内容：Edith Shao 编辑: Thomas Dai
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Part I: Speaker
We Owe Our Pumpkins to Pooping Megafauna Christopher Intagliata | October 31, 2019
This Halloween, as you carve jack-o-lanterns and make pumpkin pie, take a moment to appreciate just how far the humble pumpkin has come.
"The wild form of a pumpkin looks like a tennis ball and it tastes like one. It's incredibly bitter, it's got a really hard rind, and it's incredibly unpalatable to humans."
Logan Kistler, an archaeologist at the Smithsonian's National Museum of Natural History. He says, as unpalatable as those early squashes were, they made a tasty tidbit for mastodons.
"And we know that because there are deposits of mastodon dung in Florida that are over 30 thousand years old. And so in those mastodon dung deposits, sure enough what we can find are wild squash seeds."
Kistler says mastodons probably weren't put off by the gourds' bitter taste. Because a few years back, his team analyzed the genomes of more than 40 mammals. And they found that the larger the animal, the fewer copies of a bitter-taste-perception gene they tended to have.
"Turns out there's this absolutely beautiful correlation between body size and the ability to taste bitter compounds. So what we think is going on, is that these are really plants adapted to a landscape with large herbivores. They evolved this bitter toxicity in order to deter small mammals who would destroy the seeds, but they've evolved it at just the right level where large mammals are not put off by the bitterness and they can disperse the seeds."
Through their poop.
Kistler reported those findings in the Proceedings of the National Academy of Sciences in 2015. [Logan Kistler et al, Gourds and squashes (Cucurbita spp.) adapted to megafaunal extinction and ecological anachronism through domestication]
Along with dispersing seeds, mastodons, like modern elephants, probably stomped around a lot and vacuumed up vegetation—creating the sort of disturbed environments where squash plants thrive. So it was a beneficial match.
But then, of course, the mastodons died out. And humans, Kistler says, which also tend to disturb the environments around them, creating great squash habitat—may have taken the mastodon's place. The details are murky.
"The way that the domestication of squashes started is still a little bit of a mystery. Because they're bitter and toxic in the wild and they get to this place of palatability."
Perhaps, he says, humans grew the gourds first to use them as storage vessels…and later tamed the bitterness. Either way, squash seeds, stems and rinds discovered in a cave in Oaxaca, Mexico provide evidence that, at least 10,000 years ago, ancient people had already begun domesticating a squash that would, eventually, carve a place for itself as our modern pumpkin.
Source: Scientific American https://www.scientificamerican.com/podcast/episode/we-owe-our-pumpkins-to-pooping-megafauna/ [Rephrase 1, 02:36]
Vampire bat friendships endure from captivity to the wild Jonathan Lambert | October 31, 2019
[Time 2] Are friendships formed with those we truly like? Or do we settle for whoever happens to be around?
This question is hard to answer in humans, and even harder in other animals. But a new study of vampire bats suggests that bat “friendships” go beyond mere convenience. Many social bonds built between captive bats persist when the bats are released into the wild, researchers report October 31 in Current Biology.
“This study convincingly shows that vampire bats can form stable bonds,” says Joan Silk, an anthropologist at Arizona State University in Tempe, who wasn’t involved in the study. While she cautions against assuming that other animals’ friendships are anything like our own, she says that this study adds to a growing body of research that critters can form friendshiplike bonds.
As their name suggests, vampire bats (Desmodus rotundus) drink nothing but the blood of other animals. If the bats are lucky, they’ll get a tablespoon each night. “It’s pretty difficult for bats to extract blood from an animal, so they often go without a meal,” says Gerald Carter, an evolutionary biologist at Ohio State University in Columbus. Three straight nights without a meal, though, and the bats can die.
“But there’s a social safety net,” Carter says. “Other bats will regurgitate portions of their blood meal to feed bats who didn’t get their own meal.” Previous lab work has revealed that bats that aren’t related to one another can form long-term cooperative bonds that loosely resemble friendships.
