Key Takeaways on Ant PoopDo ants poop? Yes. Any creature that eats will poop and ants are no exception. Because ants live in close quarters, they need to protect the colony from their feces so bacteria and fungus doesn't infect their health. This is why they use toilet chambers. Whether they isolate it in a toilet chamber or kick it to the curb, ants don’t keep their waste around. But some ants find a use for that stuff. One such species is the leafcutter ant that takes little clippings of leaves and uses these leaves to grow a very particular fungus that they then eat.Like urban humans, ants live in close quarters. Ant colonies can be home to thousands, even tens of thousands of individuals, depending on the species. And like any creature that eats, ants poop. When you combine close quarters and loads of feces, you have a recipe for disease, says Jessica Ware, curator and division chair of Invertebrate Zoology at the American Museum of Natural History. “Ant poop can harbor bacteria, and because it contains partly undigested food, it can grow bacteria and fungus that could threaten the health of the colony,” Ware says. But ant colonies aren’t seething beds of disease. That’s because ants are scrupulous about hygiene.Ants Do Poop and Ant Toilets Are RealAnt colony underground with ant chambers. (Image Credit: Lidok_L/Shutterstock)To keep themselves and their nests clean, ants have evolved some interesting housekeeping strategies. Some types of ants actually have toilets — or at least something we might call toilets. Their nests are very complicated, with lots of different tunnels and chambers, explains Ware, and one of those chambers is a toilet chamber. Ants don’t visit the toilet when they feel the call of nature. Instead, worker ants who are on latrine duty collect the poop and carry it to the toilet chamber, which is located far away from other parts of the nest. What Does Ant Poop Look Like? This isn’t as messy a chore as it sounds. Like most insects, ants are water-limited, says Ware, so they try to get as much liquid out of their food as possible. This results in small, hard, usually black or brownish pellets of poop. The poop is dry and hard enough so that for ant species that don’t have indoor toilet chambers, the workers can just kick the poop out of the nest.Ants Use Poop as FertilizerWhether they isolate it in a toilet chamber or kick it to the curb, ants don’t keep their waste around. Well, at least most types of ants don’t. Some ants find a use for that stuff. One such species is the leafcutter ant. “They basically take little clippings of leaves and use these leaves to grow a very particular fungus that they then eat,” says Ware. “They don't eat the leaves, they eat the fungus.” And yep, they use their poop to fertilize their crops. “They’re basically gardeners,” Ware says. If you’d like to see leafcutter ants at work in their gardens and you happen to be in the New York City area, drop by the American Museum of Natural History. They have a large colony of fungus-gardening ants on display.Other Insects That Use ToiletsAnts may have toilets, but termites have even wilder ways of dealing with their wastes. Termites and ants might seem similar at first sight, but they aren’t closely related. Ants are more closely related to bees, while termites are more closely related to cockroaches, explains Aram Mikaelyan, an entomologist at North Carolina State University who studies the co-evolution of insects and their gut microbiomes. So ants’ and termites’ styles of social living evolved independently, and their solutions to the waste problem are quite different.“Termites have found a way to not distance themselves from the feces,” says Mikaelyan. “Instead, they use the feces itself as building material.” They’re able to do this because they feed on wood, Mikaelyan explains. When wood passes through the termites’ digestive systems into the poop, it enables a type of bacteria called Actinobacteria. These bacteria are the source of many antibiotics that humans use. (Leafcutter ants also use Actinobacteria to keep their fungus gardens free of parasites.) So that unusual building material acts as a disinfectant. Mikaelyan describes it as “a living disinfectant wall, like a Clorox wall, almost.”Insect HygieneIt may seem surprising that ants and termites are so tidy and concerned with hygiene, but it’s really not uncommon. “Insects in general are cleaner than we think,” says Ware. “We often think of insects as being really gross, but most insects don’t want to lie in their own filth.”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 American Society of Microbiology. The Leaf-cutter Ant’s 50 Million Years of FarmingAvery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.

