Ultraviolet wing signal affects territorial contest outcome in a sexually dimorphic damselfly

Ultraviolet (UV) reflectance and UV vision are both common among animals and are known to function in mate choice and male–male competition among numerous vertebrates. In comparison, examples of functional UV reflectance among invertebrates are scarce. In a territorial damselfly (Megaloprepus caerulatus), data from natural territorial contests indicated that males assessed the male wing band of rivals. We investigated the functions of (1) UV reflectance of the male-specific white wing band in territorial contests by staging contests between size- and age-matched, control and UV-reduced males, and (2) UV reflectance of the female-specific wing tip in sex recognition by presenting control and UV-reduced females to territorial males. Results showed that males whose UV reflectance of the white wing bands was reduced were more likely to lose contests. This effect dissipated late in the reproductive season, when breeding sites typically decrease in value. UV reflectance of the female wing tips did not affect male sex recognition, nor did it affect the detectability of a female at a male territory. Our study provides the first direct evidence from invertebrates, and one of the few among all animals, that UV reflectance of wing ornamentation affects the outcome of male contests in the field. •We investigated the role of sex-specific UV-reflective wing ornamentation in Megaloprepus caerulatus.•Contest duration and dynamics of males were consistent with opponent-only assessment of wing band size.•UV reflectance of male wing band serves as a ‘biological billboard’ for body size.•UV reflectance of female white tips did not affect sex recognition.•First direct evidence that UV reflectance of wing ornament affects contest outcome in invertebrates.