Harbor porpoises are the world’s smallest cetaceans. The marine mammals, which look something like a small, beakless dolphin, live in colder waters of the Northern Hemisphere and tend to stick closer to shore — a trait that led to their name. Because small bodies would lose heat quickly in cold water, scientists have thought that harbor porpoises must eat a lot, consuming as much as 10 percent of their body weight daily, to stay warm and well fed.

Now scientists have figured out just how good harbor porpoises are at finding a meal. These animals can go after hundreds of tiny fish each hour, and they are very successful hunters.

Between September 2012 and August 2014, Danuta Maria Wisniewska of Aarhus University in Denmark and colleagues tagged five harbor porpoises that had been caught as bycatch by fishermen off the Danish coast. The tags, attached via suction cup 5 centimeters behind a porpoise’s blowhole, recorded the animal’s movements and the clicking noises it made as it hunted prey. After about 24 hours, the tag detached and was retrieved.

“Tagged porpoises foraged nearly continuously, targeting small prey with remarkably high capture success rates,” the researchers report May 26 in Current Biology.

The tags revealed that the porpoises encountered up to 200 fish during the day and 50 to 550 after dusk, when the animals tended to hunt in deeper waters. The porpoises mostly targeted fish just 3 centimeters to 10 centimeters long, and were successful at catching them more than 90 percent of the time.

Harbor porpoises have features that help them manage this continuous munching: They have small stomachs that can accommodate only 1.9 kilograms of food maximum at a time, and their digestive tracts are short and move meals through the body in just 140 minutes. (Humans require an average of 40 hours.)

But this need to feed continuously to support a high metabolism has harbor porpoises “operating on an energetic knife-edge,” the researchers note. Because they have to eat and eat and eat to survive, it doesn’t take much to disturb their routine and put them on the path to starvation. “Individual porpoises have been reported to starve to death in less than a week,” the team notes.

Anthropogenic disturbances and changes to the marine ecosystem could be especially dangerous to this species, one in which starvation rates have been rising.

All about aliens
New telescopes and spacecraft will soon help researchers scour our galaxy for signs of extraterrestrial life. But what might aliens look like? And if they do exist, why haven’t they returned our calls? These are just some of the questions addressed in the Science News special report “In Search of Aliens” (SN: 4/30/16, p. 24).

Readers enjoyed Tina Hesman Saey’s feature “Will we know ET when we see it?” (SN: 4/30/16, p. 28) and described what they think aliens might look like.
“The search for extraterrestrials is a subject which needs to be taken more seriously by mainstream science. Thanks for looking at it and please have more on the topic,” Wade Carmen wrote. He suggested aliens might look similar to creatures that dwell in the deep ocean. “H.G. Wells did it right by envisioning Martians as cephalopods in War of the Worlds,” he added. “The first extraterrestrial forms of life likely to be encountered are microbes or even something amorphous like the Blob.”

Other readers thought the search for ET might require new definitions for life: “Maybe ribose is ET,” said Cabell Smith, drawing a connection between the special issue and Christopher Crockett’s “Ribose could have formed in space” (SN: 4/30/16, p. 18), which reported that the key sugar in RNA can form in lab-made “interstellar” ice. Annselm Morpurgo took the idea a step further. “I am still waiting for some astrophysicist to declare that there may even be life on the surface of the sun,” she wrote. “Forget biology. Information exchange between self-replicating structures of any kind, such as electromagnetic ‘bumps’ in a chaotic ‘soup,’ might also qualify.”
Great Plains shake-up
Earthquakes are no longer just a natural hazard. For the first time, the U.S. Geological Survey included human-made earthquakes from activities such as wastewater injection in its annual hazard forecast, Thomas Sumner reported in “Quake risk high in parts of central United States” (SN: 4/30/16, p. 20).

Online reader Pro-Marx wondered if more frequent small, human-made earthquakes could relieve built up stress and prevent the occassional massive quake.

Smaller earthquakes can relieve stress on a fault but not enough to reduce the intensity of big quakes, Sumner says. As earthquakes’ magnitudes increase, the energies they release rise drastically. A magnitude 6 earthquake, for instance, releases nearly 32 times as much energy as a magnitude 5 quake and about 32,000 times as much energy as a magnitude 3 earthquake, according to the USGS. Scientists do not consider triggering artificial quakes a good prevention strategy. “Accumulating all of those smaller quakes would be a tall task, especially when you run the risk of accidentally triggering a damaging quake,” Sumner says.