Extra-sensory perception The nine-armed octopus and the oddities of the cephalopod nervous system A mix of autonomous and top-down control manage the octopus's limbs. Kenna Hughes-Castleberry – Jun 7, 2025 8:00 am | 19 Credit: Nikos Stavrinidis / 500px Credit: Nikos Stavrinidis / 500px Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more With their quick-change camouflage and high level of intelligence, it’s not surprising that the public and scientific experts alike are fascinated by octopuses. Their abilities to recognize faces, solve puzzles, and learn behaviors from other octopuses make these animals a captivating study. To perform these processes and others, like crawling or exploring, octopuses rely on their complex nervous system, one that has become a focus for neuroscientists. With about 500 million neurons—around the same number as dogs—octopuses’ nervous systems are the most complex of any invertebrate. But, unlike vertebrate organisms, the octopus’s nervous system is also decentralized, with around 350 million neurons, or 66 percent of it, located in its eight arms. “This means each arm is capable of independently processing sensory input, initiating movement, and even executing complex behaviors—without direct instructions from the brain,” explains Galit Pelled, a professor of Mechanical Engineering, Radiology, and Neuroscience at Michigan State University who studies octopus neuroscience. “In essence, the arms have their own ‘mini-brains.’” A decentralized nervous system is one factor that helps octopuses adapt to changes, such as injury or predation, as seen in the case of an Octopus vulgaris, or common octopus, that was observed with nine arms by researchers at the ECOBAR lab at the Institute of Marine Research in Spain between 2021 and 2022. By studying outliers like this cephalopod, researchers can gain insight into how the animal’s detailed scaffolding of nerves changes and regrows over time, uncovering more about how octopuses have evolved over millennia in our oceans. Brains, brains, and more brains Because each arm of an octopus contains its own bundle of neurons, the limbs can operate semi-independently from the central brain, enabling faster responses since signals don’t always need to travel back and forth between the brain and the arms. In fact, Pelled and her team recently discovered that “neural signals recorded in the octopus arm can predict movement type within 100 milliseconds of stimulation, without central brain involvement.” She notes that “that level of localized autonomy is unprecedented in vertebrate systems.” Though each limb moves on its own, the movements of the octopus’s body are smooth and conducted with a coordinated elegance that allows the animal to exhibit some of the broadest range of behaviors, adapting on the fly to changes in its surroundings. “That means the octopus can react quickly to its environment, especially when exploring, hunting, or defending itself,” Pelled says. “For example, one arm can grab food while another is feeling around a rock, without needing permission from the brain. This setup also makes the octopus more resilient. If one arm is injured, the others still work just fine. And because so much decision-making happens at the arms, the central brain is freed up to focus on the bigger picture—like navigating or learning new tasks.” As if each limb weren’t already buzzing with neural activity, things get even more intricate when researchers zoom in further—to the nerves within each individual sucker, a ring of muscular tissue, which octopuses use to sense and taste their surroundings. “There is a sucker ganglion, or nerve center, located in the stalk of every sucker. For some species of octopuses, that’s over a thousand ganglia,” says Cassady Olson, a graduate student at the University of Chicago who works with Cliff Ragsdale, a leading expert in octopus neuroscience. Given that each sucker has its own nerve centers—connected by a long axial nerve cord running down the limb—and each arm has hundreds of suckers, things get complicated very quickly, as researchers have historically struggled to study this peripheral nervous system, as it’s called, within the octopus’s body. “The large size of the brain makes it both really exciting to study and really challenging,” says Z. Yan Wang, an assistant professor of biology and psychology at the University of Washington. “Many of the tools available for neuroscience have to be adjusted or customized specifically for octopuses and other cephalopods because of their unique body plans.” While each limb acts independently, signals are transmitted back to the octopus’s central nervous system. The octopus’ brain sits between its eyes at the front of its mantle, or head, couched between its two optic lobes, large bean-shaped neural organs that help octopuses see the world around them. These optic lobes are just two of the over 30 lobes experts study within the animal’s centralized brain, as each lobe helps the octopus process its environment. This elaborate neural architecture is critical given