Water fleas (Daphnia cucullata) change shape depending on their environment. Usually, they develop into their common form (bottom row). However, if they sense chemicals given off by predators, they develop into a distinctive, protective form (top row). A water flea that has taken this form has a long, pointed helmet on its head and a spike on its tail, making it difficult for invertebrates to catch and eat it. As it turns out, this ability to change features in response to the environment—a phenomenon known as phenotypic plasticity—occurs in all major groups of organisms, from bacteria to mammals. Its widespread existence raises such vexing questions as: How does such developmental flexibility evolve? Can environmentally induced features be passed on to the individual’s offspring? And does plasticity affect evolution? In “Evolution and the Flexible Organism,” evolutionary biologist David Pfennig addresses these and other questions about this fascinating but often misunderstood phenomenon. (Image by Christian Laforsch/Science Source.)

Singing together helps people build community and cope with anxiety and loss. Yet singing together has been notoriously risky during the COVID-19 pandemic. So, singers devised ways to carry on making music virtually, finding workarounds to deal with the audiovisual latency inherent in online communications. In “Virtual Community Singing During the COVID-19 Pandemic,” music historian Esther M. Morgan-Ellis summarizes her investigation of the experiences of people gathering online to sing together during the pandemic. Low-latency communications platforms for musicians have existed for about 15 years but are not yet broadly accessible. Great technological strides have been made during the pandemic, with programmers improving the software people use for virtual music-making and musicians becoming more aware of available technologies. Eventually, most people who want to sing together virtually will have access to technology that will facilitate real-time, multidirectional audiovisual communication. (Illustration by Chanelle Nibbelink.)

Each of the birds pictured—the scarlet macaw, the New Caledonian crow, and the extinct moa—has a large brain. But to truly understand the evolution of avian intelligence, one must consider the relationship between brain size and body size, as explained in “Bird Brain Evolution,” by biologist Daniel T. Ksepka (pages 352–359). When estimating “intelligence” from brain size—the only way to make such an estimate from a fossil—biologists look at how much larger or smaller the braincase is than the expected size for a species of that body mass. Scaled in this way, relative brain size is a proxy for intelligence. Thus, although the moa had a large brain in absolute terms, the bird was also tremendous in size. As the moa evolved, its brain size lagged behind its body size, until it ended up with the smallest relative brain size of any bird ever. By contrast, the large relative brain sizes of parrots and crows stand out from all other birds, but they got there through different pathways. Parrots decreased their body size rapidly while retaining relatively large brains. Corvids, who have the highest rate of brain-body evolution, evolved larger bodies, but their brains enlarged even faster than their bodies, much as the brains of hominids did on the mammalian family tree. (Illustration by Emma Skurnick, https://emmaskurnick.com.)

Bora Bora in French Polynesia has the reputation of being a pristine paradise, but tourism has altered the island’s ecosystem. A coral atoll encircles the island group, creating a gentle lagoon in the middle of the Pacific Ocean. Luxury hotels have built villas directly over the water, and one resort even created an artificial island to accommodate more guests. Snorkeling tourists might not recognize the signs of an unhealthy ecosystem, but remote sensing technologies such as satellites provide ecologists with an overview of the changes. This view from above can help researchers identify areas in distress so that they can develop intervention strategies before it is too late. In “The Shift to a Bird’s-Eye View” (pages 288–295), ecologists Elizabeth M. P. Madin and Catherine M. Foley detail how remote sensing technologies have changed their approaches to researching isolated coral reefs and inaccessible penguin populations. The techniques they describe are not limited to ecological projects; remote sensing tools have also helped document humanitarian crises, and they are fundamental to the transition that many researchers have made from the field to their homes in order to maintain social distancing during the COVID-19 pandemic. (Cover image from GeoEye/Science Source.)

Scientific advances have led to vast improvements in daily life—from computers to vaccines to clean energy. But science itself is a work in progress and still has a lot of potential to be more equitable, ethical, and responsive to the needs of different communities. (Cover illustration by Clare Nicolas.)

