Sexually dimorphic blue bands are intrasexual aposematic signals in nonterritorial damselflies

Sexually dimorphic traits in males are thought to evolve via female preference or male–male competition. Alternatively, in species without overt male displays or female mate choice, dimorphic coloration may function as a warning signal to conspecific males thereby avoiding costly harassment. We aimed to determine the function of sexual dimorphic coloration in the damselfly Xanthagrion erythroneurum in which males, but not females, have conspicuous blue bands on the tip of the abdomen. We show that the male blue bands and female black abdomen are chromatically and achromatically discriminable against their natural background. Moreover, the male blue bands and their adjacent abdominal segments generate higher internal contrast than female abdominal segments. We conducted two sets of experiments to test alternative hypotheses that the male blue bands are (1) the target of female mate choice, or (2) an intrasexual aposematic signal to avoid male mating harassment. We hid male blue bands by painting them black and measured female preference between the manipulated and the nonmanipulated (control) males. We found no difference in mating success between the control and manipulated males, thereby rejecting the female preference hypothesis. To test whether the blue bands function as a warning signal, we manipulated the females by painting male-like blue bands on their abdomen and measured the male response to those females relative to control females. Females with artificial blue bands on the terminal abdomen were mated less frequently than control females. However, when we painted blue bands on the anterior abdominal segments, the males did not discriminate between control and painted females. Our study demonstrates that dimorphic coloration advertises the males' unprofitability as mates to conspecifics thereby reducing intrasexual harassment.