Salamander insights
Europe’s spookily pale and blind cave salamanders, called olms, include a dark form with what look like functional eyes, Susan Milius reported in “What’s odd about a dark, big-eyed salamander” (SN: 4/30/16, p. 4).

Online reader John Turner wondered why dark olms evolved those traits. “Maybe every few generations, every few tries, a pigmented and sighted salamander makes it across open ground from one cave system to another cave system to spread the species,” he wrote. “That would be the reward for keeping a few black sheep in the family, no?”

The evolution of dark skin and vision likely resulted from olms spending more time in shallow caves where it’s lighter, says olm researcher Stanley Sessions of Hartwick College in Oneonta, N.Y. Their evolution, of course, “does not have a ‘purpose’ such as allowing [olms] to migrate over the surface,” he says. Although spring floods occasionally wash both dark and light olms aboveground, “they are all thoroughly aquatic with delicate gills, and die very quickly if they are exposed to air for long,” he says.

Sharks have a sixth sense that helps them locate prey in murky ocean waters. They rely on special pores on their heads and snouts, called ampullae of Lorenzini, that can sense electric fields generated when nearby prey move. The pores were first described in 1678, but scientists haven’t been sure how they work. Now, the answer is a bit closer.

The pores, which connect to electrosensing cells, are filled with a mysterious clear jelly. This jelly is a highly efficient proton conductor, researchers report May 13 in Science Advances. In the jelly, positively charged particles move and transmit current.

Marco Rolandi of the University of California, Santa Cruz and colleagues squeezed jelly from the pores of one kind of shark and two kinds of skate and tested how well protons could flow through the substance. Good proton conductors, including a protein found in squid skin, occur in nature. But the jelly is the best biological proton conductor discovered so far. In fact, even humankind’s best technology isn’t wildly better. The most efficient proton conductor devised by people — a polymer known as Nafion — is a mere 40 times better than the stuff sharks are born with.

During last year’s outbreak of Middle East respiratory syndrome in South Korea, about 20 percent of hospital patients in close contact with a single MERS case, known as Patient 14, contracted the disease, South Korean researchers found after reviewing medical records and security footage.

From May to July 2015, South Korea experienced the biggest outbreak of MERS outside of the Middle East. A few people dubbed “superspreaders” transmitted the virus to many others. Patient 14, a 35-year-old man, unwittingly transmitted the virus to 82 other people — including 33 patients, 41 visitors and eight health care workers — between May 27 and 29 while in the emergency room of Samsung Medical Center in Seoul. The man did not know he had been exposed to the virus while at another hospital.

During his time in the ER, the man came in contact with 675 patients, 218 health care workers and about 683 visitors, researchers report July 8 in The Lancet. People who spent time in the same zone of the emergency room with Patient 14 were most likely to catch the virus from him. But even people who weren’t in close contact had about a 5 percent chance of getting ill. That includes three people who overlapped with the man for a short time in the radiology suite. Even four of the 500 patients who were never in the same part of the emergency room as Patient 14 caught the virus.

The most likely place to catch MERS from Patient 14 was the hospital’s waiting room, designated Zone II, even though people in that part of the emergency room were exposed to him for only about three hours. On average, it took about seven days after exposure to the virus for people to develop symptoms.

ORLANDO, Fla. — Fruit flies can break down a popular herbicide with help from friendly microbes, new research suggests.

Drosophila melanogaster fruit flies have no genes needed to process an herbicide called atrazine, statistical biologist James “Ben” Brown of Lawrence Berkeley National Lab in California said July 16 at the Allied Genetics Conference. Researchers expected that the inability to break down the herbicide might make the chemical, which is often sprayed on cornfields, toxic to the flies. But after feeding atrazine to fruit flies for 72 hours, Brown and colleagues detected very little response to the chemical.
Bacteria and other microbes that live in the intestines are often able to digest chemicals that their hosts cannot. So the researchers looked for atrazine-processing genes in bacteria that live in the fruit flies’ guts.

“When we look for those genes, we see them immediately,” Brown said. “Notably, no one microbe has all these genes. They are present in a consortium of microbes.” The result indicates that intestinal microbes work together to break down the chemical.

Brown and colleagues are testing atrazine and other pesticides to determine how the chemicals affect health. In the experiments, the researchers use fruit flies as surrogates for honeybees. It is not known whether honeybees also carry microbes that can break down the herbicide.

A three-way tectonic tango may have led to the birth of what is now the largest chunk of Earth’s crust.

