Starving bacteria (cyan) use a microscopic harpoon—called the Type VI secretion system—to stab and kill neighboring cells (magenta). The prey burst, turning spherical and leaking nutrients, which the killers then use to survive and grow. (Image Credit: Glen D'Souza/ASU/Screen shot from video)NewsletterSign up for our email newsletter for the latest science newsBacteria are bad neighbors. And we’re not talking noisy, never-take-out-the-trash bad neighbors. We’re talking has-a-harpoon-gun-and-points-it-at-you bad neighbors. According to a new study in Science, some bacteria hunt nearby bacterial species when they’re hungry. Using a special weapon system called the Type VI Secretion System (T6SS), these bacteria shoot, spill, and then absorb the nutrients from the microbes they harpoon. “The punchline is: When things get tough, you eat your neighbors,” said Glen D’Souza, a study author and an assistant professor at Arizona State University, according to a press release. “We’ve known bacteria kill each other, that’s textbook. But what we’re seeing is that it’s not just important that the bacteria have weapons to kill, but they are controlling when they use those weapons specifically for situations to eat others where they can’t grow themselves.” According to the study authors, the research doesn’t just have implications for bacterial neighborhoods; it also has implications for human health and medicine. By harnessing these bacterial weapons, it may be possible to build better targeted antibiotics, designed to overcome antibiotic resistance. Ruthless Bacteria Use HarpoonsResearchers have long known that some bacteria can be ruthless, using weapons like the T6SS to clear out their competition. A nasty tool, the T6SS is essentially a tiny harpoon gun with a poison-tipped needle. When a bacterium shoots the weapon into another bacterium from a separate species, the needle pierces the microbe without killing it. Then, it injects toxins into the microbe that cause its internal nutrients to spill out.Up until now, researchers thought that this weapon helped bacteria eliminate their competition for space and for food, but after watching bacteria use the T6SS to attack their neighbors when food was scarce, the study authors concluded that these tiny harpooners use the weapon not only to remove rivals, but also to consume their competitors’ leaked nutrients.“Watching these cells in action really drives home how resourceful bacteria can be,” said Astrid Stubbusch, another study author and a researcher who worked on the study while at ETH Zurich, according to the press release. “By slowly releasing nutrients from their neighbors, they maximize their nutrient harvesting when every molecule counts.” Absorbing Food From NeighborsTo show that the bacteria used this system to eat when there was no food around, the study authors compared their attacks in both nutrient-rich and nutrient-poor environments. When supplied with ample resources, the bacteria used their harpoons to kill their neighbors quickly, with the released nutrients leaking out and dissolving immediately. But when resources were few and far between, they used their harpoons to kill their neighbors slowly, with the nutrients seeping out and sticking around. “This difference in dissolution time could mean that the killer cells load their spears with different toxins,” D’Souza said in another press release. While one toxin could eliminate the competition for space and for food when nutrients are available, another could create a food source, allowing bacteria to “absorb as many nutrients as possible” when sustenance is in short supply.Because of all this, this weapon system is more than ruthless; it’s also smart, and important to some species’ survival. When genetically unedited T6SS bacteria were put in an environment without food, they survived on spilled nutrients. But when genetically edited T6SS bacteria were placed in a similar environment, they died, because their ability to find food in their neighbors had been “turned off.”Harnessing Bacterial HarpoonsAccording to the study authors, the T6SS system is widely used by bacteria, both in and outside the lab. “It’s present in many different environments,” D’Souza said in one of the press releases. “It’s operational and happening in nature, from the oceans to the human gut.” The study authors add that their research could change the way we think about bacteria and could help in our fight against antibiotic resistance. In fact, the T6SS could one day serve as a foundation for targeted drug delivery systems, which could mitigate the development of broader bacterial resistance to antibiotics. But before that can happen, however, researchers have to learn more about bacterial harpoons, and about when and how bacteria use them, both to beat and eat their neighbors.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.1 free article leftWant More? Get unlimited access for as low as $1.99/monthSubscribeAlready a subscriber?Register or Log In1 free articleSubscribeWant more?Keep reading for as low as $1.99!SubscribeAlready a subscriber?Register or Log In

