Young Yosemite toads survive winter, then emerge with a deadly fungus

Some young Yosemite toads emerge from their first winter underground infected with the deadly chytrid fungus Batrachochytrium dendrobatidis, or Bd, researchers report September 23 in Functional Ecology. Although the fungus typically spreads through water, the finding points to an unexpectedly risky period as toads wait out harsh winters in burrows beneath feet of snow. Bd has hit amphibian populations hard.

The fungus attacks amphibians’ skin, preventing electrolytes and fluids from entering the body, and often leads to heart failure. Infections, which can kill the frogs, are responsible for declines in at least 500 species and 90 possible extinctions, according to a 2019 estimate. The disease is one reason why Yosemite toads (Anaxyrus canorus) are listed as threatened under the U.S.

The toads live in California’s Sierra Nevada mountains, where the toads spend most of their time on dry land. Mature adults breed in shallow pools of water. But high-elevation living also means they spend roughly half the year underground during brumation, a hibernation-like state for reptiles and amphibians.

How a water-loving fungus sticks around even after toads spend a long, cold winter underground is a mystery, says David Daversa, an ecologist at UCLA. The team captured Yosemite toads of all ages — including the youngest toads both before they went into their burrows and after they emerged — and swabbed their skin to test for Bd. Surprisingly, the fungus was more prevalent in juvenile toads after their first winter than in adults breeding in the water.

An average of roughly 23 percent of juveniles tested positive for Bd before their first brumation. In the spring, more than 90 percent of those young toads emerged from their burrows with sometimes severe infections. Less than half of breeding adults tested positive.

Daversa says it’s unclear why the fungus thrives underground. It’s possible that young toads burrow with undetectable infections that grow over time, or that the fungus spreads when many toads burrow together. Also unknown is whether other brumating amphibians face similar risks during the winter.