By scrutinizing what little geologic evidence remains from 190 million years ago, researchers reconstructed the origins of the Pacific tectonic plate, which now covers a fifth of Earth’s surface. The plate formed during the early Jurassic period from a single point where three tectonic plates once met, the work suggests. The plate’s birthplace sat above the gravesite of a section of tectonic plate that sank into the planet’s depths, the researchers report July 27 in Science Advances. The remnants of that sunken plate remain embedded in Earth’s mantle.
This origin story of tectonic life and death is unique in Earth’s known history, says study coauthor Lydian Boschman, a geologist and geodynamicist at Utrecht University in the Netherlands. All of the other modern plates formed by one plate splitting into two, she says. “We’re not sure why this happened, but we now know how it happened.”

A network of shifting tectonic plates covers Earth’s surface. While pieces of continental plates can date back billions of years, the oldest oceanic crust is only about 200 million years old. Anything older has been swallowed into Earth’s interior by subduction.
The oldest part of the Pacific Plate dates back 190 million years and is triangle-shaped. That shape led scientists to postulate that the Pacific Plate formed at a convergence of three other plates. Some scientists proposed that these three plates were in a Y-shape configuration, with the plates spreading away from a central point. As the plates drifted, the ground split and new crust formed as molten rock extruded onto the seafloor and cooled. But such a setup of three spreading ridges wouldn’t create a new plate — the new material would have just made the existing plates bigger.
Reviewing the ages of the seafloor across the Pacific Ocean, Boschman and Douwe van Hinsbergen, a geophysicist also at Utrecht University, instead propose a different story.

The story begins with three tectonic plates that slid together, with one of the plates steamrolling over a section of another, forcing the plate segment into Earth’s depths. That plate segment’s disappearance briefly led to a three-way junction. But instead of the plates moving away from a central point, the three plates bumped and scraped alongside one another in a pinwheel motion. (One plate gliding past another plate is the type of movement seen along California’s San Andreas Fault.)

With all three plates now sliding past one another, a triangular gap opened in the middle. Fresh molten rock from Earth’s interior rushed up to fill the void, creating the nucleus of the Pacific Plate. Spreading ridges formed on each side of this budding plate, adding additional rock to the plate as the gap between the three original plates grew.

“The Pacific Plate is the largest on Earth, but it started out as the smallest,” Boschman says.

Using seismic imaging, scientists previously identified the possible remains of a sunken tectonic plate west of Costa Rica. That lost plate could mark the birthplace of the Pacific Plate, Boschman says.

Verifying this story will be “extremely challenging,” says geophysicist Dietmar Müller of the University of Sydney. No seafloor lingers from before the Pacific Plate formed, and the sunken tectonic plate may have moved over time, making it difficult to concretely tie the plate to the proposed scenario. Boschman says the composition of the Pacific Plate’s oldest rocks may reveal whether they formed over a submerged plate, though collecting such rocks could be expensive and difficult.

Even so, the work provides the first plausible look at the tectonic movements that may have led to the Pacific Plate’s formation, says marine geophysicist William Sager of the University of Houston. “It was a dark room and they turned on the light. It’s a dim light, so we can’t see very far, but it’s something.”

The black holes that produced the first detected gravitational waves may have exotic origins in the early universe.

When the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, glimpsed gravitational waves from two merging black holes, scientists were surprised at how large the black holes were — about 30 times the mass of the sun (SN: 3/5/16, p. 6). Inspired by this unusual finding, two papers published in Physical Review Letters propose that the hefty black holes were born in the universe’s infancy.
Unlike run-of-the-mill black holes that form from collapsing stars, such primordial black holes could have formed when dense regions of the very early universe collapsed under their own gravity, some theories suggest. If they exist, primordial black holes could also solve another puzzle: the identity of dark matter, the unknown source of mass in the universe that holds galaxies and galaxy clusters together. Primordial black holes could make up the universe’s missing mass, an idea that counters the more popular theory that dark matter is made up of undetected particles.

A Japanese team of astrophysicists reported August 2 that LIGO’s black holes may be primordial, and that, if so, they could make up some portion of the universe’s dark matter. Johns Hopkins University scientists reported May 19 that LIGO’s estimated rate of black hole mergers matches with that expected from primordial black hole dark matter.

LIGO’s black holes’ bigger than expected mass had astrophysicist Simeon Bird and colleagues at Johns Hopkins University wondering, “Gosh — it’s unexpected — what else could it be?” Bird says. Previous research had ruled out primordial black hole dark matter for all but a narrow range of masses. But that allowed range happens to overlap with the masses of the black holes LIGO found.

