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In a three-year field study of Yosemite toads in California’s Tioga Pass area, researchers found that infection with the amphibian chytrid fungus was more than four times as prevalent in first-year toads after their first winter dormancy as it was just before. The result links winter dormancy with a sharp rise in infection, but the researchers did not observe the fungus spreading inside snow-covered burrows, and the cause of the increase remains unknown.
What the study found
The pathogen is Batrachochytrium dendrobatidis (Bd), commonly called the amphibian chytrid fungus. The host in this study was the Yosemite toad (Anaxyrus canorus), a high-elevation California species—not frogs in general.
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Researchers surveyed toads at six sites in the Tioga Pass area of Yosemite National Park from 2021 to 2023, collecting more than 1,800 samples. In first-year toads, Bd prevalence was more than fourfold higher immediately after their first terrestrial winter dormancy than immediately before it. Infection prevalence and intensity also peaked after dormancy across multiple sites and years. The study appeared in Functional Ecology in 2026.
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Yosemite toads breed in seasonal pools, but after metamorphosis they spend winter in underground terrestrial refuges beneath snow. The post-dormancy infection peak coincided with the aquatic breeding season. Yet breeding adults sampled then had fewer and weaker infections than the terrestrial juveniles. These are comparisons between different life stages and sampling windows, not a controlled experiment showing that age alone caused the difference.
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Did Bd spread inside the burrows?
The field pattern shows that infection rose across the first winter dormancy; it does not show where or how that rise happened. The team did not sample toads inside their snow-covered refuges or track the animals’ temperatures there. As a result, the study cannot establish that Bd transmitted between toads underground.
Swab testing and quantitative PCR can fail to detect very low-level infections. One possibility is that some toads already carried Bd before winter, but infections were too faint to register and intensified during or after dormancy. The paper calculates that, under uniform sampling, more than 600 metamorphs would have needed such undetected infections for within-host intensification alone to account for the observed post-dormancy prevalence. That is a conditional estimate, not a measured count of infected animals.
Another possibility is new infection during dormancy—either from contaminated refuge material or through contact among animals 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 conditions inside the refuges were not measured. In-vitro Bd growth is thought to cease at very low temperatures, but that finding does not establish what happens in a living host under its natural winter conditions.
The study therefore leaves open whether the increase reflects intensification of infections already present, new transmission, or a combination. David Daversa, the study’s first author and a UCLA ecologist, described the finding as evidence that “major outbreaks of infection grow and spread during winter hibernation on land” in UCLA’s account of the study. The field data document the striking seasonal change; they do not directly trace transmission within burrows.
What is known about harm to the toads
The researchers did not observe illness or deaths during their field surveys. They note that the high Bd infection intensities measured after dormancy are at levels known to be lethal in Yosemite toads and other hosts, making hidden mortality a concern. But whether Bd kills metamorphs during winter dormancy—and how often—has not been established.
That distinction matters: a high infection rate among toads found after winter does not reveal how many animals, if any, died before they could be sampled. The study identifies a risk and a question for further work, not a winter death toll.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the finding matters for conservation
The Yosemite toad is federally threatened and designated a California Species of Special Concern, according to the study. UCLA reports that the findings informed its captive-rearing and reintroduction program: the program now focuses releases on toads that have reached maturity after several years of growth, rather than releasing metamorphs or younger animals. That is a reported response for this program, not a universal rule for amphibian releases.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11The researchers identify several questions that would help explain the pattern: whether infection intensifies within hosts or new transmission occurs; which behaviors, immune conditions, or refuge environments allow infection to rise; and whether Bd causes metamorph deaths during dormancy. The study also raises the possibility that dormancy-associated disease dynamics could matter in other wildlife, but it does not demonstrate the same risk in other amphibians or species.
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