Discoveries on the island of Borneo illustrate that cave art emerged in Southeast Asia as early as in Western Europe, and with comparable complexity, researchers say.

A limestone cave in eastern Borneo features a reddish-orange painting of a horned animal, possibly a type of wild cattle that may have been found on the island at the time. The painting dates to at least 40,000 years ago, concludes a team led by archaeologist Maxime Aubert of Griffith University in Southport, Australia. This creature represents the oldest known example of a painted figure anywhere in the world, the scientists report online November 7 in Nature.
The same cave walls contain two hand outlines framed in reddish orange pigment that were made at least 37,200 years ago and a similar hand stencil with a maximum age of 51,800 years.

Three nearby caves display instances of a second rock art style that appeared around 20,000 years ago, the investigators say. Examples include purple-hued, humanlike figures and hand stencils, some decorated with lines or dots. Painted lines link some hand stencils to others.

Age estimates rest on analyses of uranium in mineral deposits that had formed over and underneath parts of each cave painting. Scientists used known decay rates of radioactive uranium in these deposits to calculate maximum and minimum dates for the paintings.

Aubert’s group previously used this technique, called uranium-series dating, to calculate that people on the nearby Indonesian island of Sulawesi created hand stencils on cave walls nearly 40,000 years ago (SN: 11/15/14, p. 6).
“Cave art could have potentially been exported from Borneo to Sulawesi and all the way to Papua and Australia,” Aubert says. Australian cave paintings of humanlike figures resemble those found on Borneo, he says. But the ages of the Australian finds remain uncertain.

No Southeast Asian cave paintings have been found from when humans first arrived in the region, between 70,000 and 60,000 years ago. At that time and up to the end of the last Ice Age around 10,000 years ago, Borneo formed mainland Eurasia’s easternmost tip thanks to lowered sea levels.

Those first Southeast Asians may have created cave art that hasn’t been discovered, Aubert says. Or, small groups of early colonizers may not have painted on cave walls until their populations expanded, leading to more complex social and ritual behaviors. It’s also possible that another human migration from elsewhere in Asia brought rock art to Borneo roughly 50,000 years ago.
Whatever the case, “Western European and Southeast Asian cave art seem to first appear at about the same time and with remarkable similarities,” says archaeologist Sue O’Connor of Australian National University in Canberra, who did not participate in the new study.
Other investigators have used the uranium-series technique to date a painted red disk in a Spanish cave to at least 40,800 years ago (SN: 7/28/12, p. 15). Another report this year suggested that Neandertals painted abstract shapes and hand stencils on the walls of several Spanish caves at least 64,800 years ago (SN: 3/17/18, p. 6).

Aubert’s team has criticized that study, saying the researchers may have unintentionally dated mineral deposits that are much older than the artworks. If so, humans rather than Neandertals could have created the Spanish cave art.

Meanwhile, scientists who conducted the Neandertal cave art study express their own doubts about the reliability of dates for the Borneo paintings. Descriptions of sampled mineral deposits from the Borneo caves leave it unclear whether, for example, Aubert’s group dated the horned animal figure or adjacent paint remnants of some other, unidentified figure, says archaeologist João Zilhão of the University of Barcelona.

Zilhão and Neandertal paper coauthor Paul Pettitt of Durham University in England don’t doubt that cave painting emerged in Southeast Asia at least 40,000 years ago. But they and Aubert’s team disagree about how to collect mineral samples for dating rock art.

There’s something big lurking beneath Greenland’s ice. Using airborne ice-penetrating radar, scientists have discovered a 31-kilometer-wide crater — larger than the city of Paris — buried under as much as 930 meters of ice in northwest Greenland.

The meteorite that slammed into Earth and formed the pit would have been about 1.5 kilometers across, researchers say. That’s large enough to have caused significant environmental damage across the Northern Hemisphere, a team led by glaciologist Kurt Kjær of the University of Copenhagen reports November 14 in Science Advances.
Although the crater has not been dated, data from glacial debris as well as ice-flow simulations suggest that the impact may have happened during the Pleistocene Epoch, between 2.6 million and 11,700 years ago. The discovery could breathe new life into a controversial hypothesis that suggests that an impact about 13,000 years ago triggered a mysterious 1,000-year cold snap known as the Younger Dryas (SN: 7/7/18, p. 18).
Members of the research team first spotted a curiously rounded shape at the edge of Hiawatha Glacier in northwest Greenland in 2015, during a scan of the region by NASA’s Operation IceBridge. The mission uses airborne radar to map the thickness of ice at Earth’s poles. The researchers immediately suspected that the rounded shape represented the edge of a crater, Kjær says.
For a more detailed look, the team hired an aircraft from Germany’s Alfred Wegener Institute that was equipped with ultra-wideband radar, which can send pulses of energy toward the ice at a large number of frequencies. Using data collected from 1997 to 2014 from Operation Icebridge and NASA’s Program for Arctic Regional Climate Assessment, as well as 1,600 kilometers’ worth of data collected in 2016 using the ultra-wideband radar, the team mapped out the inner and outer contours of their target.

