Beyond Temperature: Relative Humidity Systematically Shifts Juvenile Thermal Performance and Projected Population...

Understanding how temperature-sensitive organisms respond to environmental change is central to addressing challenges in public health, biodiversity conservation and food security. For many ectotherms, abiotic and biotic factors shape their abundance and distribution by generating stage-specific variation in life history traits. Although previous studies have examined temperature, rainfall, competition and habitat quality in relation to maximal population growth rate ( $r_m$ ), relative humidity has rarely been incorporated into trait-based thermal performance frameworks. Using laboratory experiments, we show that relative humidity alters juvenile life history trait responses in Anopheles stephensi, an important malaria vector. We then integrate these humidity-dependent juvenile trait responses into an analytic $r_m$ model to examine how relative humidity shifts the temperature dependence of projected population growth. Heuristic climate-suitability comparisons further illustrate that temperature-humidity interactions acting through juvenile traits alone can alter qualitative inference about when and where temperature-only models may over- or underestimate environmental suitability. These results highlight the importance of incorporating humidity alongside temperature when assessing ectotherm responses to climatic variation.