A toad’s winter refuge could be a biohazard.
Scientists have discovered that Yosemite toads (Anaxyrus canorus) are facing a critical threat from the deadly chytrid fungus Batrachochytrium dendrobatidis ( Bd ), which continues to devastate amphibian populations worldwide. The research, published in Functional Ecology on September 23, reveals a puzzling pattern: while the fungus typically spreads through water contact, unexpectedly survives throughout long periods of winter underground—a bunker where dozens of toads share space during harsh cold months.
Bd poses a severe threat to skin function, blocking electrolyte absorption and frequently leading to heart failure. Since at least 500 species have declined due to chytridiomycosis and 90 possible extinctions have been estimated based on early 2019 data, affected toads represent one component of a broader ecological crisis.
To address this urgent concern, Daversa and his team conducted extensive sampling across Yosemite National Park, capturing toads of all life stages—including juveniles before and after their first winter underground—and swabbing their skin for fungal detection.
Contrary to expectations, the outbreak revealed a surprising trend. Juvenile toads exhibited significantly higher infection rates after their initial winter compared to adult breeding pairs. On average, approximately 23 percent of juvenile toads tested positive for Bd prior to their first brumation period. However, by spring emergence, over 90 percent of those young individuals carried either moderate to severe infections, while less than half of adult breeding adults showed detectable fungus loads.
“We know it’s still unclear why Bd thrives so effectively below ground,” explained David Daversa, an ecologist at UCLA who led the study. “It remains a waterborne pathogen, yet we observe it persisting through prolonged periods of cold dormancy. One possibility is that juvenile toads harbor undetectable infections during winter that gradually develop, or that collective burrowing conditions facilitate fungal exchange among crowded groups.”
The authors speculate that conservation strategies may benefit from targeting this vulnerable pre-adolescent stage. Rather than focusing solely on established breeding populations, future initiatives—exemplified by a joint reintroduction effort between the San Francisco Zoo and the U.S. National Park Service—now prioritize releasing toads that have survived multiple winter cycles without establishing chronic infections.
“The most promising window for intervention appears to be these early post-winter months, when Bd proliferates most aggressively,” noted Daverse. “If we can interrupt the fungus at this developmental stage—if we prevent infection during those first vulnerable winters—it might halt the spread entirely.”
This work underscores a pressing need to extend monitoring beyond aquatic breeding sites and into terrestrial habitats where toads shelter during harsh seasons, offering hope for protecting a species currently designated as threatened under the U.S. Endangered Species Act.