The object is almost certainly an impact crater, the researchers say. “It became clear that our idea had been right from the beginning,” Kjær says. What’s more, it is not only the first crater found in Greenland, but also one of the 25 or so largest craters yet spotted on Earth. And it has held its shape beautifully, from its elevated rim to its bowl-shaped depression.

“It’s so conspicuous in the satellite imagery now,” says John Paden, an electrical engineer at the University of Kansas in Lawrence and a member of the team. “There’s not another good explanation.”

On the ground, the team hunted for geochemical and geologic signatures of an asteroid impact within nearby sediments. Sampling from within the crater itself was impossible, as it remains covered by ice. But just beyond the edge of the ice, meltwater from the base of the glacier had, over the years, deposited sediment. The scientists collected a sediment sample from within that glacial outwash and several from just outside of it.

The outwash sample contained several telltale signs of an impact: “shocked” quartz grains with deformed crystal lattices and glassy grains that may represent flash-melted rock. The sample also contained elevated concentrations of certain elements, including nickel, cobalt, platinum and gold, relative to what’s normally found in Earth’s crust. That elemental profile points not only to an asteroid impact, the researchers say, but also suggests that the impactor was a relatively rare iron meteorite.
Determining when that iron meteorite slammed into Earth is trickier.
The ice-penetrating radar data revealed that the crater bowl itself contains several distinct layers of ice. The topmost layer shows a clear, continuous sequence of smaller layers of ice, representing the gradual deposits of snow and ice through the most recent 11,700 years of Earth’s history, known as the Holocene. At the base of that “well-behaved” layer is a distinct, debris-rich layer that has been seen elsewhere in Greenland ice cores, and is thought to represent the Younger Dryas cold period, which spanned from about 12,800 to 11,700 years ago. Beneath that Younger Dryas layer is another large layer — but unlike the Holocene layer, this one is jumbled and rough, with undulating rather than smooth, nearly flat smaller layers.

“You see folding and strong disturbances,” says study coauthor Joseph MacGregor, a glaciologist with Operation IceBridge. “And below that, we see yet deeper, complex basal ice.” Radar images of that bottommost ice layer within the crater show several curious peaks, which MacGregor says could represent material from the ground that got incorporated into the ice. “Putting that all together, what you have is a snapshot of an ice sheet that looked fairly normal during the Holocene, but was quite disturbed before that.”

Those data clearly suggest that the impact is at least 11,700 years old, Kjær says. And the rim of the crater appears to cut through a preexisting ancient river channel that must have flowed across the land before Greenland became covered with ice about 2.6 million years ago.

That time span — essentially, the entire Pleistocene Epoch — is a large range. The team is working on further narrowing the possible date range, with more sediment samples, simulations of the rate of ice flow and possibly cores collected from within the crater.

The date range does include the possibility that the impact occurred near the onset of the Younger Dryas. “It’s the woolly mammoth in the room,” MacGregor says.
Planetary scientist Clark Chapman of the Southwest Research Institute in Boulder, Colo., notes that “there are plenty of roughly circular landforms on Earth of many different sizes, most of which are not impact craters.” Still, he says, the paper presents several lines of evidence that strongly support the conclusion that the object is a crater, including the shocked quartz and the topography.

As for the idea that a crater may have formed within the last couple of million years, Chapman says, it’s “quite unlikely.” Such strikes are rare in general, he adds, and asteroids barreling into Earth are far more likely to land somewhere in an ocean. “[And] it would be at least a hundred times less likely that it could have happened so recently as to have affected the Younger Dryas.”

Regardless of when the crater formed, it is “a straight-up exciting discovery,” MacGregor says. “And we’re just happy not to have to keep it a secret anymore.”

Losing one variety of gut bacteria may lead to type 2 diabetes as people age.

Old mice have less Akkermansia muciniphila bacteria than young mice do, researchers report November 14 in Science Translational Medicine. That loss triggers inflammation, which eventually leads cells to ignore signals from the hormone insulin. Such disregard for insulin’s message to take in glucose is known as insulin resistance and is a hallmark of type 2 diabetes.

Researchers have suspected that bacteria and other microbes in the gut are involved in aging, but how the microbes influence the process hasn’t been clear. Monica Bodogai of the U.S. National Institute on Aging in Baltimore and colleagues examined what happens to mice’s gut bacteria as the rodents age. The mice lose A. muciniphila, also called Akk, and other friendly microbes that help break down dietary fiber into short-chain fatty acids, such as butyrate and acetate. Those fatty acids signal bacteria and human cells to perform certain functions.
Losing Akk led to less butyrate production, Bodogai’s team found. In turn, loss of butyrate triggered a chain reaction of immune cell dysfunction that ended with mice’s cells ignoring the insulin.

Treating old mice and elderly rhesus macaques with an antibiotic called enrofloxacin increased the abundance of Akk in the animals’ guts and made cells respond to insulin again. Giving old animals butyrate had the same effect, suggesting that there may be multiple ways to head off insulin resistance in older people in the future.

The next NASA Mars rover will hunt for signs of ancient life in what used to be a river delta, the agency announced on November 19.