the octopus’s dual role in the ecosystem as both predator and prey. Without natural defenses like a hard shell, octopuses have evolved a highly adaptable nervous system that allows them to rapidly process information and adjust as needed, helping their chances of survival. Some similarities remain While the octopus’s decentralized nervous system makes it a unique evolutionary example, it does have some structures similar to or analogous to the human nervous system. “The octopus has a central brain mass located between its eyes, and an axial nerve cord running down each arm (similar to a spinal cord),” says Wang. “The octopus has many sensory systems that we are familiar with, such as vision, touch (somatosensation), chemosensation, and gravity sensing.” Neuroscientists have homed in on these similarities to understand how these structures may have evolved across the different branches in the tree of life. As the most recent common ancestor for humans and octopuses lived around 750 million years ago, experts believe that many similarities, from similar camera-like eyes to maps of neural activities, evolved separately in a process known as convergent evolution. While these similarities shed light on evolution's independent paths, they also offer valuable insights for fields like soft robotics and regenerative medicine. Occasionally, unique individuals—like an octopus with an unexpected number of limbs—can provide even deeper clues into how this remarkable nervous system functions and adapts. Nine arms, no problem In 2021, researchers from the Institute of Marine Research in Spain used an underwater camera to follow a male Octopus vulgaris, or common octopus. On its left side, three arms were intact, while the others were reduced to uneven, stumpy lengths, sharply bitten off at varying points. Although the researchers didn’t witness the injury itself, they observed that the front right arm—known as R1—was regenerating unusually, splitting into two separate limbs and giving the octopus a total of nine arms. “In this individual, we believe this condition was a result of abnormal regeneration [a genetic mutation] after an encounter with a predator,” explains Sam Soule, one of the researchers and the first author on the corresponding paper recently published in Animals. The researchers named the octopus Salvador due to its bifurcated arm coiling up on itself like the two upturned ends of Salvador Dali’s moustache. For two years, the team studied the cephalopod’s behavior and found that it used its bifurcated arm less when doing “riskier” movements such as exploring or grabbing food, which would force the animal to stretch its arm out and expose it to further injury. “One of the conclusions of our research is that the octopus likely retains a long-term memory of the original injury, as it tends to use the bifurcated arms for less risky tasks compared to the others,” elaborates Jorge Hernández Urcera, a lead author of the study. “This idea of lasting memory brought to mind Dalí’s famous painting The Persistence of Memory, which ultimately became the title of the paper we published on monitoring this particular octopus.” While the octopus acted more protective of its extra limb, its nervous system had adapted to using the extra appendage, as the octopus was observed, after some time recovering from its injuries, using its ninth arm for probing its environment. “That nine-armed octopus is a perfect example of just how adaptable these animals are,” Pelled adds. “Most animals would struggle with an unusual body part, but not the octopus. In this case, the octopus had a bifurcated (split) arm and still used it effectively, just like any other arm. That tells us the nervous system didn’t treat it as a mistake—it figured out how to make it work.” Kenna Hughes-Castleberry is the science communicator at JILA (a joint physics research institute between the National Institute of Standards and Technology and the University of Colorado Boulder) and a freelance science journalist. Her main writing focuses are quantum physics, quantum technology, deep technology, social media, and the diversity of people in these fields, particularly women and people from minority ethnic and racial groups. Follow her on LinkedIn or visit her website. 19 Comments

The bearded vulture (Gypaetus barbatus) is a very large bird of prey in the monotypic genus Gypaetus. It is vernacularly known as the Homa, a bird in Iranian mythology. The bearded vulture is the only known vertebrate whose diet consists of 70 to 90 per cent bone. It lives and breeds on crags in high mountains in Iran, southern Europe, East Africa, the Indian subcontinent, Tibet, and the Caucasus. The bearded vulture population is thought to be in decline; since 2014, it has been classified as near threatened on the IUCN Red List. Bearded vultures are 94 to 125 centimetres (37 to 49 inches) long, with a wingspan of 2.31 to 2.83 metres (7.6 to 9.3 feet). This bearded vulture was photographed carrying a piece of carrion in the Alps in Switzerland, where the species was reintroduced in the late 20th century after having become locally extinct in the early 20th century. Photograph credit: Giles Laurent Recently featured: London King's Cross railway station Daft Punk Eastern quoll Archive More featured pictures

