Museum Frogs Solve a 50-Year Mystery Behind a Brazilian Die-Off

For nearly fifty years, biologists who worked at Brazil's Boraéia Biological Station have had little more than a puzzle on their hands: between 1979 and 1982, frog populations in that stretch of the Atlantic Forest collapsed, and no one could say with confidence why. Some blamed drought or cold spells. Others suspected a disease that, at the time, had not even been identified by science.

A study published October 14 in Molecular Ecology has now supplied an answer. Researchers from the Smithsonian's National Zoo and Conservation Biology Institute and Pennsylvania State University tested preserved frogs collected before, during and after the crash and found that the chytrid fungus Batrachochytrium dendrobatidis, known as Bd, arrived in the area at exactly the moment the die-off began. The disease it causes, chytridiomycosis, attacks the skin of amphibians and disrupts the way they regulate water and electrolytes, often with fatal results.

What the Frozen Frogs Revealed

The team drew on more than 370 preserved specimens from 17 frog species, all gathered at Boraéia by William Ronald Heyer, a former curator of amphibians and reptiles who led field expeditions there in the 1970s and 1980s and whose collections are held at the Smithsonian's National Museum of Natural History. Researchers swabbed the specimens for fungal DNA and used a technique called droplet digital PCR, cross-checking results against skin tissue examined under a microscope at the University of Florida, as a separate account of the study also noted.

The pattern was stark. None of the frogs collected before 1979 carried a trace of the fungus. By the 1980s, more than half of those tested came back positive. Oddly, the species that crashed hardest in the wild showed almost no detectable infection among the specimens that had been collected — while several species whose numbers held steady were riddled with it. The researchers' explanation is that the most vulnerable frogs likely died too fast to be caught and preserved in large numbers, while hardier species carried the fungus and survived.

• More than 370 museum specimens from 17 species were tested, spanning field trips from the 1970s and 1980s.
• Zero infections were detected in frogs collected before 1979; more than half of 1980s specimens tested positive.
• Genetic analysis places the arrival of the globally invasive Bd lineage at Boraéia between 1979 and 1982, the exact window of the recorded die-off.
• Follow-up surveys between 2022 and 2024 found 12 of the original 17 species still present, with infection rates now similar between species that once crashed and those that never did.

A Pathogen That Outran Its Own Discovery

The chytrid fungus is thought to have originated in Asia and spread around the world through the 1970s and 1980s, hitchhiking on traded amphibians and contaminated water. It was not formally identified and described by scientists until 1999 — two decades after it is now believed to have swept through Boraéia, and after it had already devastated amphibian populations on several continents. That lag helps explain why so many historical die-offs, including this one, went unsolved for so long: there was no test for a pathogen nobody yet knew existed.

The scale of the damage the fungus has done worldwide is well established. A 2019 global assessment published in Science linked chytridiomycosis to declines in at least 501 amphibian species and the presumed extinction of 90 of them, calling it the most destructive pathogen ever recorded for vertebrate biodiversity. Brazil's Atlantic Forest, a narrow coastal strip reduced to a fraction of its original extent by deforestation, is one of the planet's richest amphibian hotspots and had already lost an unusually large share of its frog diversity before the cause was ever pinned down.

"After almost 50 years, we still aren't confident what caused many historical amphibian die-offs, making these declines some of the most enduring cold cases in wildlife epidemiology," said Jack Boyette, a graduate fellow at the Smithsonian's National Zoo and Conservation Biology Institute and a doctoral candidate at Pennsylvania State University, who led the study.

Conservation Scientists and a Reservoir of Frozen Evidence

The findings matter most directly to amphibian biologists and conservation managers trying to understand which populations are vulnerable to chytrid and which have developed some tolerance — information that shapes decisions about captive breeding, reintroduction and where to focus limited conservation funding. They also matter to museums, whose decades-old specimen jars are increasingly being mined as genetic archives for pathogens that were invisible to the people who first collected them.

Michael Campana, a genomicist at the Smithsonian's National Zoo and Conservation Biology Institute who worked on the study, framed the find as a vindication of that kind of collecting. "Museum collectors could not have anticipated how important their collections would be to our modern conservation challenges, but without them, we would never be able to detect pathogens that went unnoticed for decades and reconstruct disease outbreaks that would otherwise remain a mystery," he said, according to the Smithsonian's announcement of the research.

Boyette cautioned that the picture at Boraéia is not simply one of villain and victims. "It's important to keep in mind that not all species suffered catastrophic losses at Boraéia," he said. "The species that didn't decline may have naturally tolerated the fungus." That distinction — tolerance versus susceptibility — is central to why, decades later, a return expedition found a largely intact frog community rather than an empty forest.

What Comes Next

Between 2022 and 2024, Boyette's team made six return visits to Boraéia with researchers from the University of São Paulo, swabbing hundreds of live frogs. They located 12 of the 17 species Heyer had originally documented; the other five remain unaccounted for, their status unresolved. Roughly 29.5 percent of frogs sampled now carry Bd, a rate that no longer distinguishes historically hard-hit species from those that never declined — a sign, the authors argue, that amphibian populations and the fungus have reached something closer to an uneasy truce.

The researchers caution against extrapolating too far: their data describe one site, not the entire Atlantic Forest, and Heyer's original collecting trips were not designed with statistical sampling in mind. Still, the team plans to extract and sequence DNA from the historical specimens to look for genetic signatures of disease resistance, and to pursue environmental DNA techniques that could fill in gaps at sites where no preserved specimens exist. The Smithsonian also flagged a related concern: a cousin pathogen, Batrachochytrium salamandrivorans, is already devastating salamander populations in Europe and has raised alarm among biologists who fear it could eventually reach North America, home to the world's greatest salamander diversity.