Key takeaways
UCLA biologists have found the hiding place of a silent killer stalking threatened Yosemite toads: their own burrows. The Batrachochytrium dendrobatidis (Bd) fungus decimating amphibian populations around the world has long been thought to live in and infect the toads through their skin when they are in the water.
However, the researchers discovered a high infection rate in previously healthy toads when they emerged from winter hibernation, suggesting that the fungus proliferates in their underground burrows during the harsh Sierra winter. The finding has informed a captive rearing and reintroduction program at Yosemite.
“The results of our Bd surveillance study of Yosemite toads were completely unexpected,” said UCLA ecologist Dave Daversa, the 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. The discovery sharply contrasts with current thinking about the high-risk season for Bd infection, which has often been assumed to occur when amphibians are breeding in water. Our study flies in the face of that expectation.”
The Yosemite toad ( Anaxyrus canorus ) lives almost exclusively in Yosemite and in Sequoia and Kings Canyon national parks. They are only about 1 to 3 inches long and walk, rather than hop. Females are larger than males and have bumpy skin mottled attractively in varying shades of green, while males are less colorful. They breed in small pools but spend much of their time chilling (literally) and hiding from predators in abandoned rodent burrows. The toads survive harsh winters in the High Sierra by hibernating in their burrows.
But climate change is making their habitats drier and hotter, and fewer tadpoles are growing to adulthood. Infection with Bd — which is fatal — is widespread. As a result, 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. The toads were being reared at the San Francisco Zoo and returned to the wild to restore the population, but it wasn’t known how Bd might be complicating these efforts.
From 2021–23, a group of researchers from UCLA and Yosemite National Park surveyed toads to determine how Bd prevalence and severity varied across seasons and life stages, comparing them before and after cohorts of toads entered winter dormancy. They captured metamorphs — juvenile toads that recently completed metamorphosis — from high altitude locations near Tioga Pass and took skin swabs immediately after emerging from hibernation in May, during the summer and again before hibernation in September. They returned the following spring during early snowmelt and sampled the same toads as they emerged from hibernation. In a parallel survey, they sampled toads at all life-cycle phases, for a total of 1,800 samples over three years and six sites.
They screened the swabs for Bd using established techniques, including PCR tests similar to the ones used to test for COVID.
“The COVID pandemic made it easier to explain to people I encountered along trails what I was doing when I was swabbing toads,” Daversa said.
The results unambiguously identified the winter months of underground dormancy as the key season of rapid Bd proliferation, particularly in metamorphs.
The finding came as a shock because Yosemite toads were thought to be at lower risk for Bd because the fungus was thought to spread predominantly in water, and the toads spend most of their time on land. It was believed that the toads only contracted the fungus during the brief window of the mating season, when they are primarily aquatic. But the new data clearly showed that toads were developing strong infections on land.
Bd was thought to grow best in more moderate conditions, so the discovery that it proliferated during harsh winters contradicted everything scientists thought they knew.
“It certainly appears that the Bd fungus has the ability to move between individuals,” said UCLA conservation biologist and co-author Brad Shaffer. “The ability to increase in number on land at super low temperatures is a real surprise.”
The researchers don’t know for sure that frogs are contracting new infections in the burrow. It could be that they go underground with a very light, undetectable infection, and for some reason, each individual grows a heavy fungal load. Studying the burrows during hibernation is difficult because they are buried under many feet of snow.
“How pathogens persist in host populations with incredibly seasonal life histories is one of the puzzles we study,” said co-author Jamie Lloyd-Smith, a UCLA wildlife disease ecologist. “This fungus isn’t supposed to grow happily at such low temperatures — either the temperatures above ground or the temperatures we suspect exist in the burrows. 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 researchers hope to find answers in follow-up studies, but the research has nonetheless changed the reintroduction strategy for captive-reared animals. Although the toad reintroductions would be cheaper and easier if done at the metamorphic stage or younger, managers now only release animals after a few years of growth, when they are at less risk of overwinter Bd infection.
Functional Ecology