The Fearsome Megalodon Ate Basically Whatever It Wanted to Reach Its Daily 100,000-Calorie Need, Study Suggests Scientists previously assumed the giant, prehistoric sharks mostly feasted on whales, but it turns out they probably weren’t so picky An artistic reconstruction of the extinct megalodon. Scientists' ideas about how the megalodon looked are based on its fossilized teeth. Hugo Saláis via Wikimedia Commons under CC BY 4.0 Between 3 million and 20 million years ago, the largest predatory fish ever known hunted in Earth’s oceans. Called theOtodus megalodon), this giant shark grew up to 79 feet long, had teeth the size of human hands and could bite with the strength of an industrial hydraulic press. But what scientists know about the extinct creature has been almost entirely determined from fossil teeth—since paleontologists have yet to discover a complete megalodon, and the animals’ cartilaginous skeletons don’t preserve well. Now, new research on the mineral content of their teeth suggests megalodons ate pretty much whatever they wanted. Until recently, scientists assumed that megalodons satisfied their estimated 100,000-calorie daily needs by mostly eating whales. A study published Monday in the journal Earth and Planetary Science Letters, however, suggests the prehistoric shark had a much more diverse diet than previously thought—akin to the great white shark’s “if it moves, it’s food” hunting strategy of today, writes Vice’s Ashley Fike. Jeremy McCormack with a fossilized megalodon tooth. Uwe Dettmar for Goethe University An international team of researchers reached this conclusion after analyzing the ratio of different variants, called isotopes, of the mineral zinc in 18-million-year-old megalodon teeth. Animals absorb zinc only through food, so this could offer a hint to their diets. Muscles and organs absorb more of the isotope zinc-64 than zinc-66, meaning that the higher up the food chain an animal is—or the more meat and fish it eats—the less zinc-66 it absorbs, and its ratio of zinc-66 to zinc-64 is lower, in turn. “Since we don’t know how the ratio of the two zinc isotopes at the bottom of the food pyramid was at that time, we compared the teeth of various prehistoric and extant shark species with each other and with other animal species. This enabled us to gain an impression of predator-prey relationships 18 million years ago,” Jeremy McCormack, a scientist from Goethe University Frankfurt and lead author of the study, says in a statement. Unsurprisingly, the isotope ratios in the teeth put the megalodon at the top of the food chain, alongside close shark relatives such as Otodus chubutensis. At the same time, however, the scientists noticed there wasn’t a huge difference between the megalodon and the lower-tiered animals, suggesting the sharks feasted on creatures from all rungs of the ladder. “They were not concentrating on certain prey types, but they must have fed throughout the food web, on many different species,” McCormack tells CNN’s Jacopo Prisco. “While certainly this was a fierce apex predator, and no one else would probably prey on an adult megalodon, it’s clear that they themselves could potentially feed on almost everything else that swam around.” The results also indicate that megalodon populations living in different habitats had slightly contrasting diets, potentially because of differing prey availability. More broadly, the study invites comparisons between the megalodon and its iconic extant relative, the great white shark. These comparisons, however, may have previously led to some overreaching assumptions. “Previous studies simply assumed that megalodon must have looked like a gigantic version of the modern great white shark without any evidence,” Kenshu Shimada, a vertebrate paleontologist at DePaul University and co-author of the new study, told National Geographic’s Jason Bittel back in March. He and colleagues had just published a different paper that reassessed the prehistoric shark’s size, suggesting that it had a more slender body than its smaller, modern cousin. The new study thus joins a host of research challenging widely held ideas about megalodons and their relatives, says Alberto Collareta, a paleontologist at the University of Pisa in Italy who was not involved in the research, to CNN. “These have led us to abandon traditional reconstruction of the megatooth sharks as ‘inflated’ versions of the modern white shark. We now know that the megalodon was something else—in terms of size, shape and ancestry, and of biology, too,” he adds. In fact, with both species eating generalist diets, great white sharks might have outcompeted megalodons for food and ultimately played a role in their demise. “Even ‘supercarnivores’ are not immune to extinction,” Shimada says in the statement. Get the latest stories in your inbox every weekday.