Enormous collections of invisible dark matter are thought to have seeded the formation of galaxies in the early universe. This computer simulation, created by a team led by Hsi-Yu Schive of National Taiwan University in Taipei, maps dark matter based on its density to make its structure obvious. Despite substantial evidence for dark matter, nobody has yet detected it directly. In “Enter the Axion” (pages 158–165), physicist Chanda Prescod-Weinstein explores the increasingly popular idea that dark matter consists of particles called axions. Over large scales, axions could act like waves rather than collections of particles, producing the rippled forms seen on the cover. Prescod-Weinstein is studying the cosmological implications of this phenomenon and assisting efforts to determine whether axions really exist. (Cover image courtesy of H.-Y. Schive et al., with permission from Nature Physics.  https://doi.org/10.1038/nphys2996)

A respiratory droplet harbors SARS-CoV-2, the virus that causes COVID-19, in this painting by biologist and artist David S. Goodsell. Respiratory droplets are expelled when we breathe, which is why face masks that catch those droplets can slow the spread of the pandemic. The surface of SARS-CoV-2 is covered with spike proteins, giving it a distinctive, crownlike silhouette (magenta). The droplet consists primarily of water, but it also includes molecules that are normally found in the respiratory tract, including mucus molecules (green), pulmonary surfactant molecules from the surfaces of respiratory cells (blue), and protective molecules from the immune system (tan). Goodsell describes his process of researching and creating molecular art in “Painting a Portrait of SARS-CoV-2”

Maat Mons, an enormous shield volcano on Venus, is a prime example of the planet’s familiar yet utterly alien landscape, as Paul K. Byrne describes in “Unveiling Earth’s Wayward Twin”. The volcano’s overall structure is reminiscent of the ones that make up the Hawaiian islands, but the planet’s high surface temperature—above 450 degrees Celsius—and lack of water resulted in extensive, distinctive lava flows. The image itself is also unusual. Because of the perpetual cloud cover on Venus, planetary scientists have to use radar to map its surface. This radar snapshot, acquired by NASA’s Magellan orbiter in 1989, is colorized to resemble surface views from the Soviet Venera landers and is vertically exaggerated to highlight the landforms. In reality, Maat Mons stands 5 kilometers above the surrounding plains; circumstantial evidence suggests it might still be erupting. Going back for better data on Venus, says Byrne, might help us understand how it got this way and why Earth didn’t, and it could help us better characterize exoplanets. (Image by NASA/JPL)

Paranthropus boisei, a little-known member of the hominin family, lived in East Africa from about 2.3 to 1.3 million years ago. Neither an ape nor a direct ancestor of modern humans, P. boisei shared the landscape with several of the earliest species of the genus Homo. The relatively few fossils that have been found so far indicate an unusual combination of physical features: upright posture together with adaptations for climbing, a braincase approximately one-third the size of our own but molar and premolar teeth several times larger than ours, and tiny canine and incisor teeth. This dental pattern suggests a diet consisting mainly of tough grasses and seeds, a notion that is corroborated by stable-isotope analysis of the thick enamel coating on P. boisei teeth. (Illustration by Andrew Recher, pensandbeetles.com.)

The white-sand beaches of K’gari, an island just off Australia’s eastern coast near Brisbane, provide a surreal backdrop for a mysterious creature. Dingoes were thought to be dogs by the first European explorers who encountered them, but the canids dwell somewhere between domesticated and wild—some of them may hang around humans and their settlements, but they exhibit no interest in pleasing humans the way dogs do. (Dingoes on K’gari that are aggressive toward humans, usually while scavenging food, are given an ear tag, as shown in the cover photograph.) Indeed, it has long been debated whether dingoes were originally domesticated canids that went feral once they reached Australia, or whether they were some altogether different, wild species that arrived on the island continent. Evidence from burial sites shows that dingoes have been in Australia for about 4,000 years, and genetics now indicate that their lineage is independent from that of other canid species. In “The Elusive Dingo” (pages 292–297), anthropologist Pat Shipman describes the latest research on what dingoes are, and the scientific enigmas about them that remain. (Photograph by James Orr, Trinity College Dublin; www.jamesorrphoto.com)

Art and science have long had a false divide, but they are intrinsically joined by their strident search for creativity, innovation, and new connections. The two fields are working more closely together than ever to spark insights. (Cover illustration by Michael Morgenstern). See the July–August 2020 issue. 