Drawing on scientists’ knowledge of dark matter’s properties, Bird and colleagues estimated how often LIGO would expect to see merging primordial black holes, assuming they were the source of dark matter. This rate matched LIGO’s estimated detection rate, made by assuming the one unexpectedly massive black hole merger LIGO has seen so far wasn’t a fluke. Although both estimates have large errors, their agreement indicates that dark matter may be composed of primordial black holes.

Likewise, the Japanese team reported that LIGO could have detected primordial black holes. But the researchers found that such primordial black holes could explain only a small fraction of dark matter. This disparity boils down to differing assumptions about how primordial black holes group into pairs before merging.
“The important thing is that this can be tested,” says astrophysicist Misao Sasaki of Kyoto University in Japan. More data from LIGO or further studies of the cosmic microwave background — remnant light from the aftermath of the Big Bang — could exclude primordial black holes as a possibility.

To better understand LIGO’s black holes, “we’re going to need to make more detections,” says LIGO scientist Chad Hanna of Penn State University. (LIGO also detected a second black hole merger (SN: 7/9/16, p. 8), but those black holes were smaller, indicating that they formed from stars.)

Eventually, subtle signs of primordial black holes may appear in gravitational wave data, says Bernard Carr of Queen Mary University of London. The eccentricity of the black holes’ orbits around one another — how elliptical their paths are — could indicate whether the black holes are primordial or standard black holes, Carr says. “It’s a bit more exciting to my mind if they turn out to be primordial black holes.”

WASHINGTON — Bacteria assassinating each other when crowded together ironically can favor the evolution of cooperation.

When a Vibrio cholerae bacterium jostles neighbors in crowds on crab shells, it fires a spring-loaded toxin injection. Siblings with the same immunity genes don’t die, but genetically different strains of V. cholerae can succumb.

In both laboratory battles and computer simulations, these neighbor-to-neighbor harpoonings over time can separate a random mix of strains into a patchy landscape of same-strain clumps. The change from all mixed up to irregular patches works like a separation process of phases of metals (called the Model A order-disorder transition) and has not been reported before in living things, William Ratcliff of Georgia Institute of Technology in Atlanta said August 5 at the 2nd American Society for Microbiology Conference on Experimental Microbial Evolution.
This resulting clumpydistribution, despite its murderous origin, favors the rise of cooperation, such as secreting substances useful to a whole community, said Georgia Tech colleague Brian Hammer. In an analysis of more than 400 strains of bacteria from 26 genera, the more elaborate the weapons made by microbial strains are, the more of a strain’s genes were devoted to secretions.

Cooperative patches of bacteria are of interest because they form scary defenses like biofilms. They also matter to biologists trying to weed or reseed the microflora of the gut.

Earth might have a kindred planet orbiting the star next door. A world at least 1.3 times as massive as Earth appears to orbit the closest star to the sun: Proxima Centauri, a dim red orb about 4.2 light-years away.

Dubbed Proxima b, the planet is cozied up to its star, needing just 11.2 days to complete one orbit. But despite the proximity to its star — just 5 percent of the distance from Earth to the sun — Proxima b is potentially habitable. Its temperature is just right for liquid water to flow on its surface, Guillem Anglada-Escudé, an astronomer at Queen Mary University of London, and colleagues report in the August 25 Nature. That makes Proxima b the closest known world outside our solar system where life might exist.
“It’s an incredible discovery — it’s almost a gift,” says David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. With Proxima b, researchers might now have their best chance at characterizing the atmosphere of an Earthlike world in another solar system and probing for hints of life elsewhere in the galaxy.
Proxima Centauri, which lies in the southern constellation Centaurus, is a runt of a star. Temperatures at the surface run about 2,800 degrees Celsius cooler than our sun, giving Proxima a feeble, ruddy glow. The star is much closer in size to Jupiter than the sun, and even though it’s relatively close to Earth, Proxima is invisible to the naked eye — it wasn’t discovered until 1915. Part of a triple star system known as Alpha Centauri, it’s not clear whether Proxima is gravitationally bound to its brighter companions (taking hundreds of thousands of years to complete one orbit around both) or just passing by.