The rover is expected to launch in July 2020 and to land on Mars around February 18, 2021. It will seek out signs of past life in the sediments and sands of Jezero crater, which was once home to a 250-meter-deep lake and a river delta that flowed into the lake.
“This is a major attraction from our point of view for a habitable environment,” said Mars 2020 project scientist Ken Farley of Caltech in a news conference discussing the site. “A delta is extremely good at preserving biosignatures.” Any evidence of life that may once have existed in the lake water, or even evidence that came from the river’s headwaters and flowed downstream, could be preserved in the rocks that are there today.

The 2020 rover’s design is similar to that of the Curiosity rover, which has been exploring a different ancient crater lake, Gale crater, since 2012 (SN: 5/2/15, p. 24). But where Curiosity has an onboard chemistry lab for studying the rocks and minerals in its crater, Mars 2020 will have a specialized backpack for sample storage. A future mission will pick up the cached samples and return them to Earth for more detailed study, possibly sometime in the 2030s.

“The samples will come back to the best labs — not the best labs we have today, but the best labs we will have then,” said science mission directorate administrator Thomas Zurbuchen of NASA headquarters in Washington, D.C.

Mars 2020 will also use a souped-up version of Curiosity’s landing system called Sky Crane, in which a hovering platform lowers the rover onto the ground with a cable. Mars 2020’s version will include a navigation system that will help it avoid hazards on the ground, like cliff faces and boulders.
Jezero crater is within striking distance of another site on scientists’ wish list. That region, called Midway, is just 28 kilometers away from Jezero and contains some of the most ancient rocks on Mars. At the final landing site selection workshop in October, scientists floated the idea of visiting both sites in one mission, a feat seen as ambitious but achievable. But a decision on that will have to wait until after the rover is safely on Mars, Farley said.

For the first time, humans have built a set of pushy, destructive genes that infiltrated small populations of mosquitoes and drove them to extinction.

But before dancing sleeveless in the streets, let’s be clear. This extermination occurred in a lab in mosquito populations with less of the crazy genetic diversity that an extinction scheme would face in the wild. The new gene drive, constructed to speed the spread of a damaging genetic tweak to virtually all offspring, is a long way from practical use. Yet this test and other news from 2018 feed one of humankind’s most persistent dreams: wiping mosquitoes off the face of the Earth.

For the lab-based annihilation, medical geneticist Andrea Crisanti and colleagues at Imperial College London focused on one of the main malaria-spreading mosquitoes, Anopheles gambiae. The mosquitoes thrive in much of sub-Saharan Africa, where more than 400,000 people a year die from malaria, about 90 percent of the global total of malaria deaths.

To crash the lab population, the researchers put together genes for a molecular copy-and-paste tool called a CRISPR/Cas9 gene drive. The gene drive, which in this case targeted a mosquito gene called doublesex, is a pushy cheat. It copies itself into any normal doublesex gene it encounters, so that all eggs and sperm will carry the gene drive into the next generations. Female progeny with two altered doublesex genes develop more like males and, to people’s delight, can’t bite or reproduce.

In the test, researchers set up two enclosures, each mixing 150 males carrying the saboteur genes into a group of 450 normal mosquitoes, males and females. Extinction occurred in eight generations in one of the enclosures and in 12 in the other (SN: 10/27/18, p. 6).

This is the first time that a gene drive has forced a mosquito population to breed itself down to zero, says Omar Akbari of the University of California, San Diego, who has worked on other gene drives. However, he warns, “I believe resistance will be an issue in larger, diverse populations.” More variety in mosquito genes means more chances of some genetic quirk arising that counters the attacking gene drive.

But what if a gene drive could monkey-wrench a wild population, or maybe a whole species, all the way to extinction? Should people release such a thing? To make sense of this question, we humans will have to stop talking about “mosquitoes” as if they’re all alike. The more than 3,000 species vary considerably in what they bite and what ecosystem chores they do.

The big, iridescent adults of Toxorhynchites rutilus, for instance, can’t even drink blood. And snowmelt mosquitoes (Ochlerotatus communis) are pollinators of the blunt-leaved orchid (Platanthera obtusata), ecologist Ryo Okubo of the University of Washington in Seattle said at the 2018 meeting of the Society for Integrative and Comparative Biology.
Estimating what difference it would make ecologically if a whole mosquito species disappeared has stirred up plenty of speculation but not much data. “I got pretty fed up with the hand-waving,” says insect ecologist Tilly Collins of Imperial College London. So she and colleagues dug through existing literature to see what eats An. gambiae and whether other mosquitoes would flourish should their competitor vanish.

So far, extermination of this particular mosquito doesn’t look like an ecological catastrophe, Collins says. Prey information is far from perfect, but diets suggest that other kinds of mosquitoes could compensate for the loss. The species doesn’t seem to be any great prize anyway. “As adults, they are small, not juicy, and hard to catch,” she says. The little larvae, built like aquatic caterpillars with bulging “shoulders” just behind their heads, live mostly in small, temporary spots of water.
The closest the researchers came to finding a predator that might depend heavily on this particular mosquito was the little East African jumping spider Evarcha culicivora. It catches An. gambiae for about a third of its diet and likes the females fattened with a human blood meal. Yet even this connoisseur “will readily consume” an alternative mosquito species, the researchers noted in July in Medical and Veterinary Entomology.