Malaria is a devastating disease transmitted from person to person by mosquitoes. It kills more than 400,000 people per year, more than half of those deaths being children 5 years old or younger. CRISPR (clustered regularly interspaced short palindromic repeats) is a new gene-editing technology that acts like a pair of molecular scissors: It is used to “cut and paste” DNA sequences to alter gene function. In “Gene Editing and the War Against Malaria”, Ethan Bier and Elliott Sober describe how biologists can now alter genes in a malaria-transmitting mosquito population by engineering a CRISPR gene drive, which mimics a natural evolutionary process. With this tool, a new gene can be inserted into the genome so that the alteration is rapidly passed down to successive sexually reproducing generations. Two gene-drive strategies have been found feasible: The first drives a local malaria-transmitting population to extinction; the second renders mosquitoes unable to transmit malaria, which breaks the malaria transmission cycle. (Illustration by Barbara Aulicino.)

NASA’s Juno spacecraft arrived at Jupiter in 2016, and since then it has been stunning scientists and the public with views of Jupiter unlike any seen before. Juno’s visible-light images are processed by citizen scientists and often are color-stretched to emphasize detail and contrast. These images have led to discoveries, such as the state of Jupiter’s shrinking Great Red Spot. But Juno’s other instruments have measured properties beyond the view of visible light and have produced dramatic data about the interior of the gas giant planet, as well as revelations about its gravity, radiation, and magnetic field. In “Inside Jupiter” (pages 98–105), Scott Bolton details how all of these discoveries not only tell us about our enormous neighbor but also expand our understanding of the formation of our own planet and the rest of our Solar System, and are essential to better characterizing exoplanets in other systems. (Cover image courtesy of NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.)

Remote sensing satellites make it possible to study the Earth in new ways, such as by documenting changes to the planet over time, including changes in climate. These space-based pictures have led to scientific discoveries, but the U.S. Geological Survey, which runs the Landsat program, also produces images for educational and reference purposes. Here, scientists enhanced a portion of a Landsat 8 image to show the patterns of tropical vegetation, sediment, and nutrients at an estuary in northwestern Australia. This kind of top-down dissemination of knowledge can be traced to remote sensing’s military origins in technologies designed to provide decision-makers with information on enemy whereabouts. In “How Climate Science Could Lead to Action” (pages 34–41), Samantha Jo Fried describes this history and explains how rethinking the relationship to remote-sensing satellites could prompt more public engagement in addressing climate change. (Cover image produced by Geoscience Australia, a Landsat Science Team Member; courtesy of U.S. Geological Survey.)

In 1869, 150 years ago, chemist Dmitri Ivanovich Mendeleev developed his first version of the periodic table of the elements. Mendeleev wasn’t the only scientist trying to organize the elements, and many scientists have contributed to the periodic table since (in the cover background are Lothar Meyer, Ida Noddack, and John Newlands). But as Abhik Ghosh and Paul Kiparsky note in Perspective, the 150th anniversary is worth celebrating because of Mendeleev's appreciation of the elements’ periodic properties; he left gaps in his table for elements not discovered yet. Mendeleev used Sanskrit names of numbers to mark these gaps. Why not Latin, or Greek, or German? In “The Grammar of the Elements,” Ghosh and Kiparsky make a compelling case that Mendeleev was exposed to Sanskrit and the grammar of that ancient language could have influenced his organizational thinking about the periodic table. (Artwork by Wayne Brezinka.)

Water transcends boundaries, cycling through land, atmosphere, rivers, and living things. This everyday substance is fundamental to biological, chemical, meteorological, and geological processes; one cannot study science without learning about its extraordinary properties. These interwoven attributes of water make the challenges it presents complicated and convoluted. The research used to problem-solve water issues is necessarily interdisciplinary. (Cover design and illustration by Barbara Aulicino.)

In her widely touring installations of infinity mirror rooms, the artist Yayoi Kusama uses colored lights and multiple reflective surfaces to create both intricate repetitive patterns and optical illusions that immerse the visitor. A central tool in her artwork, the mirror, is a simple optical device that just reflects the light that hits it. However, what humans see in the mirror—of themselves, their fellow humans, or the world around them—is more complicated, and the final outcome is a product of neuroscience, body morphology, habitual expectations, and circumstances. As Michael C. Corballis explains in “What Mirrors Do” (pages 224–231), the perennial question we ask is why mirrors reverse left and right, but the answer is that they really don’t, we just see it that way because of our viewpoint. (Photograph of Yayoi Kusama installation “Gleaming Lights of the Soul” by Colin McPherson.)