The Alpha Centauri system is no stranger to claims of exoplanets. In 2012, astronomers reported in Nature that the star Alpha Centauri B hosts a planet roughly as massive as Earth, though too warm to be habitable (SN: 11/3/12, p. 5). Other researchers are skeptical; a 2015 report in Monthly Notices of the Royal Astronomical Society Letters, for example, found no evidence for the planet. The claim for Proxima b appears to be much stronger.
Anglada-Escudé and colleagues found their quarry by looking for a minute wobble in the speed of Proxima Centauri, the sign of a gravitational tug from the orbiting planet. An intensive two-month observing campaign in early 2016 using two telescopes in Chile — the European Southern Observatory’s 3.6-meter and Very Large telescopes — confirmed earlier suspicions of a planet.

“It’s not clear if the planet will be Earthlike,” Anglada-Escudé says. Not much is known about Proxima b, such as its size or what its atmosphere is like. Even its mass is just a minimum estimate. Without knowing how the planet’s orbit is tilted relative to us, the researchers can say only that Proxima b is no lighter than 1.3 Earths — it could be heavier and have more in common with Neptune than Earth.

Even though it’s just one star away, “we will likely have to wait a long time in order to learn anything more about the planet,” says Heather Knutson, a planetary scientist at Caltech.
The best bet, says Knutson, is to hope that the planet, when viewed from Earth, passes in front of Proxima Centauri, allowing starlight to filter through the planet’s atmosphere. Molecules in the atmosphere would betray their presence by absorbing specific wavelengths of light. Substances such as oxygen, methane and carbon dioxide are widely considered to be chemical markers of life.

If the planet does cross in front of the star, NASA’s James Webb Space Telescope, scheduled to launch in late 2018, should be able to characterize its atmosphere, says Mark Clampin, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Hundreds of hours of telescope time would need to be dedicated to the task. “It will be an extremely challenging observation, but not impossible,” he says.

Scientists can also estimate the planet’s size by measuring how much light the planet blocks. The size combined with the mass would let researchers determine the density of Proxima b and figure out if the planet is puffy like Jupiter or rocky like Earth.

Kipping has already been monitoring Proxima Centauri with the Canadian MOST satellite, looking for a periodic dip in light caused by the planet partially blocking its sun. There’s only a 1.5 percent chance, however, that the planet lines up just-so with the star. And if it does line up, the inherent variability in Proxima Centauri’s light will make any drop in brightness from the planet hard to detect.

Without a fortuitous alignment, “things get much more difficult,” Knutson says. Astronomers would have to rely on light coming from the planet — either an intrinsic infrared glow or visible light reflected from its sun. James Webb might be able to barely sense infrared light emanating from Proxima b, but it could be a decade or more before any other observatory is up to the challenge (SN: 4/30/16, p. 32). And even then, there are no guarantees. “It’s going to be very difficult to characterize the planet without sending a probe there,” Kipping says.

Breakthrough Starshot, a group funded by Russian entrepreneur Yuri Milner, wants to do just that. In April the group announced a plan to put $100 million toward developing technology that would send a fleet of nanocraft — robotic probes weighing just a few grams — toward Alpha Centauri, nudging them along with an Earth-based 100 gigawatt laser. Accelerating to roughly 20 percent the speed of light, the armada would arrive at Alpha Centauri about 20 years after launch. In comparison, the fastest spacecraft ever to leave Earth — the New Horizons mission to Pluto — would need roughly 90,000 years to complete the journey, traveling at its current speed of about 52,000 kilometers per hour.

“The discovery is likely to energize the project,” says Harvard University astrophysicist Avi Loeb, chairman of Breakthrough’s advisory committee. “A spacecraft equipped with a camera and various filters could take color images of the planet and infer whether it is green (harboring life as we know it), blue (with water oceans on its surface) or just brown (dry rock).”

If anything is alive on Proxima b, it’s probably quite different from anything on Earth. Photosynthesizing organisms would have to deal with a faint, cool star that emits mostly infrared light. Proxima Centauri is also known for exuberant flares, which would buffet any orbiting planets with bursts of ultraviolet radiation and X-rays. “Conditions on such a planet would be very interesting for life,” says Lisa Kaltenegger, an astrophysicist at Cornell University.
Given such an alien environment, life might show its presence in unusual ways. Kaltenegger, along with Cornell astronomer Jack O’Malley-James, proposes looking for biofluorescence, a glow from organisms triggered by ultraviolet light, in the wake of stellar flares. Critters on Proxima b could have evolved biofluorescence as protection, taking harmful UV radiation and transforming it into more palatable visible light — a flicker that might be detectable from an Earth-based telescope. “The idea that we could spot a glow seems to be right out of a [science fiction] novel,” says Kaltenegger, whose proposal appears online August 24 at arXiv.org.

