When cold-bloods heat up: meta-analytical evidence that climatic variability mediates behavioural fever in amphibians and reptiles

Fever is an adaptive defence response that enhances immune performance by elevating body temperature above normal values. In ectotherms, fever is expressed behaviourally through the selection of warmer microhabitats following infection, yet its magnitude and determinants vary widely across species and environments. We performed a phylogenetically informed meta-analysis to test whether behavioural fever expression is shaped by climatic thermal variability, pathogen identity and taxonomy in amphibians and reptiles. We predicted that species from thermally variable environments would exhibit stronger behavioural fever than those from stable environments and reptiles would exhibit stronger fever responses than amphibians owing to lower hydrothermal constraints. Across 47 studies encompassing 101 effect sizes, behavioural fever emerged as a widespread but context-dependent response. Behavioural fever was strongest in species from thermally variable habitats, suggesting that thermal opportunity and behavioural plasticity mediate the expression of fever. Contrary to our prediction, amphibians exhibited stronger fever responses than reptiles, possibly reflecting differences in thermoregulatory biology, environmental opportunity or methodological artefacts. Bacterial infections elicited larger body temperature increases than fungal or viral challenges. Our findings demonstrate that temperature is not a background condition for host–pathogen interactions, but an active and environmentally contingent component of immune defence in ectothermic tetrapods.