Influence of population density and vegetation on the expression of ventral ornaments in male Sceloporus lizards.

Sexual selection can drive the evolution of elaborate traits that enhance reproductive success, such as colorful ornaments. In Sceloporus lizards, ventral ornaments represent an example of such traits, yet the ecological and demographic factors driving their evolutionary trajectories are not fully understood. We examined the association between ornament traits, vegetation cover, and population density in three Sceloporus species (S. grammicus, S. olivaceus, and S. spinosus). Site-specific population densities were estimated using N-mixture models and ornament morphology and coloration of male lizards were analyzed as functions of vegetation cover and population density. Higher population densities were associated with brighter and bluer ornaments in S. grammicus and S. olivaceus, consistent with stronger sexual selection under high-density conditions. In contrast, we observed the opposite response to high population density with less bright and less blue ornaments that approached green and purple hues in S. spinosus. In two species, S. grammicus and S. spinosus, we observed bluer and brighter ornaments in sites with sparse trees, supporting the sensory drive hypothesis, which proposes that blue hues should be more detectable where light originates primarily from the open sky. Our findings emphasize that the evolution of sexually selected ornaments is influenced by the interplay between demographic processes and vegetation structure.