That’s assuming anything could survive on the planet. If Earth were placed in the same orbit as Proxima b, it would be stripped of its protective ozone roughly three times per Earth year, Kipping says. “That’s kind of bad,” he says. That rate doesn’t give the atmosphere time to recover, “but it’s not a showstopper,” he adds. A strong planetary magnetic field or a dense atmosphere might be able to withstand the blows. And if life has taken shelter underground or underwater — or is impervious to a lack of oxygen — it might still survive.

Whether or not critters crawl on Proxima b, the discovery of the planet “could really usher new energy into the search for other nearby worlds,” says Margaret Turnbull, an astronomer with the SETI Institute and based in Madison, Wis. Most exoplanets are hundreds to thousands of light-years away. But little is known about the possible planet families huddled up to the stars nearest to us. “I’d love to see interstellar travel,” says Turnbull. “To really inspire that kind of effort, we need interesting destinations like this.”

An experimental drug swept sticky plaques from the brains of a small number of people with Alzheimer’s disease over the course of a year. And preliminary results hint that this cleanup may have staved off mental decline.

News about the new drug, an antibody called aducanumab, led to excitement as it trickled out of recent scientific meetings. A paper published online August 31 in Nature offers a more comprehensive look at the drug’s effects.

“Overall, this is the best news that we’ve had in my 25 years doing Alzheimer’s clinical research,” study coauthor Stephen Salloway of Brown University said August 30 at a news briefing. “It brings new hope for patients and families most affected by the disease.”
The results are the most convincing evidence yet that an antibody can reduce amyloid in the brain, says Alzheimer’s researcher Rachelle Doody of Baylor College of Medicine in Houston, who was not involved in the study.

Still, experts caution that the results come from 165 people, a relatively small number. The seemingly beneficial effects could disappear in larger clinical trials, which are under way. “These new data are tantalizing, but they are not yet definitive,” says neuroscientist John Hardy of University College London.

Like some other drug candidates for Alzheimer’s, aducanumab is an antibody that targets amyloid-beta, a sticky protein that accumulates in the brains of people with the disease. Delivered by intravenous injection, aducanumab appeared to get inside the brains of people with mild Alzheimer’s (average age about 73) and destroy A-beta plaques, the results suggest. After a year of exposure to the drug, A-beta levels had dropped. This reduction depended on the dose — the more drug, the bigger the decline in A-beta. In fact, people on the highest dose of the drug had almost no A-beta plaques in their brains after a year.

“I know of no other antibody that leads to this degree of amyloid removal,” study coauthor Alfred Sandrock of Biogen in Cambridge, Mass., said at the news briefing. For several decades, scientists have been trying to figure out whether A-beta is a cause, or just a symptom, of Alzheimer’s (SN: 3/12/11, p. 24). With its ability to reduce A-beta plaques in the brain, aducanumab may help settle the debate.

The bigger question is whether the drug can preserve thinking skills and memory. The new study was not designed to detect improvements in mental performance. Yet it turned up hints that aducanumab may help.
Compared with participants who received a placebo, people who took aducanumab showed less decline on standard tests of memory and thinking skills over the course of a year. And like the reductions in brain amyloid, better performance seemed to come with higher doses. During the study, people who received the placebo lost just under three points on average on a 30-point cognitive test. In contrast, people on the highest dose of aducanumab lost a little over half a point.

“One needs to take the cognitive data with a grain of salt at the moment, given the small number of people who enrolled and completed the study,” says neuroscientist Eric Reiman of the Banner Alzheimer’s Institute in Phoenix. But if larger studies show a similar benefit, “it would be a game changer for the field,” says Reiman, who wrote an accompanying commentary in Nature.

Aducanumab targets several forms of A-beta — including both small, soluble bits called oligomers and larger clumps called fibrils — that make up plaques. Both forms may cause trouble. Once aducanumab sticks to A-beta, specialized brain cells called microglia may come in and remove the buildup, lab experiments suggest.

Twenty-seven people in the study had an adverse drug reaction known as ARIA, marked by changes in brain fluid detected by brain scans. The side effect is often without symptoms, but can cause headaches or more serious trouble in some people. ARIA was more common at higher doses, the researchers found. The larger studies of aducanumab that are under way may help scientists pinpoint the most effective dose with the fewest side effects.