Biology session

Amphibian decline is part of the current sixth mass extinction event, and the amphibian pathogenic fungus, Batrachochytrium dendrobatidis (Bd) was identified among its main causes. Typically, tadpoles only show mild symptoms in case of chytridiomycosis infection, while in newly metamorphosed and adult amphibians the disease can lead to death. It is known that preferred body temperature of amphibians at all developmental stages is somewhat higher, than the thermal optimum of Bd. Additionally, diseased amphibians can show behavioural fever, when infected individuals spend more time at warm places than healthy conspecifics.

In our study, we investigated the thermal preference of common toad (Bufo bufo) tadpoles, tested for the presence of behavioural fever in response to Bd infection, and for a negative relationship between temperature preferences and infection intensity.

During larval development, we repeatedly infected half of experimental tadpoles with Bd. Subsequently, we placed infected and uninfected tadpoles into 2 m long half-pipes for 4 days individually, providing a thermal gradient between 18-32 °C or a homogeneous 20°C, that is ideal for the fungus. We recorded tadpole behaviour using video-cameras and measured Bd prevalence and infection intensity using qPCR methods.

Preferred mean body temperature of tadpoles was between 25-27 °C, but there was no difference in the behaviour of infected and uninfected individuals. Interestingly, we found significant differences (up to 1.5°C) in preferred temperatures between families. Only two of the 16 initially infected tadpoles which had the opportunity to choose their body temperature were infected at the end of the experiment, while five of the 16 infected individuals reared at a constantly low temperature remained Bd positive.

According to our results, behavioural fever triggered by Bd infection was not detectable in case of common toad tadpoles, but their preferred temperatures exceeded the temperature range that is optimal for Bd. In addition, thermal preference of tadpoles varied among families, so that there may always be individuals that select a high enough temperature to minimise Bd infection.