Beetle gloss is a cue of conspecifics, but only in sunny conditions.

Illumination conditions have a tremendous influence on the appearance of colours in nature. This is especially true for glossy objects, which generate bright highlights in direct sunlight, but the visibility of these highlights is noticeably reduced in shaded or overcast conditions. Despite this dramatic change in appearance with illumination, we know little about how illumination influences the visibility of glossy objects, and no study has directly tested the functional consequences. Here, we investigated whether jewel beetles (family: Buprestidae) use glossiness as a long-distance cue of conspecifics and how weather conditions determine the visibility of glossy beetles. We documented beetle attraction to glossy and matte interception traps in their natural habitat. Male beetles were more attracted to glossy than matte traps, but only on sunny days. To determine whether surface gloss is also used to locate conspecifics at short-range, we documented beetle behaviour towards glossy and matte replica beetle models placed on host trees. Beetles were more likely to approach glossy than matte replica beetles. Our results indicate that jewel beetles use glossiness as both long- and short-range cues of potential conspecifics, but only in sunny conditions. This demonstrates that environmental conditions determine when these brilliant visual cues are functionally relevant.