Thermal therapy, a standard treatment for the lethal chytrid fungus decimating amphibian populations, may carry a hidden reproductive cost for male frogs. New research shows that while heat treatment effectively clears infection in green and golden bell frogs (Ranoidea aurea), it significantly reduces sperm concentration, with effects persisting for months.
The findings, published August 4 in the Journal of Experimental Biology, highlight a potential trade-off in conservation strategies that aim to treat and release endangered amphibians.
“We’ve looked at the survival of frogs after these treatments… but it’s less common that we’ve taken into account effects on reproduction,” says Cori Richards-Zawacki, an amphibian biologist at the University of Pittsburgh who was not involved in the study. “That seems important if the idea is to revive populations by treating frogs and then putting them back into the wild.”
Over the past half-century, at least 500 amphibian species have declined due in part to Batrachochytrium dendrobatidis (Bd), a chytrid fungus that colonizes skin and disrupts electrolyte balance, ultimately causing cardiac arrest. Conservationists commonly combat Bd with antifungal baths or heat therapy; temperatures above 29°C (84°F) kill the fungus within days, while 37°C (99°F) can eliminate it in hours.
Reproductive biologist Rose Upton witnessed a Bd outbreak in July 2024 at a captive breeding facility at Australia’s University of Newcastle. To treat infected frogs, Upton and colleagues gradually raised enclosure temperatures from 25°C to 37°C over several days, held the peak temperature for six hours, then ramped down. The team seized the opportunity to investigate whether this thermal regimen impaired fertility—a known risk of heat stress across taxa, including humans.
Researchers collected sperm from 13 mature males—four uninfected and nine Bd-positive—before treatment and again five weeks afterward. Regardless of infection status, sperm concentrations dropped by nearly half following thermal therapy. Six months later, levels remained substantially depressed in four infected males that were retested, a result Upton called the “biggest surprise.” The proportion of motile sperm initially increased post-treatment but declined slightly over the following months.
Richards-Zawacki urges caution in interpreting the long-term data. “We don’t have a really good handle on what causes fluctuations in sperm production in amphibians,” she notes, adding that it is difficult to disentangle lingering effects of the heat treatment, the infection itself, or other factors.
Upton acknowledges the study’s preliminary nature: it lacked a control group of untreated frogs, did not include females, and did not measure actual reproductive output. It also remains unclear whether lower, fungus-lethal temperatures would produce similar sperm deficits.
“I’m certainly not suggesting that we should stop heat treatments or other mitigative actions,” Upton emphasizes. “I think it’s just good to be monitoring reproduction so that we know ahead of time if there is a dip in that fertility.”