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.

Comparison of a megalodon tooth and a great white shark tooth, not associated with the study. (Image Credit: Mark_Kostich/Shutterstock) NewsletterSign up for our email newsletter for the latest science newsMegalodon teeth have always been key to understanding the ancient marine predator. Fossilized teeth are all that remain to prove the existence of these massive sharks, and the name megalodon is from the Greek for “big tooth.”A new study, published in Earth and Planetary Science Letters, highlights the importance of the megalodon’s human-hand-sized teeth once again. Thanks to extracting and analyzing the traces of zinc left in the fossilized teeth, researchers now know that the megalodon’s diet was much broader than scientists once believed.“Megalodon was by all means flexible enough to feed on marine mammals and large fish, from the top of the food pyramid as well as lower levels – depending on availability,” said Jeremy McCormack from the Department of Geosciences at Goethe University, in a press release.What Did the Megalodon Eat?Clocking in at 78 feet in length and weighing about twice as much as a semi truck, the megalodon was a big fish with a big appetite. It is suggested that a member of the Otodus shark family would require about 100,000 kilocalories per day to survive. Due to this extreme number, scientists have often assumed that the megalodon’s main source of calories came from whales.This new study suggests that whales were not the only item on the megalodon’s daily menu and that these sharks were actually quite adaptable when it came to their food. The research team analyzed 18-million-year-old giant teeth that came from two fossil deposits in Sigmaringen and Passau. What they were looking for was the presence of zinc-66 and zinc-64, two isotopes commonly ingested with food. Typically, the higher up in a food pyramid an animal is, the lower the presence of zinc. As they are oftentimes at the top of the food chain, species such as Otodus megalodon and Otodus chubutensis have a low ratio of zinc-66 to zinc-64 compared to species lower on the food chain.“Sea bream, which fed on mussels, snails, and crustaceans, formed the lowest level of the food chain we studied,” said McCormack in the press release. “Smaller shark species such as requiem sharks and ancestors of today’s cetaceans, dolphins, and whales, were next. Larger sharks, such as sand tiger sharks, were further up the food pyramid, and at the top were giant sharks like Araloselachus cuspidatus and the Otodus sharks, which include megalodon.”Surprisingly, the zinc levels in the megalodon teeth weren’t always that different from the zinc levels in species lower down the food chain. This result means that the commonly held scientific belief that megalodons focused their attention on eating large marine mammals may be incorrect. Instead, McCormack refers to the megalodon as an “ecologically versatile generalist” that adapted to environmental and regional constraints that changed the availability and variety of their prey.A New Method in Teeth TestingUsing the zinc content of fossilized teeth is a relatively new method of analysis, and the research team working on the megalodon couldn’t be happier with their results. The methods used in this study have not only been used for prehistoric shark and whale species but also modern-day shark species, and have even been used on herbivorous prehistoric rhinoceroses.Overall, these new methods have begun to rewrite the history of megalodon’s eating habits and may help to explain more about why these giants of the food chain went extinct. “[Determining zinc isotope ratios] gives us important insights into how the marine communities have changed over geologic time, but more importantly the fact that even ‘supercarnivores’ are not immune to extinction,” said Kenshu Shimada, a paleobiologist at DePaul University and a coauthor of this study, 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:Earth and Planetary Science Letters. Miocene marine vertebrate trophic ecology reveals megatooth sharks as opportunistic supercarnivoresAs the marketing coordinator at Discover Magazine, Stephanie Edwards interacts with readers across Discover's social media channels and writes digital content. Offline, she is a contract lecturer in English & Cultural Studies at Lakehead University, teaching courses on everything from professional communication to Taylor Swift, and received her graduate degrees in the same department from McMaster University. You can find more of her science writing in Lab Manager and her short fiction in anthologies and literary magazine across the horror genre.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

Nature, Published online: 21 May 2025; doi:10.1038/s41586-025-08944-wRe-examination of the presumed Cambrian fossil fish Anatolepis reveals previous misidentification of aglaspidid sensory structures as dentine, a vertebrate sensory tissue, showing it to be an arthropod, and shifting the origin of vertebrate hard tissues to the Middle Ordovician.