Bacteria wouldn’t be so bad if we could tell them what to do. “Stop spreading! Stop sticking together! Stop fending off our antibiotics!” A new method is starting to allow scientists to do just that, letting them use light to control certain functions of bacteria. Introduced in a paper published in The European Physical Journal Plus, the preliminary approach could have several potential applications, including a possible avenue for combating antibiotic resistance.The Problem of Antibacterial Resistance Bacteria are behind a variety of diseases, from strep to staph to pneumonia and meningitis, and they attack our bodies in a variety of ways, as well, including through the production of toxins that damage and disrupt our cells. Some of these infections stop on their own, but others are too stubborn, or too serious, to leave untreated. These are the infections that we target with antibiotics — that is, as long as our antibiotics are working.But, because bacteria are constantly changing, they can develop defenses against the antibiotics that we use to stave them off, making these treatments much less effective. That’s the gist of the growing threat posed by antibiotic resistance, which has contributed to millions of deaths since 1990 and is anticipated to contribute to millions more by 2050. Setting out to find a new solution to this growing problem, scientists from the Italian Institute of Technology and the Polytechnic University of Milan embarked on the Engineering of Bacteria to See Light (EOS) project. The project aims to use light to control bacteria, primarily for the fight against antibiotic resistance. And the new method pushes the project closer to achieving that aim. Using light and light-sensitive molecules to adjust the electrical signals that are transmitted across the bacterial membrane, the method impacts the biological activity of bacteria without any alterations to their genetic makeup.“This interplay between light and electrical [signaling] allows us to control key biological processes such as movement, biofilm formation, and antibiotic sensitivity,” said Giuseppe Maria Paternò, a study author and a professor at the Polytechnic University of Milan, according to a press release. “We can influence antibiotic uptake and restore or even enhance the effectiveness of treatments against resistant strains.”Coating Bacteria to Curb Antibiotic ResistanceTo control bacteria, the method takes advantage of a light-sensitive molecule called Ziapin2, which sticks to the bacterial surface. By covering bacteria with this light-sensitive molecule and by subjecting the covered bacteria to light, the scientists were able to modify the electrical signals that were transmitted across their bacterial membranes, transforming the bacteria’s basic functioning. Testing their method on one of the most studied bacterial species, the scientists changed the electrical signaling across the membranes of Bacillus subtilis, a popular model organism that’s often used as a stand-in for Staphylococcus aureus, the bacterium that causes staphylococcus, or staph, infections.When tested, the method modulated the bacteria’s susceptibility to Kanamycin, an intracellular antibiotic that’s frequently used as a treatment for severe bacterial infections after other treatments fail. “Under blue light,” Paternò said in the release, “the effectiveness of Kanamycin was significantly reduced,” indicating that the electrical signaling on the bacterial membrane “plays a crucial role in the drug’s uptake.”Additional research is required to tailor the method to increase the effectiveness of Kanamycin and other antibiotics against bacteria. But for now, it seems that such an outcome could be possible. “This initial assessment […] represents a first step in a completely new field of study,” the scientists state in their paper. “This proof-of-concept study underscores the potential of non-genetic, light-based interventions to modulate bacterial susceptibility in real time. Future work will expand this approach […] ultimately advancing our understanding of bacterial bioelectric regulation and its applications in antimicrobial therapies.”This article is not offering medical advice and should be used for informational purposes only.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:The European Physical Journal Plus. Photocontrol of Bacterial Membrane Potential Regulates Antibiotic Persistence in B. SubtilisSam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.