The places most likely to harbor the highest numbers of threatened species are those with many species that have small geographical ranges, especially in locations where habitat destruction is high. Local extinction rates increase as forests decrease in area, so strategically connecting patches of habitat can maximize conservation efforts. In “Connecting Habitats to Prevent Species Extinctions” (pages 162–169), ecologists Stuart L. Pimm and Clinton N. Jenkins use satellite imagery and geographical information systems to home in on small fragments of forest that are inhabited by high numbers of species with small ranges. They then work with local conservation partners to create corridors between these fragments so that threatened species with restricted habitats can expand their ranges and move into uplands as necessary when the climate changes. One such corridor now connects habitat fragments of an endangered monkey in Brazil called the golden lion tamarin, creating a reserve of sufficient size to maintain viable populations of the monkey, in addition to many other inhabiting species in the long term; observers have seen golden lion tamarins moving through the new corridor. (Illustration by Michael Morgenstern. Photos by Pr bernardes/CC-BY-SA 4.0; Felipe Toledo; Stuart L. Pimm; Lars Falkdalen Lindahl/CC-BY-SA 3.0). Read this issue. 

Louisiana’s Highway 1 passes through miles of eroding wetlands in Jefferson Parish. Ridges called cherniers mark where the barrier island beach once was. This stretch of road traverses along the barrier island southwest of Barataria Bay, between Grand Isle and Port Fourchon. This island helps protect the bay and has benefited from the Caminada Headland restoration, which enhanced beaches and dunes near the highway, a critical hurricane evacuation route. In “Renewed Hope for Coastal Marshes in Louisiana.” Paige Byerly, Bethann Garramon Merkle, and Megan Hepner talk about the restoration of another Louisiana barrier island, Whiskey Island, as an example of the efforts to curb the state’s rapid loss of saltmarshes. These marsh ecosystems harbor a diversity of flora and fauna, and also provide protection from storm surges and erosion. Before the Deepwater Horizon oil spill in 2010, many of these plants and animals were not monitored. Today, new data are emerging to provide a fuller picture of marsh ecology. (Photograph taken by Ben Depp in 2014.) Read this issue.

For about 2,000 years, starting in about 1000 BCE, the Maya city of El Ceibal was a bustling urban center with an estimated peak population of up to 52,000 inhabitants. Today this important archaeological site is almost inaccessible beneath farmlands, modern settlements, and protected forest, but archaeologists can create a detailed map of the site, thanks to a laser-based technique known as airborne lidar, short for light detection and ranging. As explained in “Estimating Ancient Populations by Aerial Survey” (pages 30–37), the technique calls for aircraft with specialized laser equipment to fly slowly over the site, collecting topographic information from laser reflections from the ground, archaeological structures, and vegetation. Computer programs then create a contour map of the area that includes buildings, earthworks, irrigation canals, and roads, with decimeter accuracy. (Lidar data collected and image created by the National Center for Airborne Laser Mapping.) Read this issue. 

Astronomers employ computational techniques to manage the ever-increasing flood of data from state-of-the-art ground- and space-based observatories. (Cover design and illustration by Barbara Aulicino and Tom Dunne. Galaxy based on spiral galaxy Messier 101; image courtesy of NASA, ESA, CXC, SSC, and STScI.) Read this issue. 

Fossil evidence of reptile parenting? In 2014, an exquisite fossil of a small caiman-like reptile, Philydrosaurus, was found in today’s Liaoning, China, in rock from a lake bed dating to the Early Cretaceous period. Mere inches away from the fossilized adult were seven juveniles of the same species, all large enough to be at least a few weeks old. This exciting discovery implies that this group of aquatic reptiles, choristoderes, cared for their young. Because crocodilians and dinosaurs share a common ancestor with choristoderes and also care for their young, it may be that this behavior is the ancestral condition in all reptiles. In “Mystery of the Lost Reptiles” (pages 222–229), paleontologist Daniel T. Ksepka writes about this diverse and long-overlooked group of aquatic reptiles, and explores why they survived the extinction that wiped out the nonavian dinosaurs, but then declined mysteriously during the Cenozoic era. (Illustration by Emma Skurnick.)