The nature of these relationships, however, is a topic of debate. Do bats — or other animals — pair up with individuals they prefer? Or are those bonds transient, transactional relations between two individuals seeking the best deal they can get on any given day? [290 words]
[Time 3] Carter and his colleagues devised a way of distinguishing between these two extremes in vampire bats. If social bonds are like friendships, those bonds should persist across radically different contexts — captivity and the wild. But if bats form bonds strictly out of convenience, then friendships forged in captivity should dissolve in the wild.
The researchers took 23 vampire bats from a colony of about 200 in a tree hollow in western Panama and brought them to a lab at the Smithsonian Tropical Research Institute in the central Panamanian city of Gamboa. As would happen in nature, only some bats were fed blood meals on certain nights. Then, as some bats stepped up to feed their hungry friends, researchers could witness the formation of social bonds. Over time, certain bonds grew stronger, as measured by time spent grooming each other.
“There were definitely bats that liked each other more or less,” Carter says.
After 22 months, the researchers drove five hours back to the tree hollow and released the bats. Each bat was fitted with a tiny computer sensor, glued to the animal’s back, that measured its proximity to other sensors — giving the researchers an unprecedented look at behavior inside the colony. The team caught an additional 27 bats and fitted them with sensors too, and then tracked which bats spent more time together over the next eight days.
It turned out that bat friends stayed bat friends. [237 words]
[Time 4] Captive bats spent more time hanging out with other captive bats, while control bats showed no such association. “Most importantly, the bats who had stronger bonds in captivity had stronger bonds in the wild,” Carter says, though he notes the bats were tracked for only eight days. The researchers aren’t sure if social bonds endured beyond that.
Persistent social bonds have been seen before in other animals, including baboons, crows and whales. But this study shows these bonds aren’t just circumstantial, says Damien Farine, a biologist at the Max Planck Institute of Animal Behavior in Konstanz, Germany, who was not involved in the research. “There’s a benefit to maintaining them, or a cost to breaking them,” he says.
Carter says that benefit could come down to trust. It may make sense for a bat to stick with a partner it knows, rather than find a new one that may not reciprocate with as much grooming or food.
The researchers also looked at bonds between mother and offspring, which surprisingly did not persist for bats born in captivity in the study. Carter says that in the wild, the mother-offspring relationship is even stronger than other cooperative bonds. While the six bats born in captivity bonded normally with their mothers in the lab, they all left the site before the study ended. “We aren’t sure why this happened,” Carter says. The bats could have flown back to where they were born, a behavior common in vampire bats, or they could’ve been forced out by other wild bats in the colony.
Carter says the study shows that animal sociality is complicated. Animals can form durable, social bonds. But those bonds may be broken when circumstances change — perhaps not so differently from human relationships. [290 words]
Source: Science News https://www.sciencenews.org/article/vampire-bat-friendships-endure-captivity-wild
Here’s what it will take to adapt the power grid to higher wildfire risks Maria Temming | November 1, 2019
[Time 5] Efforts to prevent wildfires, which are once again raging across California, have plunged vast parts of the state into darkness.
Millions of people lost power in October in a series of deliberate blackouts intended to preempt power lines from sparking wildfires in especially dry, windy conditions. Cell towers died, leaving many without phone service. Traffic lights blinked out. Hospitals scrambled to keep lifesaving equipment running on backup generators.
While disruptive, the cautionary electricity outages starting October 9 were meant to ward off something even more disastrous. In 2017 and 2018, wildfire season caused record-breaking destruction. Hundreds of fires in California in 2018 alone are thought to have been sparked by equipment run by power supply companies. Many were relatively small and easily put out, but others were more catastrophic — including the deadliest fire in California’s history, known as the Camp Fire, which killed more than 80 people and leveled the town of Paradise last November.
With wildfire risks increasing as climate change leaves the landscape increasingly parched, mass power outages could become the new normal — unless the electricity system can be made more fire-safe. Science News asked two experts in energy infrastructure how to improve the power grid so it poses less of a threat amid heightened wildfire risks.
