Previously healthy Yosemite toads leave their winter burrows infected with chytrid fungus
UCLA biologists found chytrid infections growing in Yosemite toads during winter hibernation on land, in a three-year survey of 1,800 samples. That makes the months a threatened species spends underground part of the high-risk season for the fungus.
The Scientist · Science desk
What happened
- From 2021 to 2023, researchers from UCLA and Yosemite National Park swabbed young toads near Tioga Pass in May, in summer and in September, just before hibernation.
- They returned during early snowmelt the following spring and swabbed the same toads again as they emerged.
- Biologists had assumed the toads caught Bd mainly during the brief breeding season in water, since the fungus was thought to spread mostly there.
- The Yosemite toad was listed as threatened under the Endangered Species Act in 2014 and is a highest-priority species of special concern in California.
- Toads reared at the San Francisco Zoo were being returned to the wild, though how Bd affected that effort was unknown.
Why it matters
- Who has to decide decision The Yosemite reintroduction program, already informed by the result, has to plan for Bd growth during overwintering as well as exposure in breeding pools.
- Who's exposed exposure Winter growth was strongest in metamorphs, so Bd presses hardest on young toads in a species where fewer tadpoles already survive to adulthood.
- What it limits constraint Until burrow transmission is separated from growth of infections carried in undetected, managers cannot tell whether to target the burrows or the toads' infection status before hibernation.
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The strongest feature of the design is that the September and spring swabs came from the same animals [11]. Between those two swabs the toads were underground, away from any breeding pool, so a rise in infection between them happened on land. Sampling different toads in each season could not have excluded a simpler explanation: that the spring animals were a sicker group from the start.
Counting both surveys, the team collected 1,800 samples over three years at six sites [12]. That averages about 100 per site per year [20], spread across every life stage. The swabs were screened with PCR, the same kind of test used for COVID [13]. The phys.org report describes a high infection rate in toads that had been healthy [1], but it does not give prevalence or fungal-load figures, so the size of the winter increase cannot be judged from it.
The thing this doesn't tell you is where the winter infections begin. According to the report, the researchers do not know for sure that toads catch new infections in the burrow. The animals may go underground carrying an infection too light to detect [19]. A toad under the detection limit in September and over it in spring looks like a new case either way. "It certainly appears that the Bd fungus has the ability to move between individuals," said Brad Shaffer, a UCLA conservation biologist and co-author [17]. I think the paired swabs show when the fungus multiplies more firmly than they show how a toad acquires it.
The temperature result is the odder one. Bd had been thought to grow best in moderate conditions [16]. "The ability to increase in number on land at super low temperatures is a real surprise," Shaffer said [18]. "Although Bd is a largely aquatic pathogen, we discovered that major outbreaks of infection grow and spread during winter hibernation on land," said Dave Daversa, the UCLA ecologist who is first author of the paper in Functional Ecology [3][4].
All of this comes from one species. The Yosemite toad, Anaxyrus canorus, is 1 to 3 inches long and lives almost entirely in Yosemite and Sequoia and Kings Canyon national parks [5]. It breeds in small pools but spends much of its life in abandoned rodent burrows, and it hibernates in those burrows through the High Sierra winter [6].
What to watch
- Sampling of burrow soil or of toads inside burrows that detects Bd directly, which would separate transmission underground from the growth of light infections carried in.
- Any change the Yosemite captive-rearing program makes to release timing or pre-hibernation testing in response to the winter result.
- Whether winter proliferation of Bd turns up in other amphibians that hibernate on land at low temperatures.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence72
- Adoption30
- Hype gap+15
- Incentives
- Not scored
- Confidence68
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
UCLA biologists found a high Bd infection rate in previously healthy Yosemite toads when they emerged from winter hibernation, suggesting the fungus proliferates in their underground burrows during the Sierra winter.
- [2]
The finding has informed a captive-rearing and reintroduction program at Yosemite.
- [3]
"Although Bd is a largely aquatic pathogen, we discovered that major outbreaks of infection grow and spread during winter hibernation on land."
- [4]
Dave Daversa is the first author of the paper, published in Functional Ecology.
- [5]
The Yosemite toad (Anaxyrus canorus) lives almost exclusively in Yosemite and in Sequoia and Kings Canyon national parks and is about 1 to 3 inches long.
- [6]
Yosemite toads breed in small pools but spend much of their time in abandoned rodent burrows, and they survive High Sierra winters by hibernating in those burrows.
- [7]
Climate change is making the toads' habitats drier and hotter, fewer tadpoles are growing to adulthood, and Bd infection, which is fatal, is widespread.
- [8]
The Yosemite toad was listed as threatened under the Endangered Species Act in 2014 and by California as a highest-priority species of special concern.
- [9]
Toads were being reared at the San Francisco Zoo and returned to the wild to restore the population, but it was not known how Bd might be complicating these efforts.
- [10]
From 2021 to 2023, researchers from UCLA and Yosemite National Park captured metamorphs near Tioga Pass and swabbed them right after emergence from hibernation in May, during the summer, and again before hibernation in September.
- [11]
The researchers returned the following spring during early snowmelt and sampled the same toads as they emerged from hibernation.
- [12]
A parallel survey sampled toads at all life-cycle phases, for a total of 1,800 samples over three years and six sites.
- [13]
The swabs were screened for Bd using established techniques, including PCR tests similar to those used to test for COVID.
- [14]
The results identified the winter months of underground dormancy as the key season of rapid Bd proliferation, particularly in metamorphs.
- [15]
Yosemite toads were thought to be at lower risk because Bd was thought to spread mainly in water and the toads spend most of their time on land; they were believed to contract the fungus only during the brief mating season, when they are primarily aquatic.
- [16]
Bd was thought to grow best in more moderate conditions.
- [17]
"It certainly appears that the Bd fungus has the ability to move between individuals."
- [18]
"The ability to increase in number on land at super low temperatures is a real surprise."
- [19]
The researchers do not know for sure that toads contract new infections in the burrow; the toads could go underground with a very light, undetectable infection.
- [20]
The surveys averaged about 100 samples per site per year.
Sources
2 independent publishers whose own reporting we read for this story.
- phys.orgYosemite toad killer may be hiding in their own homes
1 article · October 1, 2026
- sciencenews.orgYoung Yosemite toads survive winter, then emerge with a deadly fungus
1 article · October 2, 2026
Topics and entities
Topics
- Amphibian chytrid fungus
- Endangered species conservation
- Wildlife disease ecology
Entities
- University of California, Los Angeles
- Yosemite National Park
- Jamie Lloyd-Smith
- San Francisco Zoo
- Batrachochytrium dendrobatidis
- Functional Ecology