The idea that addiction is a brain disease had become prominent among neuroscientists by the 1990s. However, despite decades of research, the theory lacks evidence to support it, contend Marc Grifell and Carl L. Hart in “Is Drug Addiction a Brain Disease?” Although drug use both influences and is influenced by the brain, substance abuse is not a brain disease that responds to pharmaceutical treatments in the ways that, for example, Huntington’s or Parkinson’s diseases are. This myopic approach, they assert, which is based on intuition and political necessity more than on clinical results, overlooks socioeconomic and societal factors in research, clinical practice, and policy, despite their obvious importance. This paradigm also contributes to a misguided focus on punitive law enforcement strategies and on treating an individual’s brain. (Cover illustration by Boris Séméniako/www.borissemeniako.fr.)

The skeleton is sometimes viewed as a passive scaffold for the body, but bone is a living, growing organ. Every few years, your body completely replaces all of your bones with new growth. But what happens when damage is so extensive that bone cannot fix itself? Traditional implants and prostheses have needed to be solid simply to bear the weight of the body. But natural bone has a highly complex and layered structure, making it both tough and light. In “3D Printing of Bone Implants and Replacements” (pages 112–119), Susmita Bose, Samuel Ford Robertson, and Amit Bandyopadhyay describe a type of 3D printing that can work with biocompatible powders, closely developed light-but-strong designs, and careful control of the chemical properties of their base materials, resulting in modern implants that can span areas of separated bone and induce it to implement lasting repairs. (Cover image by Alfred Pasieka/Science Photo Library.)

Elephant numbers across Africa have dropped by as much as 60 percent since 2007. Increasing global demand for ivory, especially in China and elsewhere in Asia, has exacerbated elephant poaching in Africa and induced international outcry. This situation has prompted changes in wildlife conservation policy, often without locals’ knowledge or input. Botswana has remained a stronghold against the rise in ivory poaching. Although numbers of elephants are decreasing overall, in Botswana they are increasing or stable. The livelihoods of the people in the paths of growing numbers of elephants in Botswana are shaped by the animals’ presence. In “Living in an Elephant Landscape” (pages 34–41), Jonathan Salerno, Lin Cassidy, Michael Drake, and Joel Hartter—a group of ecologists and geographers—talk about how wildlife conservation policy in the region currently affects locals, and how their voices could be better included in the policy-making process. (Cover photo LZT/Alamy.)

Technology has extended the times of day when people can be active and has allowed for connectivity at any time of day. A growing body of research shows that the biological clock is affected by light exposure, especially at certain times of day, which in turn can affect all sorts of biological processes necessary for health, including sleep, focus, and metabolism. More people seem to be experiencing social jet lag, when our body rhythms are out of sync with the day-night cycle. This lack of synchrony can have serious public health consequences; for example, it has been linked to sleep disorders, obesity, and mental disorders. In “Adapting Your Body Clock to a 24-Hour Society” (pages 348–355), circadian biologists Alexis Webb and Erik Herzog explain how the biological clock sets and keeps time, and what solutions are under study to help people sync up with healthy daily routines for sleeping and eating. (Cover illustration by Michael Morgenstern.)

Structures made from biomaterials—such as ceramics, plastics, and complex carbohydrates—can provide a variety of mechanical, chemical, and even electrical cues that respond to external stimuli and support tissue repair. Such scaffolds can use seeded stem cells or support the growth of the body’s own cells. A fibrous polymer scaffold (cover, dark blue) signals cells to grow and differentiate. Human mesenchymal stem cells (green) are growing along these fibers, dotted with their nuclei (bright blue circles). In “Structural
Support for Damaged Tissue Repair” (pages 298–305), Treena Livingston Arinzeh, Jennifer Moy, and Gloria Portocarrero Huang discuss how their development of such scaffolds is helping them find new ways to regenerate bone, cartilage, and nerve tissue and could someday help patients with musculoskeletal injuries move better.

