Deadly Fungus Spreads Among Young Yosemite Toads During Winter Hibernation

Young Yosemite toads emerge from their first winter underground heavily infected with a deadly fungus, according to research published September 23 in Functional Ecology. While scientists traditionally expected amphibians to contract the pathogen primarily in water, the findings reveal that a deadly skin fungus called Batrachochytrium dendrobatidis, or Bd, surges while the animals hibernate beneath snow.

Surprising Infection Rates Among Hibernating Toads

Amphibian populations worldwide have suffered severe declines due to Bd, which attacks skin and prevents electrolytes and fluids from entering the body. The disease is a primary factor behind the threatened status of Yosemite toads under the U.S. Endangered Species Act. These toads live in California’s Sierra Nevada mountains, spending roughly half the year underground in a hibernation-like state known as brumation.

To understand how a water-borne pathogen persists through long winters on land, researchers from the University of California, Los Angeles, traveled to Yosemite National Park. The team captured toads of various ages, including young toads before they entered burrows and after they emerged, swabbing their skin to test for Bd. The project surveyed toads across six high-elevation meadow areas near Tioga Pass between 2021 and 2023, analyzing more than 1,800 samples.

The results showed a stark contrast between expected aquatic transmission and underground reality. An average of roughly 23 percent of juvenile toads tested positive for Bd before their first brumation. In the spring, more than 90 percent of those young toads emerged from their burrows with infections. Bd was found in 76.9 percent of first-year juveniles and 60 percent of second-year juveniles at Lower Gaylor Meadow during the initial fortnight post-winter, whereas mature breeding adults displayed substantially lower infection levels.

Microscopic Pathogens Thriving in Freezing Temperatures

The data revealed exceptionally high fungal loads on animals emerging from the snow. At Dana Meadow, infection levels were measured in 18 young toads that emerged carrying upward of 600,000 copies of the specific Bd DNA segment. Levels of this magnitude are generally linked with disease and death in Yosemite toad juveniles, although none of those 18 animals looked obviously sick upon emergence.

Scientists expressed surprise at how the fungus multiplies in sub-zero alpine conditions.

It certainly appears that the Bd fungus has the ability to move between individuals. The ability to increase in number on land at super low temperatures is a real surprise,

H. Bradley Shaffer

Researchers remain uncertain whether young toads burrow with undetectable infections that grow over time or if the fungus spreads directly when many animals cluster together underground. It is also unclear whether other brumating amphibians face similar winter risks.

Conservation Strategies Shifting to Older Toads

The findings have already altered practical conservation measures in the region. A reintroduction program run by the San Francisco Zoo and the U.S. National Park Service aimed at boosting Yosemite toad populations now focuses on releasing mature toads that have already passed their initial, highly vulnerable years.

By looking beyond aquatic environments, conservationists hope to target the disease during early life stages.

could nip chytrid in the bud in these early life stages when it seems to proliferate most,

David Daversa