Precipitation shapes colour polymorphism in a widespread spider

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 of colour variants, but effects may vary depending on the lineage‐specific functions of colour. Colour polymorphism is widespread in arachnids, in which colour functions in intra‐ and interspecific interactions such as prey‐luring; yet, the drivers of colour diversity in this lineage remain largely unexplored. Here, we investigated climatic factors influencing colour polymorphism in a widely distributed colour polymorphic spider ( Austracantha minax ) using a combination of controlled experiments, field measurements and tests for macroecological associations. We showed that although different colour morphs occupy distinct climatic niches across Australia, in the laboratory and field, dark colouration is unlikely to provide biologically relevant thermal benefits. Instead, 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 and 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. Read the free Plain Language Summary for this article on the Journal blog.