Sexual Selection of Beard Color in the Inland Bearded Dragon (Pogona vitticeps)

Many animal traits are the result of inter- or intra-sexual selection. Intersexual selection which usually takes the form of female mate choice, is the process by which an individual chooses a particular mate that exhibits desirable traits, or that provides an immediate benefit, such as access to resources. Intrasexual selection, which is most commonly in the form of male-male competition, is the selection of heritable traits that make an individual more successful when competing with other males and gaining access to mates. These processes often go hand in hand, with traits such as size and coloration being selected for through both processes. Sexually selected traits often contribute to the evolution of sexual dimorphism. The inland bearded dragon (Pogona vitticeps) is a moderately sized Australian agamid species that exhibits sexual dichromatism. Males display a dark gular region, or beard, during breeding season. The current study was designed to determine whether beard color in male bearded dragons has resulted from intersexual selection (demonstrated by a differential response by female subjects to dark- and light-bearded males), intrasexual selection (demonstrated by a differential response by male subjects to dark- and light-bearded males), or both. Thirty-two male and 32 female subjects were presented with male bearded dragons that had their beards artificially modified to be either dark black or light gray. Visual inspection (“looking”), approach, and avoidance behaviors were scored from videotapes of the trials. Eye use (right or left) by the subjects was also scored to evaluate a post hoc research question on laterality. Female subjects did not exhibit significantly different behavior when presented with either beard color, and did not favor one eye over the other. Male subjects responded similarly to both beard colors in most measures, with the exception of duration of the first look, and eye use. Males had significantly longer first looks when presented with dark bearded stimuli, and tended to use their right eye preferentially to the left when looking at male stimuli. The data suggest that male-male competition may have an effect on beard coloration in this species. The predominant use of the right eye is also interesting, as it is consistent with findings in other species. While the left eye is usually preferred in aggressive and breeding interactions, some research suggests that the right eye is preferred for categorizing items into broad categories, and in inspection of unfamiliar conspecifics. As stimuli were prevented from seeing subjects, and therefore not displaying aggressive or other typical behaviors, it is likely that there was less activation of the right hemisphere, and so little use of the left eye. The right eye was preferred, likely to more fully inspect and assess or categorize the unfamiliar stimulus male. A lateralized response in males, and a lack of one in females, supports the evidence that beard color is likely a product of intrasexual selection.