Collins also thinks about the alternatives to using genetically engineered pests as pest controls. Her personal hunch is that saddling mosquitoes with gene drives to take down their own species is “likely to have fewer ecological risks than broad-spectrum use of pesticides that also kill other species and the beneficial insects.”

Gene drives aren’t the only choice for weaponizing live mosquitoes against their own kind. To pick just one example, a test this year using drones to spread radiation-sterilized male mosquitoes in Brazil improved the chances that the old radiation approach will be turned against an Aedes mosquito that can spread Zika, yellow fever and chikungunya.

Old ideas, oddly enough, may turn out to be an advantage for antimosquito technologies in this era of white-hot genetic innovation. Coaxing the various kinds of gene drives to work is hard enough, but getting citizens to sign off on their use may be even harder.

Mysterious particles called neutrinos constantly barrel down on Earth from space. No one has known where, exactly, the highest-energy neutrinos come from. This year, scientists finally put a finger on one likely source: a brilliant cosmic beacon called a blazar. The discovery could kick-start a new field of astronomy that combines information gleaned from neutrinos and light.

It began with one high-energy neutrino spotted on September 22, 2017, by the IceCube observatory, a giant particle detector with thousands of sensors buried deep in the ice at the South Pole. Alerted by IceCube, astronomers soon spotted a flare from a blazar about 4 billion light-years away. The neutrino had come from the same area of the sky. With that matchup in time and space between the neutrino and the blazar’s light, scientists in 2018 pegged the blazar as the particle’s probable source (SN: 8/4/18, p. 6).

“People have been hoping for this kind of discovery for decades,” says astrophysicist Meg Urry of Yale University.
Blazars are active regions at the centers of galaxies that spew jets of high-energy matter and light toward Earth. Both the Earth-orbiting Fermi Gamma-ray Space Telescope and the Major Atmospheric Gamma Imaging Cherenkov, or MAGIC, telescopes in the Canary Islands reported that the blazar was violently flaring up in gamma rays, a type of high-energy light, at about the same time the neutrino was detected.

After combing through old data, IceCube researchers found evidence of even more neutrinos from near the blazar’s location in the sky. With those extra neutrinos, the researchers were finally convinced that the blazar birthed neutrinos.
Not only did the detection hint at the source of at least some high-energy spacefaring particles, it also taught physicists a few things about blazars. Scientists weren’t sure what kinds of particles blazars emit, but the detection reveals that the jets contain protons. That’s because scientists know that any neutrino from a blazar would have to be produced in combination with protons.

The discovery, scientists say, could invigorate a nascent field, dubbed multimessenger neutrino astronomy, to reveal secrets of the cosmos, whether from blazars or other sources. Now, says astrophysicist Kohta Murase of Penn State, “we can use neutrinos as very important probes” to learn more about the objects that spit them out. For example, researchers might spot neutrinos from a collision of two neutron stars, like the one detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2017 (SN: 11/11/17, p. 6). IceCube didn’t see any neutrinos from that event, but astrophysicists are hopeful that future neutron star smashups will produce a neutrino bounty.

Before scientists are fully confident that blazars can blast out high-energy neutrinos, researchers need to spot more of the wily particles, Murase says. To improve detection, an upgrade to IceCube will make the detector 10 times bigger in volume and should be ready by the mid-2020s, says Francis Halzen, leader of IceCube and an astrophysicist at the University of Wisconsin–Madison. If all goes well, the tiny particles may soon be revealing secrets from new corners of the cosmos.

Cleaning up ocean pollution is no simple task, as an effort to fish plastic out of the Pacific Ocean is revealing.

In September, scientists launched a 600-meter-long boom meant to herd plastic debris from the great Pacific garbage patch into a net (SN Online: 9/7/18). The trash accumulation, which is twice the size of Texas, swirls in waters between California and Hawaii.

But some scientists worry the system, designed by a Dutch organization called Ocean Cleanup, could harm marine wildlife. Others aren’t convinced it will even work. Four months in, some of those concerns appear to be founded: Wind and currents have pushed trash into the rig, but the setup hasn’t kept the trash corralled as planned. Now part of the rig has broken off, and the device is being towed back to shore for repairs and design tweaks.
Whether the system will eventually help remove garbage from the Pacific remains to be seen. But it’s not the only option for reducing how much plastic is dumped in the oceans — now at some 5 trillion pieces, per some estimates. Here are a few other approaches seeing success.

1. Meet Mr. Trash Wheel and friends
   It’s easier to collect trash from rivers and streams than from the open ocean. Baltimore has deployed three giant waterwheels that trap river plastic before it flows into the harbor.
   The first installation, adorned with googly eyes and dubbed Mr. Trash Wheel, debuted in 2014, followed by Professor Trash Wheel in 2016 and Captain Trash Wheel in 2018. Collectively, the wheels so far have removed more than 680,000 kilograms of trash.

