Fever is one of the best-known mechanisms by which animals combat infectious diseases. Sick animals can increase their body temperature in two ways: through physiological regulation, by altering internal metabolic processes, or through behavioral changes, such as seeking warmer environments. Ectothermic animals, including amphibians, fish and reptiles, generally rely only on the latter strategy. However, fever is not always the most effective response against every pathogen.
Researchers from ELTE Eötvös Loránd University and the HUN-REN Hungarian Research Network have now experimentally demonstrated that an ectothermic vertebrate infected with a virus actively seeks cooler, rather than warmer, environments to regulate its body temperature. This strategy, known as behavioral cooling, has so far been documented only rarely in the animal kingdom, having previously been reported in a handful of arthropod species infected with bacterial or viral pathogens and in rats infected with bacteria. The new findings suggest that behavioral cooling may also help animals combat viral infections.
Tadpoles chose cooler water
Researchers from the HUN-REN–ELTE–MTM Integrative Ecology Research Group and the Department of Evolutionary Ecology at the HUN-REN Centre for Agricultural Research investigated how the temperature preferences of agile frog (Rana dalmatina) tadpoles change following infection with ranaviruses. Ranaviruses are among the most devastating pathogens of amphibians, capable of causing mass mortality events in susceptible natural populations worldwide.
In their laboratory experiment, the researchers compared the thermoregulatory behavior of ranavirus-infected and healthy tadpoles. One group of tadpoles was free to regulate its body temperature within an artificial thermal gradient, whereas another group was maintained under constant cool conditions. The temperatures selected by the tadpoles were monitored over five consecutive days, after which molecular techniques were used to quantify the amount of virus present in their tissues.
Published in BMC Biology, the results showed that infected tadpoles kept in the thermal gradient gradually selected cooler water than their healthy counterparts. Moreover, the higher the viral load within an individual, the lower the temperature it preferred. In addition, infected tadpoles regulated their body temperature with increasing precision over time, restricting themselves to progressively narrower temperature ranges. By contrast, tadpoles maintained under constant cool conditions harbored significantly lower viral loads at the end of the experiment.
Cooling may slow the virus
"Our results suggest that infected tadpoles fine-tune their body temperature to slow viral replication while avoiding the detrimental physiological consequences of excessively low temperatures. It is well known that low temperatures can slow growth and development in tadpoles and may also reduce the effectiveness of their immune system," explains Dr. Dávid Herczeg, research fellow at the HUN-REN–ELTE–MTM Integrative Ecology Research Group and first author of the study.
At first glance, these findings may seem surprising, as behavioral fever is generally regarded as an effective defense against infection. However, the ranavirus examined in this study replicates more rapidly at higher temperatures. Consequently, lowering body temperature may provide agile frog tadpoles with a physiological advantage in combating infection.
"Our study provides the first direct experimental evidence that ectothermic vertebrates can employ behavioral cooling in response to pathogens. These findings also demonstrate that behavioral responses to disease are far more diverse than previously recognized," says Dr. Attila Hettyey, head of the Department of Evolutionary Ecology, Plant Protection Institute at the HUN-REN Centre for Agricultural Research.
Wetland temperature diversity could help
The findings are also important from a conservation perspective. Ranaviruses pose a serious threat to amphibian populations worldwide, while climate change is continuously altering the thermal characteristics of natural aquatic habitats. The results suggest that the availability of thermally diverse microhabitats within wetlands may improve the survival prospects of disease-affected amphibian populations, highlighting the importance of conserving thermal heterogeneity in freshwater ecosystems.
Publication details
Dávid Herczeg et al, Experimental evidence for behavioural cooling as a response to virus infection in an ectothermic vertebrate, BMC Biology (2026). DOI: 10.1186/s12915-026-02669-6
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Citation: When fever isn't the answer: Infected tadpoles cool themselves to fight viruses (2026, August 10) retrieved 10 August 2026 from https://phys.org/news/2026-08-fever-isnt-infected-tadpoles-cool.html
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