The past decade has seen an explosion of discoveries of planets outside of our Solar System, dubbed extrasolar planets, or simply exoplanets. Many of the first exoplanets identified were large, about the size of Jupiter, and orbiting close to their parent stars. But such planets are not likely to harbor liquid water nor, therefore, life. More advanced techniques have allowed astrophysicists to locate exoplanets such as the one illustrated here, Kepler-138b, which was the first exoplanet discovered that has a mass and size smaller than Earth (planets are not shown at scale to star). Planets in what’s called the habitable zone around their stars could have liquid water on their surfaces. But as Howard A. Smith argues in “Questioning Copernican Mediocrity” (pages 232–239), a significant number of additional factors are needed to determine whether intelligent life might exist elsewhere in the universe. (Image courtesy of SETI Institute/Danielle Futselaar and NASA.)

Drawing inspiration from forms found in nature, artist Rogan Brown uses a scalpel or laser to cut intricate patterns into hundreds of microlayers of paper. (The cover features a detail from his paper sculpture Kernel.) As new technologies increasingly dominate the popular culture, transforming the way people tend to see today, Brown fears an erosion of the human ability to contemplate art, nature, or anything else. To counter this loss, he creates sculptures whose sheer abundance of detail makes them impossible to comprehend in one glance; viewers are obliged to move closer and study the work in order to take it in fully.

Artificial endosymbioses hold promise for transferring their benefits to novel hosts. In mosquitoes, for example, a bacteria of the genus Wolbachia, which can live in the ovaries or testes of a variety of insects, are under exploration for their potential to cause population declines or to limit virus transmission in diseases such as dengue, chikungunya, and Zika. In “The Prospects of Artificial Endosymbioses” (pages 36–43), authors Ryan Kerney, Zakiya Whatley, Sarah Rivera, and David Hewitt discuss the ways that endosymbioses might be engineered and used, as well as the challenges they pose. The authors also point out that, for better or worse, symbiotically modified organisms are often seen as more “natural” by the public and given less ethical scrutiny than genetically modified organisms that have similar uses and benefits. (Cover illustration by Michael Morgenstern.)

The human family gained a previously unknown relative last year when a great trove of fossil bones and fragments was discovered in a cave outside Johannesburg, South Africa. Researchers determined that the fossils represented a new species, which they named Homo naledi; however, because the species exhibits a puzzling mixture of new and old features, its exact position on the hominin family tree remains a mystery. One skeletal structure that may provide some clues to the mystery is the pelvis, featured in this artist’s rendering of a pregnant female. In “An Updated Prehistory of the Human Pelvis” (pp. 354–361), paleoanthropologist Caroline VanSickle discusses the challenges that H. naledi’s mixture of features is posing to long-held ideas about how adaptations for two-legged walking and childbirth shaped the human pelvis. (Cover illustration by Benoît Clarys.)

In an election season, prospective voters take their impressions of the candidates not only from what they say but also on the basis of other aspects of their presentation, such as intonation, gestures, and appearance. Recent studies have identified another trait that contributes subtly but significantly to voters’ impressions: the pitch of a candidate’s voice. In “How Voice Pitch Influences Our Choice of Leaders” (pages 282—287), political scientist Casey Klofstadt and biologists Stephen Nowicki and Rindy Anderson describe their interdisciplinary research, which attempts to weigh the effect of a high- or low-pitched voice on a candidate’s electoral success. (Cover illustration by Carlos Zamora, carloszamora.com.)

The 2012 Waldo Canyon Fire in Colorado Springs, Colorado, was the worst wildfire in the state’s history. Shown here burning out of ­control on June 26, three days after the fire began, it ultimately lasted 18 days, blazing through more than 18,000 acres of public and ­private land, destroying 347 homes, and killing two people. In “Coexisting with Wildfire”, fire ecologist Max A. Moritz and historian Scott Gabriel Knowles describe the state of research on safety and damage prevention in the face of wildfires, ­dispelling common misconceptions that they say may be holding back proactive policy and contributing to reactively fighting fires. 

The authors pose solutions that they think could save lives, homes, and taxpayer money. (Photograph by R. J. Sangosti.)