Co-lead author Ravneet Sidhu examines an ancient human tooth at the McMaster Ancient DNA Centre. (Image Credit: McMaster University)NewsletterSign up for our email newsletter for the latest science newsA new study in Science suggests that changes in a gene in Yersinia pestis, the bacterium that causes plague, could’ve added to the length of two plague pandemics, including the pandemic that started with the “Black Death.” “Ours is one of the first research studies to directly examine changes in an ancient pathogen, one we still see today, in an attempt to understand what drives the virulence, persistence, and eventual extinction of pandemics,” said Hendrik Poinar, a study author and the director of the McMaster Ancient DNA Centre, according to a press release.The study suggests that less virulent plague bacteria could’ve caused longer plague pandemics — thanks to the fact that infected rodents lived (and spread plague) for longer periods of time before dying from their infections. Read More: Scientists Reveal the Black Death’s Origin StoryThe Three Plague PandemicsThe bacterium Y. pestis infects rodents and humans alike and has caused three main plague pandemics in humans, all of which continued for centuries after their initial outbreaks. The first began in the 500s; the second began in the 1300s; and the third started in the 1800s (and still continues in certain areas in Asia, Africa, and the Americas today). Although all three pandemics were devastating at their outset, the second pandemic was by far the most severe. The Black Death, its initial outburst, killed around 30 to 50 percent of the population of Europe between 1347 and 1352 and — to this day — represents the deadliest disease wave in recorded history.To learn more about how these plague pandemics changed over time, scientists at McMaster University in Canada and the Institut Pasteur in France turned to a Y. pestis virulence gene known as pla. This gene is repeated many times throughout the Y. pestis genome, and it allows the bacterium to spread undetected throughout the bodies of infected individuals. A Gene and the PlagueTo investigate this gene, the scientists studied historical strains of Y. pestis from human remains and found that the number of repetitions of pla decreased over the course of the first and second plague pandemics. Then, the scientists tested Y. pestis bacteria from the third pandemic, infecting mice with three strains that had reduced repetitions of pla. “These three samples enabled us to analyze the biological impact of these pla gene deletions,” said Javier Pizarro-Cerdá, another study author and the director of the Yersinia Research Unit at the Institut Pasteur, according to the release.The results revealed that pla depletion decreases the virulence and increases the length of plague infections in mice. According to the study authors, these changes could have caused rodents to live longer in the later stages of the first and second pandemics, allowing them to spread their infections for a longer period. “It’s important to remember that plague was an epidemic of rats, which were the drivers of epidemics and pandemics. Humans were accidental victims. ” Poinar added in another press release.The Continued Threat of Y. PestisThough the pla depletion occurred around 100 years after the first and second pandemics began, the scientists stress that both changes were random and unrelated.“Our research sheds light on an interesting pattern in the evolutionary history of the plague. However, it is important to note that the majority of strains which continue to circulate today in Africa, the Americas, and Asia are highly virulent strains,” said Ravneet Sidhu, another study author and a Ph.D. student at the McMaster Ancient DNA Centre.Though still a threat to current populations, Y. pestis infections are much more manageable now as a result of modern diagnostics and treatments.“Today, the plague is a rare disease, but one that remains a public health concern and serves as a model for gaining a broad understanding of how pandemics emerge and become extinct. This example illustrates the balance of virulence a pathogen can adopt in order to spread effectively,” Pizarro-Cerdá said in the press release.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Science. Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.1 free article leftWant More? Get unlimited access for as low as $1.99/monthSubscribeAlready a subscriber?Register or Log In1 free articleSubscribeWant more?Keep reading for as low as $1.99!SubscribeAlready a subscriber?Register or Log In

