慶應SFC 2017年 環境情報学部 英語 大問3 全文(正答済み)

 During his three controversial terms as mayor of New York, Michael Bloomberg launched all-out public health crusades against some of his constituents’ favorite vices, including cigarettes, trans fats, and sodas. Perhaps he should have declared war on loneliness instead. Scientists have repeatedly found that people who lack—or believe that they lack—close social connections have significantly higher mortality rates than those who find themselves surrounded by friends. One 2010 review of previous studies suggested that social isolation may be more dangerous than obesity and just as deadly as smoking. Loneliness and isolation appear to alter hormone levels, immune responses, and gene expression, and to increase the risk for a variety of ailments, including heart disease, depression, high blood pressure, and Alzheimer’s.

 Humans are not the only creatures that benefit from a little companionship; isolation can also enfeeble rats, mice, pigs, rabbits, squirrel monkeys, starlings, and parrots. And it takes a particular toll on Camponotus fellah, a species of carpenter ant. According to a new study by researchers at the University of Lausanne, in Switzerland, worker ants that live alone have one-tenth the life span of those that live in small groups. Scientists have known for decades that social insects fare poorly when separated from their colonies, and the Swiss group, in the course of their ongoing experiments with carpenter ants, had observed this effect firsthand. “We realized that, when we isolated them, many of them would die quickly, even if we provided them with food and water,” Laurent Keller, the biologist who led the research team, claimed.

 Keller and his colleagues weren’t initially sure why the ants were dying, but they had recently developed a tool that they thought might solve the puzzle: an automated system for tracking ant movements. They printed unique patterns on tiny squares of white paper and glued them, like bar codes, to the backs of their carpenter ants. A camera snapped high-resolution photos as the ants skittered around, and a software program used the images to calculate the position and orientation of each ant. The system wasn’t foolproof. Some ant species are difficult to mark because they remove or chew the tags, and some species are difficult to track because individuals sit on top of each other, thereby obscuring the tags. But the tags allowed the researchers to easily collect billions of data points on the active, industrious insects. The system revealed, for instance, that ants are restless workers, shifting careers—from nurses to cleaners to foragers—as they age.

 Keller’s team thought that their automated tracker might help explain why isolated ants died so quickly and how the ants’ behavior changed when their social ties had been severed. And so the researchers set out to create some lonely ants. They assembled isolation chambers, small plastic boxes containing food, water, and a capsule that was designed to serve as a nest. Then they plucked a few hundred unlucky workers out of their colonies, glued bar codes to their backs, and introduced them to their new homes. The ants were assigned to the boxes in different combinations—some moved in utterly alone, while others were housed in pairs, in groups of ten, or with several squirming larvae. Then the insects went back to their lives, and the scientists waited for them to die. As their final moments approached, the ants would begin to shake. “Finally, they lay down and stop moving,” Akiko Koto, a co-author of the new study, said.

 The effect of isolation was dramatic. The ants that lived in groups of ten survived for about sixty-six days, on average. The solitary ants died after just six and a half. Ants that lived with larvae or in pairs had intermediate life spans, averaging twenty-two and twenty-nine days, respectively. When the researchers analyzed the movement data, they discovered that the companionless ants were hyperactive, spending huge amounts of time roaming around the plastic box, especially near the walls. During the first day alone, the lonesome ants walked twice as far as those that lived in groups of ten. This ceaseless locomotion is likely a stress response, Keller said, though Koto painted a more poignant picture. “I think it’s somehow expected,” she said. “In a natural condition, in the park or in a forest, if the ants lose their colonies, they’ll try to find their mother colonies.” The ants, in other words, may have been looking for their families.

 Keller, Koto, and their colleagues speculated that this excessive exercise might be responsible for the ants’ rapid demise—perhaps the ants weren’t eating enough to fuel their lengthy activities. So they conducted a second experiment, giving both isolated and group-living ants blue sugar water to eat. Then they dissected them, which Koto claims is easier than it sounds. “Oh, it’s not so difficult!” she said. “I just need forceps for the surgery.”

 When Koto inspected the insects’ insides, she saw that they were all eating approximately the same amount of food. But the solitary ants didn’t seem to be digesting it properly. Typically, ants are social eaters. When foragers first ingest their food, they store it in a balloon-like sac called the crop, which has also been called “the social stomach.” When the ants return to the nest, they generously regurgitate some of this food to share with their relatives. Whatever food remains in the crop is then pumped into the digestive tract. But when the researchers examined the ants twenty-four hours after their meals, they discovered that the isolated ants had significantly less food in their digestive system than the grouped ants did. “The food doesn’t transit in a normal way in the digestive system,” Keller said. Stuck in the crop, the food was unable to provide the itinerant insects with much-needed energy.

 The researchers aren’t yet sure exactly what causes the digestive slowdown. Maybe it’s stress. Maybe the fluid that ants exchange when they share food stimulates digestion. Maybe they die still looking for a relative to break bread with. “They store the food in the crop to share with nestmates, but they can’t find them,” Koto said. Koto, who moved from Tokyo to Lausanne to take a position as a postdoctoral fellow in Keller’s lab, has since returned to the University of Tokyo. But she still collaborates with her Swiss colleagues; they are currently studying how isolation affects the insects’ gene expression, in hopes of identifying precisely what’s disrupting their digestion.

 Until they nail down the underlying mechanism, it’s hard to know how much to extrapolate from a handful of alienated ants. “A lot of people ask me whether the effect of social isolation is also the same in humans and other animals,” Koto said. “We have to be careful.” There are, of course, sizable differences in the social structures and eating behaviors of ants and humans—we are, one hopes, considerably less likely to regurgitate our food and feed it to our friends. In humans, in particular, the connection between isolation and health is complex, and many mechanisms, both physiological and behavioral, are likely to be involved. For instance, isolation may increase inflammation, but it may also lead people to engage in more self-destructive behavior, such as chronic drinking. Nevertheless, the Lausanne findings suggest that isolation can alter physiology in ways that we are only beginning to understand. The scientists who study it—whether in humans, parrots, mice, or other animals—would do well to peer into the gut, which has typically been overlooked. “It would not be surprising if you find similar processes in a large number of species,” Keller said. Perhaps all lonely creatures are like the solitary carpenter ant: heartburned and seeking home.




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