Could burying power lines curb wildfire risks? “Putting lines underground is a great solution. It’s also very pricy,” says Alexandra von Meier of the University of California, Berkeley. “Instead of a million dollars a mile, you’re looking at 10 million dollars. In places where there’s many miles of exposed or vulnerable terrain, that’s a big price tag.”
To avoid breaking the bank, though, utilities could bury lines only in areas that are particularly vulnerable to future wildfires, says Sayanti Mukherjee of the University of Buffalo in New York, who is working on a wildfire risk analysis for different counties in California. [310 words]
[Time 6] Why don’t blackouts target only areas with wildfire risks? Power line networks are sprawling, ferrying electricity over long distances. California’s biggest power company alone, Pacific Gas and Electric Co., supplies some 5 million customers in the state’s central and northern areas. And while some communities spread farther into fire-prone woodlands and prairies, even customers in relatively fire-safe areas are still served by transmission lines that cut through tinderlike terrain, such as grass-covered hills. So those homes have to be unplugged as well.
The grid could be redesigned to feed electricity into smaller, more isolated “microgrids,” Mukherjee says. “The key benefit of a microgrid is that it can go into ‘island mode’” — disconnecting from the main power grid and switching to a local energy source in an emergency. “That way, we can prevent mass power outages for thousands of people who aren’t even getting directly affected by the wildfires,” she says. Today, microgrids are largely used by airports, university campuses or companies, and typically rely on gas or diesel backup generators.
How do we know where it’s safe to keep the power on? Historically, utilities “have relied a lot on physical inspections” of vast power line networks, von Meier says. “There’s now really a movement to begin to apply more advanced computer science techniques” to analyze data about electrical activity across the grid to identify remote disturbances, such as tree branches getting too close to power lines. Plants touching electrical equipment sparked at least one of the blazes that contributed to the Camp Fire inferno.
Sensors installed along power lines could monitor environmental conditions, such as wind and air temperature, which can raise alarms about possible new fires or warn where they might spread.
How soon can we expect to see changes in the power grid? “I’m expecting to see more power shutoffs, and to see that continue next year and the year after,” says von Meier, who is working with colleagues to develop a microgrid system where multiple homes on a city block could share energy from household solar panels. Meanwhile, there are microgrid pilot projects, such as the San Diego Gas and Electric Company’s microgrid in Borrego Springs, which can function independently during power outages.
But replicating such efforts in more places “does take time, and it does take money,” she says. “It will be a decade before they’re widespread.” [391 words]
Source: Science News https://www.sciencenews.org/article/what-it-will-take-adapt-power-grid-higher-wildfire-risks-california
How China built a single-photon detector that works in space Emerging Technology from the arXiv | November 2, 2019
[Paraphrase 7] One of the emerging uses for single photons is to pack them with quantum information and send them to another location. This technique, known as quantum communication, exploits the laws of physics to make sure the information cannot be read by any eavesdropper.
One challenge is to find ways to send this quantum information around the world. That’s difficult because the information is fragile—any interaction between the photons and their environment destroys it. Photons cannot travel more than a hundred kilometers or so through the atmosphere or through optical fibers without the quantum information they carry being destroyed.
So Chinese physicists have come up with a workaround: beam the photons to an orbiting satellite, which relays them to another location on Earth’s surface. In this way, the uncomfortable passage through the atmosphere can be minimized. If photons are transmitted from ground stations at high altitude, their journey is mostly through the vacuum of empty space.
But there is a problem. Quantum communication requires detectors that can spot and measure single photons. In recent years, physicists have designed and built increasingly sensitive devices that can do this.
However, this sensitivity makes them vulnerable to any kind of background noise, which can overwhelm the signal from the photons themselves. And space is filled with unwanted noise in the form of high-energy particles, extreme temperatures, and extraneous light from sources such as the sun.
Building single-photon detectors that can operate in this environment is a significant challenge. So it’s no surprise that physicists have been scratching their heads over this issue for some time.