For a few months of the year, the Alaskan Arctic becomes flooded with birds. From shorebirds to waterfowl, these avians arrive in the spring to breed, nest, and raise their young, and to take advantage of the ample plants and prey (invertebrates and other animals) that thrive in Alaska’s short summers. They do it today, and they did it around 73 million years ago, too. Documenting the earliest evidence ever discovered of birds breeding and nesting in the Arctic, a new study in Science describes a collection of avian fossils and fossil fragments from around 73 million years ago. The collection comprises dozens of bones and teeth from adult and baby birds, and it shows that avians similar to modern shorebirds and waterfowl reproduced in the Arctic in the Cretaceous period, when dinosaurs still dominated the Alaskan terrain.“Birds have existed for 150 million years,” said Lauren Wilson, a study author and a student at Princeton University, who worked on the study while at the University of Alaska Fairbanks, according to a press release. “For half of the time they have existed, they have been nesting in the Arctic.”An Arctic NurseryA fossil fragment of a beak from a baby bird. (Image Credit: Photo by Pat Druckenmiller)Millions of birds travel to the Arctic, and they’ve been traveling there for millions of years. (In fact, some 250 species of birds migrate to Alaska for the spring and summer breeding and nesting seasons today.) But up until now, the earliest traces of birds reproducing in the Arctic dated back to around 47 million years ago, following the disappearance of the non-avian dinosaurs from the Arctic terrain. Now, the authors of the new study claim that birds and non-avian dinosaurs shared the Alaskan Arctic as far back as the Cretaceous period. Sifting bones and teeth from the sediment of Alaska’s Prince Creek Formation, the authors identified an assortment of Cretaceous fossils and fossil fragments, which resembled the remains of modern gulls, geese, ducks, and loons. That the specimens belonged to adult and baby birds suggests that these species were breeding, nesting, and raising their young in Alaska, more than 20 million years earlier than previously thought. “The Arctic is considered the nursery for modern birds,” said Pat Druckenmiller, another study author and a professor at the University of Alaska Fairbanks, according to a press release. “They have been doing this for 73 million years.”Finding Fossils, From Adult and Baby BirdsStudy authors Joe Keeney, Jim Baichtal, and Patrick Druckenmiller in Alaska. (Image Credit: Photo by Lauren Wilson) According to the authors, the bones and teeth of adult birds are often too fragile to survive in the fossil record, and those from baby birds are even more delicate. “Finding bird bones from the Cretaceous is already a very rare thing,” Wilson said in the release. “To find baby bird bones is almost unheard of. That is why these fossils are significant.” Though the majority of specimens that are taken from the Prince Creek Formation are large, the study authors opted to collect the smaller fossils and fossil fragments that most other studies miss. To do so, they inspected screened sediment with a microscope, which revealed their tiny finds. “We put Alaska on the map for fossil birds,” Druckenmiller said in the release. “It wasn’t on anyone’s radar.”Whether the find includes bones and teeth from the Neornithes — or the modern birds — is yet to be determined, though the authors stress that some of the fossils and fossil fragments feature skeletal and dental traits, such as fused leg bones and toothless jawbones, that are seen only in modern birds. “If they are part of the modern bird group, they would be the oldest such fossils ever found,” Druckenmiller said in the release. “But it would take us finding a partial or full skeleton to say for sure.”Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Science. Arctic Bird Nesting Traces Back to the CretaceousSam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.

Giant Sloths the Size of Elephants Once Walked Along the Ground. Here’s How the Massive Animals Evolved and Declined Researchers analyzed fossils and DNA to get a big-picture view of sloth evolution and determine what drove their immense size variation Researchers revealed that differences in sloth habitats drove the wide variation in size seen in extinct species. Diego Barletta Today, sloths are slow-moving, tree-dwelling creatures that live in Central and South America and can grow up to 2.5 feet long. Thousands of years ago, however, some sloths walked along the ground, weighed around 8,000 pounds and were as big as Asian elephants. Some of these now-extinct species were “like grizzly bears, but five times larger,” as Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History, says in a statement. In a study published last week in the journal Science, Narducci and her colleagues studied ancient and modern sloth DNA along with more than 400 sloth fossils to shed light on the shocking differences in their ancient sizes—from the elephant-sized Megatherium ground sloth to its 14-pound relatives living in trees. While it’s clear that tree-dwelling lifestyles necessitate small bodies, scientists weren’t sure why ground sloths specifically demonstrated such vast size diversity. To investigate this, the team used their genetic and fossil analyses to reconstruct a sloth tree of life that reaches back to the animals’ emergence more than 35 million years ago. They integrated data on sloths’ habitats, diets and mobility that had been gathered in previous research. With a computer model, they processed this information, which ultimately indicated that sloths’ size diversity was mostly driven by their habitats and climates. “When we look at what comes out in the literature, a lot of it is description