At any given moment, millions of computing systems connected to the Internet are fending off attacks. Security software company Kaspersky Labs has created a real-time global map of cyberthreats that its security network is subjected to (see https://cybermap.kaspersky.com). This image, showing recent attacks over the span of seconds, centers on Europe and Asia, which contain countries often on the top-10 attacked list. (The order changes, but Russia is often first, followed by the United States. Germany is sixth and France is seventh.) Colors in the attacks indicate how the threat was discovered—through a user’s regular screening (red), or in a sweep brought on by a suspicious email attachment (blue), for example. Lines follow the threat from its point of discovery to other places on the globe that are being attacked by the same threat. In “Cybersecurity Is Harder than Building Bridges” (Computing Science, pages 154–157), Peter J. Denning and Dorothy E. Denning examine why the protection of cyber systems is such a complicated, messy problem, compared with other large-scale engineering endeavors. (Image courtesy of Kaspersky Lab.)

In temperate forests, one of the first signs of spring is when the trees unfurl their leaves. As the climate warms worldwide, leaves are emerging earlier than a century ago, but not all the organisms that depend on them are changing in the same way or as quickly. The responses of different tree species to warming springtime temperatures also varies widely. In the photograph on the cover, leaves unfold on young beeches under a mixed forest canopy in the foothills of the Jura Mountains in Switzerland. In “Spring Budburst in a Changing Climate” (pages 102–109), ecologists Richard B. Primack and Amanda S. Gallinat of Boston University explain how springtime schedules in temperate forests are changing. (Photograph on cover by Yann Vitasse.)

Flames behave very differently in space than they do on the ground. An experiment called BASS-II (for Burning and Suppression of Solids) flew on the International Space Station in 2014 and explored flame dynamics using several materials, including acrylic rods (as shown on the cover), in different rates of air flow. Tests were designed to evaluate materials that might be used in spacecraft. The principal investigator for the BASS-II experiment, Sandra L. Olson, and co-investigator, Fletcher J. Miller, are coauthors of the article "Fire in Microgravity" (pages 44–51). In that article, authors Indrek Wichman, Olson, Miller, and Ashwin Hariharan detail a specific type of fire called a flamelet that can persist with very little oxygen, and then flare up to ignite dangerous fires, with implications for fire safety during longer space missions. (Cover image courtesy of ESA/NASA.)

A famous artwork is shown as it is viewed by an artificial neural network, a computer program that has been trained to recognize and classify images. A technique called deep dreaming attempts to visualize various stages in the neural network’s process of analysis. Shown here is one of the early stages, where a layer of artificial neurons is sensitive to edges in various orientations and to center-surround patterns. (The mammalian brain is known to have similar feature-detecting cells.) The deep dreaming algorithm accentuates these patterns and projects them back onto the original painting. In Computing Science (pages 380–383), Brian Hayes explores what these algorithms can tell us about how both artificial and human image processing works. (The Mona Lisa on the cover is a version in the Prado Museum in Spain; see page 404.) (Cover image by Brian Hayes.)

Oil rigs, such as this one in the North Sea off the Scottish coast, are often subjected to pummeling ocean waves. Other powerful waves, such as seismic and magnetic ones, can also cause great damage. Researchers are exploring concepts from cloaking technology that might be used to protect structures or objects from such waves. Cloaking, in theory, usually means making something invisible by redirecting light waves around it so the observer sees light that doesn’t appear to have interacted with anything. It’s possible that such a technology may never be feasible. But as Gregory J. Gbur explains in “A Protective Cloak Against Earthquakes and Storms” (pages 356–359), by changing the density of water or ground, ocean or seismic waves can be made to change speed, and thus direction, diverting them around an object and “cloaking” it from damage, if not from sight. (Photograph by Gandee Vasan/Getty Images.)

For more than two decades, the Voyager 1 and Voyager 2 probes have been headed toward the edge of the heliosphere, the enormous bubble created by the Sun’s extended atmosphere. That edge, called the heliopause, is defined by the pushback from the interstellar medium against the solar wind, the charged particles that stream from the Sun. Until Voyager 1 reached the heliopause in 2012, scientists knew little about this critical boundary, not even how far away it is; it turns out to be around 18.2 billion kilometers from Earth. Researchers expected the region before the heliopause, called the heliosheath, to be smooth and calm, but Voyager 1’s measurements show that it is anything but. In this artist’s interpretation of the data, magnetic field lines (red and blue) connect back to the Sun, but the transition region is filled with magnetic bubbles. The heliopause is not a continuous shield between the solar atmosphere and the interstellar medium after all, but a porous membrane with fingers and indentations. (Image courtesy of NASA/Goddard Space Flight Center/CI Lab.)

