Chin up: are the bright throats of male common frogs a condition-independent visual cue?

Male mating success is generally governed by either female choice or male–male competition and multiple cues may influence both processes. Anuran amphibians are best known for acoustic communication but visual cues have also been found to play a role. Little is known, however, about visual cues in explosively breeding anurans in which sexual selection is based almost exclusively on male–male competition. During their search for mates, males may not distinguish between the sexes or species and mismating attempts are known to occur. Assuming that mismatings are costly for males, one would expect selection to favour a trait that facilitates sex recognition. We determined the colour of the explosively breeding common frog, Rana temporaria, during reproduction by spectrophotometry. While reflectance of all body parts in both sexes decreased from immigration to spawning, reflectance of male throats increased significantly. The mean luminance of male throats was more than twice that of female throats. Male throats contrasted most against male flanks with luminance contrast being significantly higher than chromatic contrast. In an experiment with individually marked frogs, throat luminance was not correlated with male size, body condition or the physical presence of females, whereas temperature showed a significantly positive effect. Our results give no hint that throats indicate male status or indicate male quality. Considering the high densities during breeding, our results suggest that bright male throats can act as a visual signal among males to facilitate gender recognition and avoid attacks by other males.