of individual finds, or new taxa,” Greg McDonald, a retired regional paleontologist with the U.S. Bureau of Land Management who was not involved with the study, tells Science News’ Carolyn Gramling. The new work is “more holistic in terms of looking at a long-term pattern. Often, we don’t get a chance to step back and get the big picture of what’s going on.” The big picture suggests that since the emergence of the oldest known sloths—ground animals around the size of a Great Dane—the creatures evolved into and out of tree living a number of times. Around 14 million to 16 million years ago, however, a time of global warming called the Mid-Miocene Climatic Optimum pushed sloths to become smaller, which is a known way for animals to respond to heat stress. Warmer temperatures might have also seen more rain, which would have created more forest habitats ideal for tree-dwelling sloths. Around a million years later, however, ground sloths grew bigger as the planet’s temperature cooled. “Gigantism is more closely associated with cold and dry climates,” Daniel Casali, a co-author of the paper and a researcher of mammalian evolution at the University of São Paulo, tells New Scientist’s Jake Buehler. A larger body mass would have helped the animals traverse environments with few resources more efficiently, Narducci says in the statement. In fact, these large ground sloths spread out across diverse habitats and thrived in different regions. The aquatic sloth Thalassocnus even evolved marine adaptations similar to manatees. Ground sloths achieved their greatest size during the last ice age—right before starting to disappear around 15,000 years ago. Given that humans arrived in North America around the same time (though recent research indicates they may have arrived as far back as 20,000 years ago), some scientists say humans are the obvious cause of the sloths’ demise. While tree-dwelling sloths were out of reach to our ancestors, the large and slow ground animals would have made easy targets. Even still, two species of tree sloths in the Caribbean disappeared around 4,500 years ago—also shortly after humans first arrived in the region, according to the statement. While the study joins a host of research indicating that humans drove various large Ice Age animals to extinction, “in science, we need several lines of evidence to reinforce our hypotheses, especially in unresolved and highly debated issues such as the extinction of megafauna,” says Thaís Rabito Pansani, a paleontologist from the University of New Mexico who did not participate in the study, to New Scientist. The International Union for Conservation of Nature currently recognizes seven—following a recent species discovery—and three are endangered. As such, “one take-home message is that we need to act now to avoid a total extinction of the group,” says lead author Alberto Boscaini, a vertebrate paleontologist from the University of Buenos Aires, to the BBC’s Helen Briggs. Get the latest stories in your inbox every weekday.

Worms are decomposers. Many survive by breaking down dead things — dead bacteria, dead plants, dead animals, dead anything. So, they must be accustomed to the stench of death. Not so, a new study suggests — not when the dead organism is another worm.Published in Current Biology, the study states that C. elegans roundworms react adversely to the smell of a deceased counterpart. Not only does this smell invoke a behavioral response of corpse avoidance, but it also invokes a physiological response of increased short-term fertility and decreased long-term fitness and lifespan.“Caenorhabditis elegans prefers to avoid dead conspecifics,” or deceased members of the same species, the authors state in the study, with the worms reacting to death with a range of “aversion” and “survival” responses. Taken together, the results reveal a new signaling mechanism that’s available to worms and possibly other organisms, too, as a means of detecting and responding to death.Read More: These Fruit Flies Aged Faster After Seeing DeathWorms Signal and Detect DeathC. elegans roundworms aren’t the only small organisms that respond to the dead. Ants and bees dispose of the deceased from their colonies, for instance, while fruit flies avoid corpses (and shun flies that have seen corpses themselves). Death-exposed fruit flies even experience faster aging after seeing a deceased counterpart, and have shorter lifespans than those that have had no encounters with death. That these animals respond so strongly to the dead is widely documented. So, when the authors of the new study noticed C. elegans worms wriggle away from corpses, they saw the response as a chance to dig deeper into death signaling and detection. Indeed, while many species’ reactions to death are mediated mainly by sight, that certainly wasn’t the case for wiggling roundworms, which have no eyes and no sense of vision. “We felt this was quite a unique opportunity to start diving into what is happening mechanistically that enables C. elegans to detect a dead conscript,” said Matthias Truttmann, a senior study author and a physiologist at the University of Michigan, according to a press release.To determine how C. elegans worms