule that acts as a same-species chemical signal and influences physiology and behavior. Over the years, labs, including McClintock’s and Lundström’s, have found that this particular com
pound shows measurable effects on cognition and that it can alter levels of stress hormones such as cortisol and evoke changes in emotional response.

  In one recent study McClintock and her colleague Suma Jacob of the University of Illinois at Chicago explored androstadienone’s propensity to affect mood. They mixed a trace amount into the solvent propylene glycol and then masked any possible overt odor with oil of clove. They then exposed one study group to a solvent containing the compound and another group to a plain solvent. Subjects were asked to smell gauze pads containing one version; they were told only that they were participating in olfaction research. All the subjects went on to fill out a long and tedious questionnaire.

  Overall, the subjects exposed to androstadienone remained far more cheerful throughout the fifteen- to twenty-minute test. A follow-up study repeated the same process but included brain imaging as well. The neuroimages showed that brain regions associated with attention, emotion, and visual processing were more active in those exposed to the chemosignaling compound. McClintock sees this as a classic pheromonal effect, the kind that she speculated about decades ago.

  Even so, she and other researchers continue to carefully talk of “putative” pheromones. Humans are complicated, and any causal links between specific chemicals and changes in behavior are hard to demonstrate conclusively. Indeed, no one can say for certain yet what chemical or chemicals account for McClintock’s original discovery, the synchronization of women’s menstrual cycles. Even the phenomenon itself has proved somewhat elusive: it has been confirmed in numerous follow-up studies but contradicted by others, and it is still not accepted unanimously by the scientific community.