Males with sunscreen triumph less often

Posted on July 20, 2015

A study carried out on damselflies has shown that ultraviolet ornamentation improves their competitive success. Male members of the world’s largest damselfly species (Megaloprepus caerulatus) defend water-filled tree holes where females mate and lay their eggs. Working on Barro Colorado Island, Panama, researchers carried out behavioural field trials in which they released a male damselfly into the territory of another male while the resident male was away foraging, and then observed the contest that occurred when the resident returned. The wings of male M. caerulatus have a white band that reflects strongly in the ultraviolet wavelength. Before release, some of the introduced males were treated by painting their wing bands with sunscreen to reduce their UV reflectance. Early in the reproductive season, when the value of breeding sites was greatest, all the staged contests were won by unpainted males, but this bias disappeared as the season progressed. This is the first evidence that invertebrates can use ultraviolet markings to settle disputes. A few vertebrates are known to have similar abilities. Because the area of the white wing band increases disproportionately with damselfly body size, the signal may make it easier for males to assess the size of their opponents.

Reference: Xu, M. & Fincke, O.M. 2015. Ultraviolet wing signal affects territorial contest outcome in a sexually dimorphic damselfly. Animal Behaviour 101, 67-74.