Thermal preferences of larval anurans are not affected by infection with Batrachochytrium dendrobatidis

Attila Hettyey, Boglárka Jaloveczki, Andrea Kásler, Dóra Holly, Márk Szederkényi, Dávid Herczeg and János Ujszegi, Lendület Evolutionary Ecology Research Group, ATK NÖVI, ELKH, Budapest, Hungary

Background/Question/Methods
The efficiency of the immune system of poikilothermic animals is tightly linked to their ability to maintain optimal body temperature. In addition, infection by certain pathogens may trigger behavioral fever, i.e. when animals select microhabitats where they can reach elevated body temperatures to boost their immune system. Evidence suggests that adults of some anuran amphibian species exhibit behavioral fever in response to infection, but whether larvae also show this behavior remains practically unknown. Behavioral fever may be especially effective against pathogens exhibiting low thermal tolerance limits, such as Batrachochytrium dendrobatidis (Bd), the fungal pathogen causing chytridiomycosis. Bd grows best at temperatures between 20-24 °C, its growth becomes suppressed entirely beyond 27 °C, and its survival drops sharply around 29 °C. Simultaneously, the thermal optimum of larval amphibians is thought to generally fall between 24 and 28 °C, and most studied species can endure temperatures above 30 °C. Therefore, tadpoles may be able to keep Bd-infection under control by maintaining their body temperature close to the optimum, and even more so if they exhibit behavioral fever. We performed a laboratory experiment to test this hypothesis, where we exposed tadpoles of Bufo bufo and Rana dalmatina to a thermal gradient ranging between 18-31 °C for six days. Half of the tadpoles had been infected experimentally with Bd, and the other half were non-infected controls. Besides trials performed in thermal gradients, we also performed tests in thermally homogeneous 20 °C environments. We video-recorded movements of tadpoles over five days. After termination, we preserved individuals and assessed their infection status using qPCR.
Results/Conclusions
Our results showed that Bufo bufo tadpoles allowed to thermoregulate preferred temperatures between 25-27 °C and this resulted in 60 % lower infection prevalence than conspecifics kept in a homogeneously 20 °C thermal environment. Rana dalmatina tadpoles appeared to be highly resistant to infection (i.e. low infection prevalence even in individuals held at 20 °C) and preferred 1-2 °C cooler thermal environments than Bufo bufo larvae. We did not detect signs of behavioral fever, i.e. infected tadpoles did not prefer higher temperatures than non-infected individuals. These results suggest that the studied species and many other amphibians may be able to withstand the chytridiomycosis-pandemic as long as they can effectively thermoregulate and thereby have the opportunity to reach high enough body temperatures that enable them to fight Bd effectively.