Frogs reveal the global importance and interplay of colour-based protective and thermoregulatory functions

Abstract Recent small-scale and experimental studies have shown that colour lightness variation can have important physiological implications, with darker species having greater heating rates, as well as protection against pathogens and photooxidative damage. Using data for 41% (3,059) of all frog and toad species (Anura) from across the world, we reveal ubiquitous and strong clines of decreasing colour lightness towards colder regions and regions with higher pathogen pressure and UVB radiation. The relative importance of pathogen resistance was higher in the tropics and that of thermoregulation was higher in temperate regions. These functions strongly influenced colour lightness evolution in anurans and filtered for more similarly coloured species under climatic extremes, while concurrent mechanisms resulted in high within-assemblage variation in productive regions. The findings establish three fundamentally important functions of colour lightness that were previously unknown in anurans, and they substantially broaden support for colour lightness–environment relationships in ectotherms.