Spider colour polymorphism is shaped by precipitation, not ambient temperature

Colour polymorphism, the presence of multiple colour variants within a population, is a common example of intraspecific phenotypic variation and has served as a model for studying drivers of diversity. Climatic factors can influence the distribution and abundance of colour variants, yet research often focuses on lineages where sexual selection covaries with the climate-colouration associations. Research has also focussed disproportionately on vertebrates and a few insects, neglecting other taxa where polymorphism is widespread, like arachnids. Here, we investigated climatic factors influencing colour variation in a widely distributed colour polymorphic spider using a combination of controlled experiments, field measurements and tests for macroecological associations. We showed in the lab and field that dark colouration is unlikely to provide biologically relevant thermal benefits; yet different colour morphs occupy distinct climatic niches across Australia. Precipitation, rather than temperature, appears to be the strongest driver of colour morph frequencies: there were higher frequencies of dark individuals in regions with higher precipitation while orange individuals were more common in drier areas. We discuss potential mechanisms related to background matching or pathogen protection in humid environments. Our findings highlight the role of climatic factors in shaping colour variation, and demonstrate the value of combining macroecological, field, and laboratory approaches.