Behavioral anapyrexia as a response to virus infection in a poikilothermic vertebrate

Behavioral plasticity may contribute to the ability of wild animals to survive disease outbreaks. In absence of endogen heat control, poikilothermic animals adjust their body temperature behaviorally. While many studies reported behavioral fever, its opposite, behavioral anapyrexia - when infected animals lower their body temperature by using cool microenvironments - remains poorly documented. Here, we report the first evidence of behavioral anapyrexia in a poikilothermic vertebrate as a response to pathogenic infection. We investigated thermoregulatory responses in tadpoles of the agile frog Rana dalmatina, a cool-adapted amphibian, following experimental infection with a warm-adapted ranavirus. Tadpoles were placed either in thermal gradients or homogeneously cool environments for five days post-exposure. In thermal gradients, all tadpoles reduced their preferred temperatures over time, but this decrease was steeper in infected tadpoles, and individuals with higher infection intensities preferred cooler temperatures. Infected tadpoles became increasingly precise in thermoregulation over time, while a similar trend was not detectable in non-infected tadpoles. Infection prevalence was similar between the two thermal environments, yet infection intensities were significantly higher in the thermal gradient. These results suggest fine-tuned thermoregulation by infected tadpoles to balance out the benefits of behavioral anapyrexia for fighting a warm-adapted pathogen versus the immune-suppressive and developmental costs of low temperatures.