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Male-male interactions select for conspicuous male colouration in damselflies

Male ornamentation, such as conspicuous male colouration, can evolve through female mate choice. Alternatively, in species without overt female mate preference, conspicuous male colouration can evolve via intrasexual selection to resolve male-male competition or to prevent costly male-male mating attempts. Here, we investigated the drivers of conspicuous male colouration in an ontogenetic colour changing damselfly, Xanthagrion erythroneurum, where the juvenile males are yellow and change colour to red upon sexual maturity. We first showed that red males were chromatically and achromatically more conspicuous than the yellow males. We then quantified the condition of the males and showed that red males were larger and in better condition than yellow males. We tested female preference in a choice experiment where we artificially manipulated male colour, and found that females did not choose mates based on male colouration. We further tested whether the male colouration affected male-male interactions. We presented red and yellow males in the breeding arena, and found that red males received less intra- and interspecific aggression than yellow males. Our study experimentally showed, for the first time, that male conspicuousness is not a target of female mate choice in damselflies. Intra- and interspecific male-male interactions therefore appear to be the driver of conspicuous male colouration in damselflies.