Scientists Found 8,000 Frog Bones at a Roman-Era Site in Britain—and They Finally Know Why

For all the fantastic things the Romans left behind, they were also up to some really weird things. So when thousands of ancient frog bones turned up at a known settlement from Roman Britain, scientists were puzzled but figured there was a reason. And the culprit isn’t (unfortunately) a cryptic frog cult from Roman times. A recent study by zooarchaeologists, published in Environmental Archaeology, studied the frog bones in closer detail, putting an end to what scientists call an ancient “frog mystery.” The culprits were birds of prey, specifically short-eared owls and hen harriers. But the story doesn’t end there. According to the study, people didn’t directly interact with either the frogs or the birds, but human activity at the Fens over centuries probably created conditions that resulted in this extraordinary ditch of frog bones. So, on the flip side, the discovery points to new insights on human settlements in the area. “The mystery of these frogs has been solved—a feast of frogs that were eaten and their bones left behind by a group of owls,” Paul Clarkson, the study’s first author and a zooarchaeologist at Bournemouth University in the U.K., said in a statement. A frog murder mystery According to Bournemouth University, researchers with the Museum of London Archaeology (MOLA) first discovered the bones between 2016 and 2018. MOLA shared details about the excavation in 2022, explaining that its team had retrieved more than four metric tons of animal bones from a site at the Fens in Cambridgeshire. According to the latest paper, the site was dated to the Middle to Late Iron Ages (400 BCE to 43 CE) but also showed signs of increased activity later in the Roman period, as improving climate and retreating water levels made the area more livable for humans. One particularly striking discovery came from a single settlement, which turned up more than 8,000 amphibian bones representing at least 350 frogs and toads. The bones were inside a 46-foot (14-meter) long ditch that seems to have been part of a roundhouse. Frog bones aren’t unheard of in the archaeological record, but the sheer volume of the assemblage stood out, MOLA explained. Even stranger, parts of the site closer to the rivers of the region didn’t have that many amphibian bones, it added. What made them croak? That said, the team didn’t immediately jump to anthropogenic causes. For one, the bones didn’t have any cut marks or signs of cooking, so it was unlikely humans ate them. At the time, the researchers couldn’t find teeth marks or signs of digestion on the bones, so it was unclear whether other predators were responsible. Archaeological evidence also showed the ditch was filled within a single year. Still, humans may have been indirectly responsible for these frogs, MOLA explained. The team found charred grains near the roundhouse, meaning humans processed crops here and potentially attracted pests—a food source for frogs. Alternatively, migrating frogs may have fallen into a ditch of the roundhouse. Or maybe that’s all wrong, and more natural causes, like a bad winter or disease, were responsible. An ‘owl’s eye view’ The latest analysis affirms some of those theories, while presenting new evidence for others. The team compared the damage to the frog bones with previous studies that identified breakage patterns depending on how an animal died. As a result, the researchers determined the most likely “culprits” of this frog murder case were short-eared owls and hen harriers. In addition, measuring the size of the frogs’ arm bones revealed most of them died young, specifically in the winter of their first year on Earth. Close-up images of the breakages and chemically damaged areas observed on the amphibian bones. © Clarkson et al., 2026 Radiocarbon dating found that the remains were roughly 1,700 years old, which corresponds to the Roman period in Britain. This is an important detail, as that was when the retreating waters at the Fens created ideal conditions for frogs to breed, the team explained in the statement. At the same time, grazing livestock made the landscape more open, making it easier for grassland predators to spot the frogs. Another side of the story The prime suspect, the short-eared owl. © Sumeet Moghe via Wikimedia Commons (CC BY-SA 4.0) It’s still unclear how and why the frogs and other animal bones ended up in that one ditch, the team admitted in the study. But if short-eared owls—who prefer quiet roosting sites—were responsible for the frog murders, it could mean that humans rarely visited this area in the autumn and winter the frogs died, it added. In other words, it’s possible that humans visited this region on a seasonal basis to let their livestock graze. “These bones not only tell us about life for people living near the Roman Fens, but they also give insights into the natural world—what wild animals were getting up to in the past, and what life was like for them,” Clarkson concluded.