Young Yosemite toads face an unexpected challenge during their first winter. Research suggests that infections caused by a widespread amphibian fungus become far more common after the toads emerge from their underground shelters in spring.
The findings highlight a lesser-known stage in the species’ life cycle and may help conservation teams improve protection for these vulnerable animals.
A Fungus That Affects Amphibians
The fungus Batrachochytrium dendrobatidis, known as Bd, infects amphibians through their skin. Severe infections can interfere with essential body functions, including the regulation of water and salts, and may cause serious illness.
Bd has contributed to declines in amphibian populations worldwide. Yosemite toads are of particular conservation concern because they occupy a limited range in California’s Sierra Nevada mountains and are listed as threatened. These toads reproduce in shallow pools during warmer months. When temperatures drop, they shelter underground and enter brumation, a period of reduced activity that can last several months.
What Researchers Discovered
Researchers studied Yosemite toads at six meadow sites between 2021 and 2023. They collected more than 1,800 samples to examine infection patterns across seasons and stages of life. The team used gentle skin swabs to test for Bd. By comparing young toads before their first winter dormancy with those sampled after emerging in spring, the researchers could assess how infection rates differed across the season.
Before winter, an average of approximately 23 percent of the young toads tested positive. After winter, the proportion exceeded 90 percent. This sharp increase appeared across multiple locations and years.
Why Might Infection Increase?
Scientists are considering several possible explanations. Young toads may enter their shelters with low levels of infection that are difficult to detect. During dormancy, the fungus could multiply until it becomes easier to identify when the animals emerge. Another possibility is that the fungus spreads between toads sharing underground shelters. Close contact during the winter months could contribute to the higher infection rates observed in spring.
Neither explanation has been confirmed. Studying these shelters is difficult because they can remain covered by snow for extended periods. Researchers also need to understand how seasonal changes in the toads’ activity and immune responses influence infection. Further research will help determine whether the increase mainly reflects infections becoming more detectable, transmission between animals, or a combination of factors.
What the Findings Mean for Conservation
The results could influence how conservation teams manage Yosemite toad populations. A reintroduction program involving the San Francisco Zoo and the National Park Service has shifted toward releasing more mature toads instead of younger individuals. Allowing toads to grow older before release requires additional time and resources, but it may help them pass through a vulnerable stage before returning to the wild.
The research suggests that winter dormancy is an important period for understanding fungal infections in young Yosemite toads. Infection rates rose sharply after their first winter, although the reasons for this pattern remain uncertain.
By investigating what happens inside underground shelters and monitoring young toads throughout the year, researchers may gain valuable information for future conservation work. Understanding this hidden stage could help improve efforts to protect a threatened species and support its long-term survival.