Mr. Trash Wheel wasn’t immediately successful, though, says Adam Lindquist, director of Baltimore’s Healthy Harbor Initiative. A waterwheel installed in 2008 didn’t work well. He suggests that the Ocean Cleanup group, in tweaking its device, could also see improvements — though clearing plastic from the ocean is certainly a bigger job than cleaning a river, he says.

2. Snag it on land
   Collecting debris as it’s washed onto beaches is another way to tackle the plastic problem. “Lots of published papers show the ocean spits out trash really quickly,” says Marcus Eriksen, an environmental scientist and cofounder of the 5 Gyres Institute based in Los Angeles.

The U.S. National Oceanic and Atmospheric Administration’s Marine Debris Program, for example, has collected more than 450 tons of garbage from the Alaska shoreline since 2006.

3. Rise of the bans
   Using less plastic in the first place is the most straightforward way to cut down on ocean pollution. Many cities and 127 countries have imposed regulations on single-use plastic, such as grocery bags or plastic straws, according to a December report from the U.N. Environment Program.

These restrictions can make a difference. In 2010, thousands of volunteers collecting and tallying garbage along the California coast found that 7 percent of the trash consisted of plastic bags. In 2017, after multiple California cities imposed plastic bag bans or restrictions, the bags made up less than 2 percent of trash gathered.

Bans can also fight debris that the Ocean Cleanup’s system won’t snag — microplastics, tiny fragments that can harm the health of people and other animals. The United States banned microbeads in personal care products starting in 2017; the European Union has voted to enact a similar ban by 2020.

4. Rethink the cycle
   Plastic can take decades or even centuries to break down, so some scientists are working on alternatives that are easier to recycle. For example, a type of recyclable plastic described in Science in 2018 can be broken down into component pieces and rebuilt again and again (SN: 5/26/18, p. 12). But if recyclable plastic ends up in a landfill or in the ocean, those special properties won’t matter.

“There’s a really big gap between what can be recycled in a perfect world, and what actually gets recycled,” says Miriam Goldstein, the director of ocean policy at the Center for American Progress, a research and lobbying organization in Washington, D.C. Bridging that gap isn’t as simple as telling people to use less plastic, she says. “You cannot opt out of single-use plastics in most of the country,” and lots of products are designed in ways that make them challenging to properly recycle.

That’s one reason why an approach called extended producer responsibility is gaining popularity — essentially, holding companies that make plastic products responsible for the cost of proper disposal. “If you’re going to make anything, you need to think of the recovery,” Eriksen says. Putting a financial burden on the producer provides an incentive for designing products that are easier to recycle or reuse. One example: using just one type of plastic instead of combining plastics that would then need to be separated for proper recycling.

The fishing industry also needs incentives to join in the cleanup, Eriksen says. Surveys of detritus that washes ashore suggest that a substantial portion is left by fishers. Giving them a financial reason to retrieve old nets and buoys could help keep our oceans clean.

The monarch butterfly isn’t the only insect flying up and down North America in a mind-boggling annual migration. Tests show a big, shimmering dragonfly takes at least three generations to make one year’s migratory loop.

Ecologist Michael Hallworth and his colleagues pieced together the migration of the common green darner, described December 19 in Biology Letters, using data on forms of hydrogen in the insects’ wings, plus records of first arrivals spotted by citizen scientists.
The study reveals that a first generation of insects emerges in the southern United States, Mexico and the Caribbean from about February to May and migrates north. Some of those Anax junius reach New England and the upper Midwest as early as March, says Hallworth, of the Smithsonian Migratory Bird Center headquartered in Washington, D.C.

Those spring migrant darners lay eggs in ponds and other quiet waters in the north and eventually die in the region. This new generation migrates south from about July until late October, though they have never seen where they’re heading. Some of these darners fly south in the same year their parents arrived and some the next year, after overwintering as nymphs.

A third generation emerges around November and lives entirely in the south during winter. It’s their offspring that start the cycle again by swarming northward as temperatures warm in the spring. With a wingspan as wide as a hand, they devote their whole lives to flying hundreds of kilometers to repeat a journey their great-grandparents made.
Scientists knew that these dragonflies migrated. Dragonfly enthusiasts have spotted swarms of the green darners in spring and in fall. But which generations were doing what has been tricky to demonstrate. “Going in, we didn’t know what to expect,” Hallworth says.
Tracking devices that let researchers record animals’ movements for more than a week or two haven’t been miniaturized enough to help. The smallest still weigh about 0.3 grams, which would just about double a darner’s weight, Hallworth says. So researchers turned to chemical clues in darner tissues. Conservation biologist and study coauthor Kent McFarland succeeded at the delicate diplomacy of persuading museums to break off a pinhead-sized wing tip fragment from specimens spanning 140 years.

Researchers checked 800 museum and live-caught specimens for the proportion of a rare heavy form of hydrogen that occurs naturally. Dragonfly wings pick up their particular mix of hydrogen forms from the water where the aquatic youngsters grow up. Scientists have noticed that a form called hydrogen-2 grows rarer along a gradient from south to north in North America. Looking at a particular wing in the analysis, “I can’t give you a zip code” for a darner, Hallworth says. But he can tell the native southerners from Yankees.