detect the dead, Truttman and his team exposed the worms to conspecific corpses and to fluids taken from the deteriorating cells of those corpses. The worms responded to both with avoidance, moving away regardless of their age and sex, suggesting that the corpses and fluids carried similar signatures of death. These death cues also resulted in short-term increases in fertility, long-term decreases in fitness (represented by a reduced thrashing rate), and long-term decreases in lifespan. But what were those death cues, exactly, and how did the worms pick up on them?Sounding a Sensory AlarmTo figure out what those cues could be, the study authors recorded the activity in the worms’ sensory neurons as they encountered the corpses and fluids. The recordings revealed that AWB and ASH, two neurons that are responsible for making sense of olfactory stimuli, were activated when the corpses and fluids were present, indicating that the worms were smelling the signature of death.“The neurons we identified are well known to be involved in behavioral responses to a variety of environmental cues,” Truttmann said in the release. According to the study authors, the metabolites AMP and histidine were probably responsible for the signal of death that the C. elegans worms recognized. Though these metabolites are typically contained in living cells, they are released when living cells die and deteriorate — in this case, triggering the behavioral and physiological responses in C. elegans. “They also detect a couple of intracellular metabolites that are not typically found in the environment. If they are around, it indicates that a cell has died, popped open, and that something has gone wrong,” Truttmann said in the release.It is possible that cellular metabolites serve as a signal of death in other organisms, too, Truttmann said, as the release of metabolites from dying and disintegrating cells in one tissue can cause changes in other tissues in humans, for instance. Whether this signal sounds the alarm in other organisms is still uncertain. While further research is required to understand the role of cellular metabolites in detecting death across species, for now, it’s clear that death is a sensitive subject, even for worms like C. elegans.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Current Biology. Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.

By Natalia Mesa Published May 23, 2025 | Comments (1) | Giant ground sloths Megalocnus rodens and Megalonyx wheatleyi at the American Museum of Natural History © Dallas Krentzel Giant sloths with razor-sharp claws and as large as Asian bull elephants once roamed the Earth, snacking on leaves at the tops of trees with a prehensile tongue. Now, scientists have figured out why they became so huge—and why these massive sloths didn’t stick around—according to a new study published in Science. Today, two sloth species dwell in Central and South America. But long ago, dozens of sloth species populated the Americas, all the way from Argentina to Canada. Like modern-day sloths, the smaller species were tree-dwelling. But the larger sloths? “They looked like grizzly bears but five times larger,” Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History and coauthor of the study, in a statement. The larger sloths didn’t do much tree climbing, at risk of falling to their deaths. Instead, they survived by being terrifying; the largest sloths had long, sharp claws that they used to carve their own caves out of raw earth and rocks. But exactly why they got so large remained a mystery. To figure out how these sloths got so massive, researchers analyzed ancient sloth DNA and compared more than 400 fossils from natural history museums to create a sloth tree of life. The researchers traced the sloths’ origin to 35 million years ago. And, because the scientists were particularly interested in how sloths got their size, they estimated their weights by taking fossil measurements. The researchers concluded that the Earth’s past climate was a big factor. Thirty-five million years ago, the first ancestor of modern-day sloths, which lived in what is now Argentina, was roughly the size of a large dog. Sloths hardly changed in size for 20 million years, and lived on the ground. Then, during a warming period around 16 million years ago, sloths adapted by evolving smaller physiques due to their need to keep cool. Then, as Earth cooled down again—which it’s been doing on and off for the past 50 million years—sloths started to get bigger and bigger. They also started to migrate, fanning out from Argentina throughout North and South America, and even up to Alaska and Canada. These new habitats presented challenges that the sloths met, in part, by bulking up. This new size also helped them keep warm and stay safe from predators. “This would’ve allowed them to conserve energy and water and travel more efficiently across habitats with limited resources,” Narducci said. “And if you’re in an open grassland, you need protection, and being bigger provides some of that.” They reached their most massive size during the Pleistocene Ice Ages, which spanned roughly 3 million to 12,000 years ago, shortly before they disappeared. Scientists aren’t completely sure why sloths went extinct, but they do have some guesses. Early humans migrated to the Americas around 20,000 years ago. Larger ground-dwelling sloths likely became a prime, meaty target for early humans, and being on the ground became a liability. Larger sloths were the first to go, but tree sloths didn’t escape unscathed. Over time, more and more species of tree-dwelling sloths went extinct, too. Two species survived in the Caribbean until around 4,500 years ago—until humans wiped them out. Now, sloths mostly keep to Central and South America, but thankfully aren’t on the menu anymore. Daily Newsletter

