Dusk light environment optimizes visual perception of conspecifics in a crepuscular horned beetle

Variation of visual signal coloration has been predicted among closely related species or populations by the sensory drive hypothesis and tested in fishes, reptiles, or birds. However, despite the tremendous diversity of insect colorations and habitat preferences, sensory drive constrained by habitat transmission through different light environments has not been investigated in this group. Here, we study in a strictly crepuscular horned beetle (Coprophanaeus lancifer) the relationship between body coloration, ambient light spectra, and conspicuousness to conspecifics. We estimate brightness contrast of the mean coloration, as it is seen at relatively long distance, and color contrast of the horn seen in front of the pronotum, as it is seen during interindividual contests occurring at short distance, using a physiological model of crepuscular horned beetle vision. As predicted, brightness contrast of the mean coloration is optimal in the dusk ambient light spectrum compared with diurnal light environments. Similarly, color contrast of the horn seen in front of the pronotum is maximal in dusk light compared with other forest light environments. Dusk light favors both chromatic and achromatic detection of C. lancifer by conspecifics, which is critical for pair formation during this restricted time window. We created varying reflectance spectra and calculated their conspicuousness under different light environments: maximal color contrast is obtained for the combination beetle signal--dusk light environment. This quantitative example of adaptation of body coloration to ambient light spectrum is the first insight into sensory drive through habitat-mediated transmission of a color signal in an insect species. Copyright 2008, Oxford University Press.