Jenn Logan used to share her Denver apartment with some unusual roommates: two dozen boreal toads. In 2000, the biologist had just started a job at Colorado Parks and Wildlife’s (CPW) Native Aquatic Species Restoration Facility, a hatchery in Alamosa that was being built to breed endangered and threatened fish and amphibian species for release into the wild. Until the building was finished, however, it was up to Logan to foster the imperiled toadlets. “The sooner that recovery actions start,” she says, “the better chance we have to facilitate a species’ long-term persistence.”

Once abundant, Colorado’s population of boreal toads, which live between 8,000 and 12,000 feet in elevation, has plummeted over the past four decades to fewer than 1,000 adults. In 1998, five years after CPW added the toads to the state’s list of endangered species, researchers found the reason for their decline: amphibian chytrid fungus. The fungus causes a disease that can send amphibian immune systems into overdrive and prevent the toads from absorbing water and nutrients through their skin. Eventually, their hearts stop. It’s especially deadly in Colorado, and researchers aren’t sure why. “Toads from Utah or Montana—their immune systems don’t go into panic in the way that our toads’ immune systems do,” says Alex Wells, a community conservation coordinator at the Denver Zoological Foundation.

For years, conservation efforts focused on slowing the fungus’ spread among populations in the wild. Then, in 2016, researchers at the University of Colorado Boulder led by Valerie McKenzie, a professor of ecology and evolutionary biology, began working with Janthinobacterium lividum, a bacterium known for its antifungal properties and vibrant violet hue. McKenzie’s team found that in a lab setting, inoculating boreal toads with the microbe increased their survival rate by 40 percent by hindering fungus growth on their skin. “I grew up in the ’80s [as a] huge Prince fan,” McKenzie says. So, after the musician died in April 2016, she only had one name in mind for the bath they created from the flashy fungus fighter—Purple Rain.

CU Boulder teamed up with CPW in 2019 for the first large-scale deployment of Purple Rain. Armed with a speaker blasting Prince tunes, volunteers hauled more than 5,000 tadpoles grown in the Alamosa hatchery (descendants of Logan’s old roommates) into the Collegiate Peaks and doused them in the bacteria formerly known as J. liv. The goal was to see if the treatment helped those tadpoles survive and establish new populations in the wild. More releases followed, and researchers are currently studying Purple Rain’s potential for protecting naturally born tadpoles, too.

The results have been hard to quantify; toads’ home ranges expand greatly once they metamorphose, which makes it difficult to do population counts. Their camouflage doesn’t help, either. But there have been positive signs. This year, researchers have encountered fewer dead toads than normal as well as fewer live toads with red-colored bellies, a mark of chytrid infection. “We’ve made progress,” McKenzie says, “but there’s still a lot to be done.”