Today, Meng Yang and colleagues at the University of Science and Technology of China in Hefei say they have solved the problem. They have even tested their machine over the last two years on an orbiting satellite and say it works well.
The team’s detector exploits a phenomenon known as avalanche breakdown, which occurs in semiconductor chips under special circumstances. A semiconductor such as silicon conducts electric current in the form of free electrons and holes that can move through the material lattice under the influence of an electric field.
Under normal circumstances, these charge carriers are bound to the lattice and so cannot move. In these circumstances, the material acts as an insulator.
But if an electron is set free, perhaps by thermal fluctuations or a kick from an incident photon, it can travel through the structure, creating a current. In these circumstances, the material becomes a conductor
Of course, a single electron freed in this way creates a tiny current that is hard to detect. So the trick with avalanche breakdown is to set up a voltage that rapidly accelerates a free electron to high enough speeds to knock other conducting electrons free. This creates a chain reaction—an avalanche—that results in a much larger and more easily detectable current.
In recent years, physicists have made these devices so sensitive that a single photon of a specific wavelength can trigger this kind of avalanche. The result is a single-photon detector capable of spotting most of the photons that hit it.
However, this sensitivity comes at a price. It’s easy to see how a high-energy particle can tear through a silicon photodiode, kicking out electrons and triggering an avalanche. And in space, this kind of effect creates so much background noise—called a dark count rate—that it swamps the signal from the photons physicists hope to measure.
So the task for Yang and co was to find ways to protect and enhance the performance of commercial off-the-shelf single-photon detectors so that they can operate in space.
Their first fix was straightforward—surrounding the detector with shielding that blocks high-energy particles. This is a delicate balancing act because shielding is heavy and thus expensive to put in orbit. The interaction between the shielding and the high-energy particles can also create showers of secondary particles that make the dark rate count even worse.
Yang and co eventually settled for a shield consisting of two layers. The outer layer is 12-millimeter sheet of aluminum, and the inner layer is a 4mm sheet of the much denser and heavier element tantalum. The resulting shield reduces the radiation dose by a factor of 2.5.
This shielding also acts as a thermal insulator, which allows the team to cool the detectors to -15 °C. This also reduces dark counts by minimizing thermal fluctuations in the silicon detector.
Finally, the team developed electronic drivers that switch off the detectors during periods when they are vulnerable to background noise, a technique known as after-pulsing resistance.
The effect of all these approaches was significant. For unprotected single-photon detectors, the expected dark count rate is over 200 counts per second. This is too high for quantum communication in space.
However, the modified detectors have a dark count rate of just 0.54 counts per second. That’s two orders of magnitude better.
In 2016, Yang and co launched their detectors onboard the Chinese Micius satellite, a quantum technology demonstrator that has notched up an impressive series of breakthroughs. For example, the detectors were a key component in teleporting the first object from Earth to orbit—a single photon in 2017. The satellite also enabled the first quantum encrypted video call between continents.
These experiments have set the scene for a new generation of space-based quantum communication. “Our single photon detectors open new windows of opportunities for space research and applications in deep-space optical communications, single-photon laser ranging, as well as for testing the fundamental principles of physics in space,” say Yang and co.
In the meantime, the rest of the quantum physics world has looked on with envy. China has a clear lead in space-based quantum communication, albeit with help from European researchers in key areas.
Europe is working on an orbiting quantum technology demonstrator called the Security and Cryptographic mission, or SAGA. This is part of much larger plan to create a quantum communication network across the continent. However, no launch date has been set.
By contrast, US plans have stalled. In 2012, the military technology research agency DARPA started a program called Quiness to test quantum communication technologies in space. But the program—and the field in general—has suffered from a severe lack of funding.
An important question now is how the rest of the world, particularly the US, plans to catch up. [1059 words]
Source: MIT Technology Review https://www.technologyreview.com/s/614657/how-china-built-a-single-photon-detector-that-works-in-space/
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发表于 2019-11-4 19:32:31
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