A human can’t shrink away from the threats of climate change. A clownfish, however, can. In a new paper published today in Science Advances, a team of researchers revealed that these tiny “Finding Nemo” fish can actually shrink to survive heat stress, allowing them to overcome the threat of heatwaves.“We were so surprised to see shrinking in these fish,” said Melissa Versteeg, a study author and a Ph.D. student at Newcastle University in the U.K., according to a press release. “In the end, we discovered it was very common in this population.”Clownfish Shrink in SizeA clown anemonefish. (Image Credit: Morgan Bennett-Smith)Climate change has transformed terrestrial and marine habitats and continues to transform them, with heatwaves — or periods of abnormal warmth — having one of the most significant impacts on animals. Studies show, for example, that increasing temperatures have a strong influence on the dimensions of terrestrial and marine species, shaping their size and size variability and contributing to their overall reduction in size over time.But what, exactly, is the effect of marine heatwaves on the clownfish, also known as the clown anemonefish (Amphiprion percula)?Setting out to study how heatwaves transform these fish over time, Versteeg and a team of researchers turned to the wild clownfish population in Kimbe Bay in Papua New Guinea, where heatwaves caused temperatures to sit around 4 degrees Celcius above average over the course of the study. Measuring the water temperatures and the size of the clownfish there from February 2023 to August 2023, the team found that individual clownfish shrank over time.“We measured each fish individual repeatedly over a period of five months,” Versteeg said in the release. “During our study, 100 fish shrank out of the 134 fish that we studied.” Rather than getting slimmer, these clownfish shrank by getting shorter, with the degree of their reduction depending on the individual’s initial size and social rank. According to the researchers, the results reveal that clownfish reduce their size in response to heat stress, which, in turn, increases their chances of surviving a heatwave by 78 percent.Read More: How Volunteers Are Helping Keep Coral Reefs AliveClownfish Survival ImprovesAccording to the researchers, some clownfish shrank one time, and some clownfish shrank multiple times, with all of the fish that shrank multiple times surviving throughout the course of the study. Intriguingly, the chances of clownfish survival were also improved if a clownfish shrank alongside its breeding partner. “We witnessed how flexibly they regulated their size, as individuals and as breeding pairs, in response to heat stress as a successful technique to help them survive.” Versteeg said in the release. “It was a surprise to see how rapidly clownfish can adapt to a changing environment.”Similar shrinking abilities are seen in other animals, including marine iguanas. And while clownfish are the first coral reef fish that researchers have shown to shorten in response to heat stress, they may not be the last. In fact, the results could have implications for other coral reef fish, and for other fish overall. According to the researchers, fish on the whole are much smaller today than they once were. A 2023 study in Science found, for instance, that fish, in particular, are driving a decrease in size in the world’s animal populations. One possible explanation for this is that smaller species of fish are surviving over larger species of fish. Another is that fish species of all sizes are shrinking over time, with the smaller individuals of each species surviving (and thus procreating) more than the larger individuals of each species. It is possible, however, that there are other factors contributing to the smaller size of fish today, too, including the ability to shrink in size in times of stress. “If individual shrinking were widespread and happening among different species of fish, it could provide a plausible alternative hypothesis for why the size many fish species is declining,” said Theresa Rueger, the senior study author and a lecturer at Newcastle University, according to the press release. “Further studies are needed in this area.”Though the fish themselves are small and becoming smaller, the researchers say that their results raise big questions about animal size and about animal shrinking, more specifically. “We don’t know yet exactly how they do it,” Versteeg said. “But we do know that a few other animals can do this too.”Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.