An adult darner, regardless of where it was born, is “a green piece of lightning,” says McFarland, of the Vermont Center for Ecostudies in White River Junction. Darners maneuver fast enough to snap insect prey out of the air around ponds across North America. The front of an adult’s large head is “all eye,” he says, and trying to catch samples for the study was “like hitting a knuckleball.”

Although the darners’ north-south migration story is similar to that of monarchs (Danaus plexippus), there are differences, says evolutionary biologist Hugh Dingle of the University of California, Davis, who has long studied these butterflies. Monarchs move northward in the spring in stepwise generations, instead of one generation sweeping all the way to the top of its range.

Also, Dingle says, pockets of monarchs can buck the overall scheme. Research suggests that some of the monarchs in the upper Midwest do a whole round trip migration in a single generation. As researchers discover more details about green darners, he predicts, the current basic migration scheme will turn out to have its quirky exceptions, too.

The adolescent saber-toothed cat on a summertime hunt realized too late that she had made a terrible miscalculation.

Already the size of a modern-day tiger, with huge canine teeth, she had crept across grassy terrain to ambush a giant ground sloth bellowing in distress. Ready to pounce, the cat’s front paw sank into sticky ground. Pressing down with her other three paws to free herself, then struggling in what has been called “tar pit aerobics,” she became irrevocably mired alongside her prey.

Scenarios much like this played out repeatedly over at least the last 35,000 years at California’s Rancho La Brea tar pits. Entrapped herbivores, such as the sloth, attracted scavengers and predators — including dire wolves, vultures and saber-toothed Smilodon cats — to what looked like an easy meal. Eventually the animals would disappear into the muck, until paleontologists plucked their fossils from the ground in huge numbers over the last century.

Five million or so fossils have been found at the site. But “it’s not like there was this orgy of death going on,” says Christopher Shaw, a paleontologist and former collections manager at the La Brea Tar Pits and Museum in Los Angeles. He calculates that such an entrapment scenario, dooming 10 or so large mammals and birds, would have needed to occur only once per decade over 35,000 years to account for that bounty of fossils.

At La Brea, the collection of Smilodon fatalis fossils alone includes more than 166,000 bones, from an estimated 3,000 of the ill-fated prehistoric cats. Famed for their fearsome canines, which grew up to 18 centimeters long, S. fatalis weighed as much as 280 kilograms, bigger than most of today’s largest lions and tigers.
Fossils of S. fatalis, the second largest of three Smilodon species that roamed the Americas during the Pleistocene Epoch, have been found across the United States and in South America, west of the Andes as far south as Chile. And a recent study put S. fatalis in Alberta, Canada, about 1,000 kilometers north of its previously known range.

But the La Brea fossil site, unique in offering up so many specimens, is the source of the vast majority of knowledge about the species. There, fossils of dire wolves and saber-toothed cats together outnumber herbivores about 9-to-1, leading scientists to speculate that both predators may have formed prides or packs, similar to modern lions and wolves. Yet a small number of experts argue against cooperative behavior for Smilodon, reasoning that pack-living animals would have been too intelligent to get mired en masse.
New studies may help settle the debate about Smilodon’s sociality, and answer questions about how the cat lived and why it died out 10,000 to 12,000 years ago.

“We have an innate curiosity to understand what it was doing and why it went extinct,” says Larisa DeSantis, a vertebrate paleontologist at Vanderbilt University in Nashville. Now, she says, “we can answer these questions.”

DeSantis is studying microscopic wear on fossil teeth and chemical signatures in the enamel to reveal Smilodon’s diet. Other scientists are doing biomechanical studies of the skull, fangs and limbs to understand how the powerful cat captured and killed its prey. Some researchers are extracting DNA from fossils, while others are gathering data on the paleoclimate to try to piece together why Smilodon died out.

“It’s the T. rex of mammals … a big, scary predator,” says Ashley Reynolds, a paleontology Ph.D. student and fossil cat researcher at the University of Toronto. She presented the Alberta fossil find in October in Albuquerque at the Society of Vertebrate Paleontology conference. Explaining why Smilodon cats continue to excite researchers, she says, “They’re probably the baddest of all the cats that have ever existed.”
Safety in numbers
Whether Smilodon was a pack hunter has long been debated (SN: 10/28/17, p. 5) because living in groups is rare among large cats today. But an unusual number of healed injuries in the Smilodon bones at La Brea makes it unlikely that these cats were solitary, DeSantis and Shaw reported in November in Indianapolis at a meeting of the Geological Society of America.

More than 5,000 of the Smilodon bones at La Brea have marks of injury or illness: tooth decay, heavily worn arthritic joints, broken legs and dislocated elbows that would have occurred before the animals’ tar burial. Dramatic examples include crushed chests and spinal injuries, which the cats somehow survived. “You would actually wince to see these horribly, traumatically injured specimens,” says Shaw, who is also coeditor of the 2018 book Smilodon: The Iconic Sabertooth.

One particularly debilitating injury was a crippled pelvis, but evidence of new bone growth shows that the animal lived long enough for healing to occur. “There was a lot of infection, pain and smelly stuff, and just a really awful situation for this animal, but it survived well over a year,” Shaw says. “To me that indicates [the injured cat] was part of a group that helped it survive by letting it feed at kills and protecting it.”

