Heat Therapy That Cures Frogs' Fungal Disease Cuts Their Sperm Levels

Heat therapy commonly used to cure frogs of a deadly fungal disease may come at a reproductive cost, according to a study published August 4 in the Journal of Experimental Biology. Researchers found that thermal treatment cured green and golden bell frogs (Ranoidea aurea) of chytrid fungus infection, but their sperm concentrations dropped sharply and stayed low months later.

The fungus, Batrachochytrium dendrobatidis or Bd, colonizes frogs' skin and disrupts their uptake of electrolytes, which can eventually cause cardiac arrest. Over the past half century, Bd has driven population declines in at least 500 amphibian species, including R. aurea. To treat infected frogs, scientists often use antifungal baths or heat therapy: temperatures above 29 degrees Celsius kill the fungus within days, or within hours at 37 degrees Celsius.

Reproductive and conservation biologist Rose Upton of Australia's University of Newcastle witnessed a chytrid outbreak in July 2024 at a captive breeding facility for bell frogs on campus. Her team raised enclosure temperatures by 2 degrees Celsius a day, from 25 to 37 degrees Celsius, held that peak for six hours, then lowered it in similar steps. They collected sperm from 13 mature male frogs, four uninfected and nine infected, both before and five weeks after the heat treatment.

Sperm concentrations nearly halved after treatment, regardless of infection status. Six months later, four infected frogs that were retested still had substantially reduced sperm levels — Upton's "biggest surprise." The share of motile, or swimming, sperm rose right after treatment but declined slightly at the later check. Cori Richards-Zawacki, an amphibian biologist at the University of Pittsburgh who was not involved in the study, urged caution, noting scientists do not fully understand what drives sperm production changes in amphibians.

Upton acknowledged the study is preliminary: it lacked a comparison group of frogs that were not heat-treated, did not include female frogs, and did not track whether treated males produced fewer offspring.