Some humpback whales are born in warmer waters. Others are born on the way. That’s what a study in Frontiers in Marine Science seems to suggest, anyway, after showing that hundreds of East Australian humpback whales are actually born mid-migration, while their mothers are still traveling to their established calving and breeding grounds.“Hundreds of humpback calves were born well outside the established breeding grounds,” said Tracey Rogers, the senior study author and a biology professor at the University of New South Wales, according to a press release. “Giving birth along the ‘humpback highway’ means these vulnerable calves, who are not yet strong swimmers, are required to swim long distances much earlier in life than if they were born in the breeding grounds.” In fact, the study shows that these calves are sometimes born in the temperate waters around Southeastern Australia, New Zealand, and Tasmania, around 900 miles south of the traditionally assumed area. Challenging the theory that humpback migration is essential for the birth of these whales, the study provides valuable information for protecting humpback whale populations in the future. Humpback Whale Mid-Migration SightingsEvery year, Eastern Australian humpbacks travel from the polar waters of the Southern Ocean around Antarctica to the tropical waters of the South Pacific Ocean around northeastern Australia. For a long time, it was thought that this winter migration enabled the birth of these whales, with the whales having to be born in these warmer waters.“Historically, we believed that humpback whales migrating north from the nutrient-rich Southern Ocean were [traveling] to warmer, tropical waters, such as the Great Barrier Reef, to calve,” said Jane McPhee-Frew, the lead study author and a biology Ph.D. candidate at the University of New South Wales, according to another press release.But in July 2023, McPhee-Frew spotted a pair of Eastern Australian humpbacks — a mother and a calf — in the temperate waters around southeastern Australia, apparently on their way to their established calving and breeding grounds. “The calf was tiny, obviously brand new,” McPhee-Frew said in the press release. “What were they doing there?”Hoping to find out, McPhee-Frew, Rogers, and a team of five other researchers studied hundreds of observations of Eastern Australian humpbacks from around southeastern Australia, New Zealand, and Tasmania. Including information from citizen scientist sightings, scientist surveys, and beach strandings, the team examined 209 observations of calves, including 168 observations of living calves, many of which were made in 2023 and 2024. Surprisingly, some of the observations were made as far south as Port Arthur, Tasmania, with many of the mothers continuing to travel north with their newborns.According to the team, these mid-migration births are probably not a new phenomenon, as records seem to suggest that they occurred in the 1800s and 1900s, too, before the collapse of Eastern Australian humpback populations due to commercial whaling. “I think it’s very likely that this pattern has always existed, but the low number of whales obscured it from view,” McPhee-Frew said in the press release. “The Eastern Australia humpback population narrowly escaped extinction, but now there are [30,000, 40,000, or 50,000] in this population alone. It doesn’t happen overnight, but the recovery of humpback whales, and the return of their full range of behaviors and distribution, just goes to show that with good policies built on good science, we can have excellent outcomes.”Protection for Whale PopulationsA mother and baby humpback whale swimming in Kiama, New South Wales, Australia. (Image Credit: Vanessa Risku (Instagram: @droning_my_sorrows))The fact that these calves can be born on their way to the mothers’ calving and breeding grounds means that the purpose of humpback migration is much more of a mystery than typically thought. Indeed, if humpback mothers can deliver babies in temperate waters, why do they travel to tropical waters every year? Though the study cannot confirm this theory, the warm waters of northeastern Australia may offer other benefits beyond birth. For instance, they might be a potentially safer space for calves to learn and grow, even if they were born elsewhere. Such benefits might make the move worthwhile, the team says, despite the risks of delivering a newborn in the midst of migration. And there are a lot of risks. Without a newborn, the trip is long, spanning several thousand miles from the South Ocean to the South Pacific Ocean, and straight through some of the busiest swathes of sea. “This means these vulnerable animals are exposed to risks like boat strikes, entanglements, pollution, and just general public unawareness,” Rogers said in the press release. And adding a calf to the mix merely increases the risk, as newborn humpbacks are slower and weaker than their mothers. “They have those long, enormous fins that they need to grow into, and they’re not very strong swimmers. So they rest a lot of the time on their mum’s back,” Rogers said in the press release. “It’s heartbreaking to think of these young whales [traveling] through busy ports and dangerous shipping lanes with those long, clumsy fins.”The injuries on some of the observed newborns stress the need to do more to protect whales as they travel, the team says. Fortunately, with better information about where these calves and their mothers appear, protected areas and awareness campaigns can be better calibrated to save whales of all ages. “Regardless of the health of population now, we can’t be in a situation where we’re putting any age of whales — especially baby whales — in a situation where they’re getting caught in nets, being exposed to chemicals, being hit by boats and being harassed,” McPhee-Frew said in the press release. Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.