Yosemite toads emerged from their first winter dormancy with Batrachochytrium dendrobatidis (Bd)—the chytrid fungus—detected far more often than before they went underground. A 2026 field study found prevalence in first-year toads was more than fourfold higher after dormancy. But researchers did not observe transmission inside snow-covered burrows, so how the infections increased remains unknown.
What the Yosemite toad study found
The study followed Yosemite toads (Anaxyrus canorus) at six sites in the Tioga Pass area of Yosemite National Park from 2021 to 2023, analyzing more than 1,800 samples. The authors reported that Bd prevalence in first-year toads was more than fourfold higher immediately after their first terrestrial winter dormancy than immediately before it. Post-dormancy peaks in prevalence and infection intensity appeared across multiple sites and years. Daversa et al., Functional Ecology, first published September 23, 2026.
As a secondary account of the study, Science News reported that about 23 percent of juveniles tested positive before their first brumation and more than 90 percent afterward. Fewer than half of breeding adults tested positive. These percentages are figures from that report; the primary paper’s central comparison is the more-than-fourfold increase in first-year toads.
Did Bd spread inside the toads’ winter burrows?
The field results establish a rise in detected infection across the winter interval, not direct transmission beneath the snow. Researchers sampled toads before and after dormancy, but did not sample them inside their underground refuges or measure conditions there. The study therefore cannot determine whether Bd spread among toads during dormancy, intensified within animals that were already infected, or did both.
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Infections may have intensified below detection
Swab testing and quantitative PCR (qPCR) surveillance can miss very low-level or cryptic infections. One possibility is that some toads carried Bd before winter at levels too low to detect, then developed infections detectable after dormancy. The paper calculates that, assuming uniform sampling, more than 600 pre-dormancy metamorphs would have needed cryptic infection to explain the observed post-dormancy prevalence through within-host intensification alone. That is a conditional calculation, not a count of animals known to have been infected.
New infections may have occurred during dormancy
Another possibility is transmission from a contaminated refuge surface or between toads sharing moist, crowded burrows. The authors say environmental transmission is difficult to assess because evidence that Bd persists outside hosts in non-sterile terrestrial substrates is weak. They cannot rule it out, however, because refuge conditions were not measured. In-vitro Bd growth is thought to stop at very low temperatures, but how the fungus behaves in a host—and how a host’s behavior affects its temperature—may differ.
Why the result is striking
Yosemite toads breed in seasonal pools, while young toads spend winter in underground terrestrial refuges beneath snow. The post-dormancy infection peak coincided with the aquatic breeding season, yet breeding adults had fewer and weaker infections than terrestrial juveniles. Because the groups differ in life stage and sampling window, this comparison is not a controlled test of why juveniles had heavier infections.
“Although Bd is a largely aquatic pathogen, we discovered that major outbreaks of infection grow and spread during winter hibernation on land,” said David Daversa, the study’s first author, in a UCLA Newsroom account published October 1, 2026. The study’s seasonal pattern challenges the assumption that important Bd increases occur only in aquatic settings; it does not establish exactly where or how the fungus proliferated.
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The field surveys did not observe illness or deaths. The paper notes that infection intensities measured after dormancy are at levels known to be lethal in Yosemite toads and other hosts, making hidden mortality plausible. It remains unknown whether Bd kills metamorphs during dormancy, and the study does not establish a winter death toll.
What the finding means for Yosemite toad conservation
The Yosemite toad is federally threatened and a California Species of Special Concern. UCLA reports that the finding informed its captive-rearing and reintroduction program: releases now focus on toads that have reached maturity after several years of growth, rather than on metamorphs or younger animals. This is the reported response of that program, not a universal recommendation for amphibian releases. UCLA Newsroom, October 1, 2026.
What researchers still need to find out
- Whether infections intensify within individual toads, spread from refuge surfaces, pass between animals, or involve a combination of routes.
- Which behaviors, immune conditions, or environmental factors allow Bd to increase during dormancy.
- Whether Bd causes metamorph mortality during the winter itself.
Answering those questions requires evidence from the refuges and from individual hosts across dormancy. Until then, the strongest conclusion is specific: first-year Yosemite toads in this study had substantially higher Bd prevalence after their first terrestrial winter, while the mechanism and consequences during winter remain unresolved.
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