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

CT scan of the front of a skate, showing the hard, tooth-like denticles (orange) on its skinYara Haridy Teeth first evolved as sensory organs, not for chewing, according to a new analysis of animal fossils. The first tooth-like structures seem to have been sensitive nodules on the skin of early fish that could detect changes in the surrounding water. The finding supports a long-standing idea that teeth first evolved outside the mouth, says Yara Haridy at the University of Chicago. Advertisement While there was some evidence to back this up, there was an obvious question. “What good is having all these teeth on the outside?” says Haridy. One possibility was that they served as defensive armour, but Haridy thinks there was more to it. “It’s great to cover yourself in hard things, but what if those hard things could also help you sense your environment?” True teeth are only found in backboned vertebrates, like fish and mammals. Some invertebrates have tooth-like structures, but the underlying tissues are completely different. This means teeth originated during the evolution of the earliest vertebrates: fish. Haridy and her team re-examined fossils that have been claimed to be the oldest examples of fish teeth, using a synchrotron to scan them in unprecedented detail. Unmissable news about our planet delivered straight to your inbox every month. Sign up to newsletter They focused first on fragmentary fossils of animals called Anatolepis, which date from the later part of the Cambrian Period, which ran from 539 million to 487 million years ago, and early in the Ordovician Period, which ran from 487 million to 443 million years ago. These animals had a hard exoskeleton, dotted with tubules. These had been interpreted as being tubules of dentine, one of the hard tissues that make up teeth. In human teeth, dentine is the yellow layer under the hard white enamel and it performs many functions, including sensing pressure, temperature and pain. This led to the idea that the tubules are precursors to teeth called odontodes and that Anatolepis is an early fish. That isn’t what Haridy and her team found. “We saw that the internal anatomy [of the tubules] didn’t actually look like a vertebrate at all,” she says. After examining structures from a range of animals, they found that the tubules were most similar to features called sensilla found on the exoskeletons of arthropods like insects and spiders. These look like pegs or small hairs and detect a range of phenomena. “It can be everything from taste to vibration to changes in air currents,” says Haridy. This means Anatolepis is an arthropod, not a fish, and its tubules aren’t the direct precursors to teeth. “Dentine is likely a vertebrate novelty, yet the sensory capabilities of a hardened external surface were present much earlier in invertebrates,” says Gareth Fraser at the University of Florida in Gainesville, who wasn’t involved in the study. With Anatolepis out of the picture, the team says, the oldest known teeth are those of Eriptychius, which is only known from the Ordovician Period. These do have true dentine – in odontodes on their skin. Haridy says invertebrates like Anatolepis and early vertebrates like Eriptychius independently evolved hard, sensory nodules on their skin. “These two very different animals needed to sense their way through the muck of ancient seas,” she says. In line with this, the team found that the odontodes on the skin of some modern fish still have nerves – suggesting a sensory function. Once some fish became active predators, they needed a way to hold onto their prey, so the hard odontodes made their way to the mouth, where they could be used to bite. “Based on the available data, tooth-like structures likely first evolved in the skin of early vertebrates, prior to the oral invasion of these structures that became teeth,” says Fraser. Journal reference:Nature DOI: 10.1038/s41586-025-08944-w Topics: