While epizootics increasingly affect wildlife, it remains poorly understood how the environment shapes most host-pathogen systems. Here, we employ a three-step framework to study microclimate influence on ectotherm host thermal behaviour, focusing on amphibian chytridiomycosis in fire salamanders (Salamandra salamandra) infected with the fungal pathogen Batrachochytrium salamandrivorans (Bsal). Laboratory trials reveal that innate variation in thermal preference, rather than behavioural fever, can inhibit infection and facilitate salamander recovery under humidity-saturated conditions. Yet, a three-year field study and a mesocosm experiment close to the invasive Bsal range show that microclimate constraints suppress host thermal behaviour favourable to disease control. A final mechanistic model, that estimates range-wide, year-round host body temperature relative to local microclimate, suggests that these constraints are rule rather than exception. Our results demonstrate how amphibian innate host defences against epizootics may remain constrained in the wild, which predisposes to range-wide disease outbreaks and population declines.

The data file contains data on (i) temperature preference of fire salamanders Salamandra salamandra measured during Batrachochytrium salamandrivorans (Bsal) infection experiments in thermal gradients; (ii) temperature preference of five Belgian amphibian species measured in thermal gradients; (iii) field body temperatures of the same five amphibian species and operative temperatures, measured during nocturnal field surveys at a Belgian study site; (iv) hourly internal Salamandra salamandra body temperature measured using implanted data loggers, and associated climate data of the mesocosm in which these animals were housed; (v) Bsal AMFP13 thermal growth data; and (vi) fishnets (xy coordinates in decimal degrees) used to extract ERA5 climate data used as input for NicheMapR mechanistic models of fire salamander steady-state body temperature. All temperatures are in °C. In the README file we provide additional information and references to sections of the Ecology Letters paper Supplementary Information, in which the associated methodology is explained in detail. Related code is stored at https://github.com/wouterbeukema/Beukema_et_al_2020_Ecol_Lett.