The assemblage of mammals in Egypt has declined in species diversity over the past 10,000 years. Using clues from archaeology and paleontology, ecologists have pieced together the food web of larger-bodied mammals present before, during, and after the rise of agricultural societies in ancient Egypt. In “Modern Lessons from Ancient Food Webs” (pages 188–195), Justin D. Yeakel and Jennifer A. Dunne describe the underlying structure that appears to be consistent in food webs across space and time, as well as how we can better predict their instability or recovery after major die-offs. In the case of Egypt, as the climate became more arid and human densities increased, the food web lost most of its midsize herbivores, such as gazelles, that feed many different species of carnivore. The loss of those remaining could result in domino-effect extinctions. In this illustration, artist Dominique Navarro depicts several of the mammals once present in Egypt (and some that still are) during the African Humid Period. Navarro recently illustrated the American University in Cairo Press Nature Foldouts on Egyptian flora and fauna.

Nomadic army ants (Eciton burchelli), such as these individuals from a captive colony at the California Academy of Sciences, form living bridges with their bodies to cross gaps along their foraging trails. All ant trails are marked by species-specific pheromones, although their chemical composition remains unknown for most of the world’s 20,000 or more ant species. Many behaviors of social insects are likewise mediated by smell signals, including recognizing colony mates and the development of workers and their queen. In “How Animals Communicate Via Pheromones” (pages 114–121), Tristram D. Wyatt summarizes the long history of pheromone research and discusses the animals that have most informed this field, including social insects, moths, and mice. There is one animal whose pheromones remain an intriguing question mark: Homo sapiens. Given recent advances in the field, though, Wyatt says the chemical identification of a human pheromone might be just around the corner. (Cover photo by Alexander Wild.)

Visualizing the upcoming passage of the New Horizons spacecraft past Pluto and its large satellite Charon necessarily involved a lot of educated guesswork. Pluto is so small (about 2,360 kilometers in diameter) and so distant (currently 4.9 billion kilometers away) that it is impossible to image directly from Earth. Artist Steve Gribben at Johns Hopkins University had to rely on sparse visual data from the Hubble Space Telescope and consultations with members of the New Horizons team to guide the colors and surface features seen on the cover. As mission scientist S. Alan Stern explains in “Journey to the Solar System’s Third Zone” (pages 42–45), the Pluto flyby will offer a first look not just at this one unexplored world but at an entire class of intriguing, poorly understood bodies orbiting the Sun beyond Neptune. (Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben.)

Detail from the microetching Chaotic Connectome (2013), by Greg Dunn and Brian Edwards, offers a glimpse of the intricate network of neurons in the cerebral cortex. The composition uses precisely engraved ridges on a photosensitive surface that is gilded to reflect light differently under white, blue, and red wavelengths; thus, different neurons appear and disappear as spectators change their angle of view. In “Etching the Neural Landscape” (pages 388–341), Dunn tells how he and Edwards developed a unique blend of photolithography, traditional Asian painting motifs, and neuroscience.

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The discovery of a living species of coelacanth, a lobe-finned fish recognized as an important transition in vertebrate evolution, was a surprising and exciting find in 1938, because the fish was already widely recognized in the fossil record. Hailed as a living fossil, even though there has never been any fossil find of the two extant species of coelacanth, it is native to waters around Indonesia and in the Indian Ocean. Although fossil and extant coelacanths look strikingly similar, they do not demonstrate an absence of evolution. In “The Evolutionary Truth About Living Fossils”, Alexander J. Werth and William A. Shear relate the unseen evolution of living fossils and discuss the definition and usefulness of this term, first coined by Charles Darwin. The image on the cover shows a face-to-face encounter between a coelacanth off the coast of South Africa and the renowned diver and naturalist Laurent Ballesta. (Photograph by Laurent Ballesta.)