Previously healthy Yosemite toads leave their winter burrows infected with chytrid fungus

The strongest feature of the design is that the September and spring swabs came from the same animals ^[11]. Between those two swabs the toads were underground, away from any breeding pool, so a rise in infection between them happened on land. Sampling different toads in each season could not have excluded a simpler explanation: that the spring animals were a sicker group from the start.

Counting both surveys, the team collected 1,800 samples over three years at six sites ^[12]. That averages about 100 per site per year ^[20], spread across every life stage. The swabs were screened with PCR, the same kind of test used for COVID ^[13]. The phys.org report describes a high infection rate in toads that had been healthy ^[1], but it does not give prevalence or fungal-load figures, so the size of the winter increase cannot be judged from it.

The thing this doesn't tell you is where the winter infections begin. According to the report, the researchers do not know for sure that toads catch new infections in the burrow. The animals may go underground carrying an infection too light to detect ^[19]. A toad under the detection limit in September and over it in spring looks like a new case either way. "It certainly appears that the Bd fungus has the ability to move between individuals," said Brad Shaffer, a UCLA conservation biologist and co-author ^[17]. I think the paired swabs show when the fungus multiplies more firmly than they show how a toad acquires it.

The temperature result is the odder one. Bd had been thought to grow best in moderate conditions ^[16]. "The ability to increase in number on land at super low temperatures is a real surprise," Shaffer said ^[18]. "Although Bd is a largely aquatic pathogen, we discovered that major outbreaks of infection grow and spread during winter hibernation on land," said Dave Daversa, the UCLA ecologist who is first author of the paper in Functional Ecology ^[3][4].

All of this comes from one species. The Yosemite toad, Anaxyrus canorus, is 1 to 3 inches long and lives almost entirely in Yosemite and Sequoia and Kings Canyon national parks ^[5]. It breeds in small pools but spends much of its life in abandoned rodent burrows, and it hibernates in those burrows through the High Sierra winter ^[6].