Shaw and DeSantis looked at a series of specimens with what were probably agonizing maladies in the teeth and jaws, including fractured canines and massive infections that left animals with misshapen skulls.

“These animals probably couldn’t have gone out … to kill anything,” Shaw says. “You know how it is when you have a toothache. This is like that times 100.”
DeSantis compared microscopic pits and scratches on the surface of the teeth of injured animals with microwear on the teeth of seemingly healthy Smilodon cats. The injured cats’ dental surfaces indicated that the animals were eating softer foods, which would have been less painful to chew, “likely a higher proportion of flesh, fat and organs, as opposed to bone,” she says.

The findings are consistent with the interpretation that Smilodon was a group-living animal, she says, and that the cats “allowed each other access to food when [injured pack members] couldn’t necessarily take down their own prey.”

Reynolds agrees that the healed injuries are persuasive evidence that Smilodon lived in groups. “When you see an animal with really nasty injuries that healed somehow, it does make you wonder if they were cared for.”

Not everyone is convinced, however. Ecologist Christian Kiffner of the Center for Wildlife Management Studies in Karatu, Tanzania, has studied modern carnivores such as African lions and spotted hyenas. “Relatively long survival of Smilodon fatalis individuals after dental injuries had occurred does not necessarily provide airtight evidence for a specific social system in this species,” he says. “It is very, very difficult to use patterns in Pleistocene carnivore [fossil] assemblages to make inferences about behavior of an extinct species.”

Even if the saber-toothed cats did live in groups, the animals’ exact social structure remains an open question, Reynolds says. Modern lion prides have numerous females and several younger males led by an alpha male, with intense competition between male lions. As a result, males are much bigger than females, as the males must work hard to defend their positions.

Despite searching, scientists have not found obvious evidence of a size difference between the sexes in Smilodon; researchers can’t even tell which La Brea fossils are male or female. Size differences between the sexes, if they existed, may have been small.

“That lack of sexual dimorphism is odd,” says Blaire Van Valkenburgh, a UCLA paleontologist who studies fossil carnivores. Sex-related size differences are seen in many big cats today, most particularly lions. She thinks the lack of sexual dimorphism in Smilodon might hint at a different social structure. Perhaps males weren’t competing quite so intensely for access to females. Maybe there was no single alpha male preventing the majority of males from making a move.

Family affair
Perhaps Smilodon groups had an alpha female rather than an alpha male, or an alpha pair. Such is the case in modern wolves and coyotes, which have less pronounced size differences between sexes than lions do. The prehistoric cats “could have had extended family structures [similar to wolves] where uncles and aunts hung around, because it probably took a while to raise the young saber-toothed cats,” Van Valkenburgh suspects.

Kittens may have taken a long time, as long as 22 months, to get most of their adult teeth, she says. The upper canines took even longer, as much as three years or more, to reach their massive size, researchers reported in PLOS ONE in 2015. Modern lions, in contrast, typically have all of their adult teeth by 17 months, Van Valkenburgh says.

Smilodon kittens also probably went through a substantial learning curve before attempting to take down large prey. “It took longer for them to learn how to safely kill something without breaking their teeth or biting in the wrong place and hurting themselves,” Van Valkenburgh speculates.

Pack living would enable this slower development: “If you’re a social species, you can afford to grow at a slower rate than a nonsocial species because you have a family safety net,” Reynolds says. She is studying Smilodon fossils from Peru’s Talara tar pits for evidence of slow bone development using bone histology, examining thin cross sections under a microscope to determine such things as age and growth rate.
To understand how saber-toothed cats eventually took down prey, Van Valkenburgh joined paleobiologist Borja Figueirido of the University of Málaga in Spain and others. The group studied the biomechanics of Smilodon’s killing bite and how the animal used its sabers. That work, published in the October 22, 2018 Current Biology, adds to a consensus that the cat used its powerful forelimbs, which existed even in the youngsters (SN Online: 9/27/17), to pin prey before applying a lethal bite to the neck.

“The specialization of being a saber-toothed appears to have been partly to effectively take prey larger than yourself and to do that very quickly,” Van Valkenburgh says. With the prey tightly gripped, a Smilodon cat would position itself so that one or two really strong canine bites would rip open the pinned animal’s throat.

In contrast, lions suffocate prey — one lion may clamp its jaws around the neck, crushing the windpipe, while another uses its mouth to cover the victim’s nose and mouth. Using this slower method would have increased Smilodon’s chances of injuring or damaging those precious canine teeth.

Diverging senses
Smilodon and its extinct saber-toothed relatives are on a branch of the cat family tree that is far from today’s cats. Scientists think Smilodon’s branch diverged from the ancestors of all living cats about 20 million years ago. Given the evolutionary distance, researchers are still trying to determine how similar — or different — Smilodon was from its living feline cousins. A recent focus has been the cat’s sounds and senses.

