Plate Nº 11 · recorded October 1, 2026
Biology & EvolutionReported finding
Yosemite Toads Catch Deadly Fungus in Their Winter Burrows, Study Finds
UCLA biologists found that the deadly Bd fungus spreads in Yosemite toads during winter hibernation in underground burrows, overturning assumptions that infection happens mainly in water during breeding season.
By Nathan Brooks4 min read777 words
In brief
- UCLA researchers analyzed 1,800 skin samples from Yosemite toads across six sites between 2021 and 2023.
- The study identified winter dormancy in underground burrows, not the aquatic breeding season, as the key period of rapid Bd fungal growth.
- The finding has changed Yosemite's reintroduction program: captive-reared toads are now released only after several years of growth, reducing overwinter infection risk.
A silent killer of Yosemite toads may be hiding in their own homes. UCLA biologists have discovered that a lethal skin fungus, long assumed to attack the toads mainly in water, instead proliferates while the animals hibernate underground through the harsh Sierra Nevada winter.
The pathogen, called Batrachochytrium dendrobatidis or Bd, has been decimating amphibian populations around the world. Scientists have generally believed it infects amphibians through their skin during aquatic breeding periods. When the UCLA team tested Yosemite toads emerging from winter hibernation, however, they found surprisingly high infection rates in toads that had previously appeared healthy.
"The results of our Bd surveillance study of Yosemite toads were completely unexpected," said UCLA ecologist Dave Daversa, first author of a paper published in Functional Ecology. "Although Bd is a largely aquatic pathogen, we discovered that major outbreaks of infection grow and spread during winter hibernation on land."
A toad with a small range
The Yosemite toad (Anaxyrus canorus) lives almost exclusively in Yosemite and in Sequoia and Kings Canyon national parks. Adults measure just 1 to 3 inches (2.5 to 7.6 centimeters) and walk rather than hop. Females, which are larger than males, wear attractively mottled green skin, while males are drabber. The toads breed in small pools but spend much of their lives in abandoned rodent burrows, hiding from predators and riding out High Sierra winters in hibernation.
Their situation is precarious. Climate change is making their habitat drier and hotter, and fewer tadpoles survive to adulthood. Bd infection, which is fatal, is widespread. The federal government listed the toad as threatened under the Endangered Species Act in 2014, and California classifies it as a highest-priority species of special concern. Conservationists have been rearing toads at the San Francisco Zoo and releasing them into the wild, but until now nobody knew exactly how Bd might be undermining those efforts.
Swabbing toads at 10,000 feet
From 2021 to 2023, researchers from UCLA and Yosemite National Park tracked how Bd prevalence and severity changed across seasons and life stages. They captured metamorphs — juvenile toads that had recently completed their transformation from tadpoles — at high-altitude sites near Tioga Pass. The team took skin swabs in May, immediately after the toads emerged from hibernation, again during the summer, and once more before hibernation in September. The following spring, during early snowmelt, they sampled the same toads as they woke up.
A parallel survey covered toads at every life stage. In total, the researchers collected 1,800 samples over three years and six sites. They screened the swabs using established laboratory techniques, including PCR tests similar to those used to detect COVID-19.
"The COVID pandemic made it easier to explain to people I encountered along trails what I was doing when I was swabbing toads," Daversa noted.
The results pointed unambiguously to winter dormancy underground as the season of rapid Bd growth, especially in metamorphs. That came as a shock. Because Bd was thought to spread mainly in water, and these toads spend most of their lives on land, scientists had considered them relatively safe outside the brief breeding season. Bd was also believed to grow best in moderate conditions, not in freezing alpine winters.
"The ability to increase in number on land at super low temperatures is a real surprise," said co-author Brad Shaffer, a UCLA conservation biologist. "It certainly appears that the Bd fungus has the ability to move between individuals."
Open questions remain
The findings come with caveats. The researchers cannot yet confirm that toads pick up new infections inside the burrow. An alternative explanation is that toads enter hibernation with very light, undetectable infections that each individual then amplifies into a heavy fungal load over the winter. Direct observation is difficult because hibernating burrows sit under many feet of snow.
"This fungus isn't supposed to grow happily at such low temperatures," said co-author Jamie Lloyd-Smith, a UCLA wildlife disease ecologist. "We speculate there might be something going on with the immune system: the immune state of the animal might be going down even lower than the ability of the fungus to grow."
The team hopes follow-up studies will resolve these questions. In the meantime, the discovery has already changed practice on the ground. Releasing captive-reared toads at the metamorph stage would be cheaper and easier, but managers at Yosemite now hold animals for a few years of growth before release, when the toads face less risk from overwinter Bd infection.
Publication details: David R. Daversa et al., "Fungal Batrachochytrium dendrobatidis infections proliferate during amphibian terrestrial dormancy," Functional Ecology (2026). DOI: 10.1111/1365-2435.70444
via Phys.org Biology (Source)
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