Winter Burrows May Spread Deadly Fungus to Yosemite Toads

Young Yosemite toads emerge from their first winter underground carrying high rates of the deadly chytrid fungus Batrachochytrium dendrobatidis, according to a study published September 23 in Functional Ecology, revealing an unexpected vulnerability during cold-weather brumation.

Key Clinical Takeaways:
• Over 90 percent of young Yosemite toads tested positive for the chytrid fungus after spending their first winter underground, compared to roughly 23 percent before brumation.
• The pathogen, Batrachochytrium dendrobatidis, disrupts amphibian skin function, blocking essential electrolytes and fluids and frequently causing fatal heart failure.
• Conservation efforts by the San Francisco Zoo and the U.S. National Park Service have shifted focus to releasing mature toads that have cleared these vulnerable early life stages.

Infection Rates Among Juvenile Toads After Winter Underground

The fungus typically spreads through water, making its high prevalence in subterranean burrows an unexpected discovery for researchers studying the high-elevation amphibians. David Daversa, an ecologist at UCLA, noted the mystery of how a waterborne pathogen thrives during months spent under feet of snow in California’s Sierra Nevada mountains. To investigate this vector, Daversa and fellow researchers captured Yosemite toads across various age groups, swabbing their skin both prior to brumation and immediately after their spring emergence.

Testing revealed that while an average of roughly 23 percent of juvenile toads tested positive for the fungus before entering their winter burrows, that figure jumped to more than 90 percent upon their spring emergence. By contrast, less than half of mature breeding adults tested positive during the same period. The mechanisms driving this sharp increase remain unconfirmed. Researchers are evaluating whether young toads enter burrows carrying undetectable infections that multiply over time, or if the close quarters of shared winter burrows accelerate transmission among juveniles.

Pathological Impact of Chytrid Fungus on Amphibian Populations

The pathogen attacks the outer layers of amphibian skin, interfering directly with the transport of vital electrolytes and fluids into the body. This disruption places severe physiological stress on the animal, frequently culminating in heart failure. Historical estimates attribute population declines in at least 500 species and up to 90 possible extinctions to the disease. These widespread losses are a primary reason why Yosemite toads hold threatened status under the U.S. Endangered Species Act.

While mature adults spend active months breeding in shallow pools of water, the high-elevation environment requires all age groups to spend roughly half the year in a hibernation-like state underground. The discovery of high winter infection rates highlights a previously unrecognized high-risk window in the species’ lifecycle. It remains unknown whether other brumating amphibians encounter similar infection pressures during the winter months.

Conservation Strategies for Threatened Yosemite Toads

The findings have already altered practical conservation protocols for the species. A reintroduction initiative managed jointly by the San Francisco Zoo and the U.S. National Park Service now prioritizes releasing mature toads that have survived past the age groups most vulnerable to the fungus. By focusing efforts on older cohorts, conservationists aim to protect the population during the life stages where the pathogen proliferates most extensively.