At the October vertebrate paleontology conference, Shaw presented evidence that Smilodon may have roared, as do lions, tigers, leopards and their close relatives. The clues come from 150 La Brea fossils that were once part of the hyoid arch, or larynx, in the Smilodon throat. (Tar pits stand out for preserving tiny bones rarely found elsewhere.) The small fossils are very similar in shape and style to those of roaring cats. House cats and others that purr have a different arrangement of bones.
Smilodon may have “used this type of communication as an integral part of social behavior,” Shaw says. Roaring, however, is not a sure sign of pack living, Reynolds notes; most roaring cats today do not live in large groups.

How Smilodon’s sense of smell compared with living cats’ is something else researchers wonder about. To probe this part of the extinct animal’s biology, a team lead by Van Valkenburgh looked at Smilodon’s cribriform plate — a small, perforated bone inside the skull. Smell-sensing nerve cells pass through holes in the plate from the olfactory receptors in the nose to the brain. The size and number of holes are thought to correlate with the number of receptors and, therefore, the extent of an animal’s sense of smell.

To confirm this link, Van Valkenburgh’s team combined CT scans and 3-D images of skulls from 27 species of living mammals with information on the number of olfactory receptor genes. A CT scan of a skull revealed that Smilodon may have had slightly fewer olfactory receptor nerve cells than a domestic cat, the researchers reported at the paleontology conference. Smilodon’s sense of smell was perhaps 10 to 20 percent less keen than a modern lion’s, says Van Valkenburgh, whose team reported the findings in the March 14, 2018 Proceedings of the Royal Society B.

Smilodon “might have relied more heavily on their eyes and their ears,” she says. Perhaps, in an ancient evolutionary divergence, Smilodon’s level of reliance on smell went in a slightly different direction than in modern big cats.

Saber-toothed swan song
As the pieces of the Smilodon puzzle fall into place, perhaps the biggest remaining mystery is why the animal disappeared 10,000 to 12,000 years ago. Debate about the extinction of some of North America’s large mammal species swings between blaming humans and climate change (SN: 11/10/18, p. 28). While humans, who probably arrived on the continent more than 15,000 years ago, and Smilodon certainly knew one another in the Americas, they may not have overlapped at La Brea, Shaw says. The earliest evidence of people in the Los Angeles Basin is about 11,000 years ago, by which time Smilodon may or may not already have gone. Nevertheless, human hunting of large prey elsewhere in the Americas could have led to a scarcity of food for the big cats, he says.

One theory holds that Smilodon went through tough times at La Brea when lack of prey forced the saber-toothed cats to consume entire carcasses including bones. This has been posited as the reason for all those broken teeth among the La Brea fossils. But DeSantis isn’t convinced; she thinks breakages happened during scuffles with prey. She says dental microwear suggests that Smilodon was not eating great quantities of bone.
Some opportunistic carnivores, such as cougars, did eat bone and managed to survive to the modern day. Perhaps Smilodon couldn’t adapt to hunting smaller prey when larger herbivores disappeared, also around 10,000 to 12,000 years ago (SN: 11/24/18, p. 22).

“A lot of the large prey on the landscape go extinct,” DeSantis says. “You lose out on the horses, camels, giant ground sloths, mammoths and mastodon. That’s got to have had an impact.”

The challenge of dating fossils from the tar pits has been one hurdle to understanding exactly what was going on with Smilodon over time. Bones deposited over many thousands of years get jumbled by movement in the tar, for reasons experts don’t fully understand. Plus, the tar itself becomes embedded in each specimen, complicating carbon dating.

However, new methods of chemically pretreating fossils to remove the tar have made carbon dating much easier and cheaper — and a multi-institutional project is now dating hundreds of Smilodon and other bones. Researchers will soon be able to track changes in Smilodon over the 35,000 years of prehistory recorded at La Brea and correlate fossil changes to known changes in climate over that time.

“We’re going to have a much better handle,” Van Valkenburgh says, “on what was going on towards the end of their existence.”

This article appears in the March 30, 2019 issue of Science News with the headline, “The Baddest Cat of All: Fresh details say saber-toothed Smilodon helped injured pack members.”

A tweaked laboratory protocol has revealed signs of thousands of newborn nerve cells in the brains of adults, including an octogenarian.

These immature neurons, described online March 25 in Nature Medicine, mark the latest data points in the decades-old debate over whether people’s brains churn out new nerve cells into adulthood. The process, called neurogenesis, happens in the brains of some animals, but scientists have been divided over whether adult human brains are capable of such renewal (SN Online: 12/20/18).
Researchers viewed slices of postmortem brains of 13 formerly healthy people aged 43 to 87 under a microscope, and saw thousands of what appeared to be newborn nerve cells. These cells were in a part of the hippocampus called the dentate gyrus, a suspected hot spot for new neurons. Brain samples from 45 people with Alzheimer’s disease, however, had fewer of these cells — a finding that suggests that neurogenesis might also be related to the neurodegenerative disease.

Most of the brain samples used in the study were processed within 10 hours of a donor’s death, and spent no more than 24 hours soaking in a chemical that preserves the tissue. Those factors may help explain why the new neurons were spotted, the researchers write. Some earlier experiments that didn’t find evidence of neurogenesis used samples that were processed later after a donor’s death, and that had sat for longer in the fixing chemical.