Did sexually dimorphic dorsal coloration evolve by a pre-existing bias in males in the lizard Sceloporus minor?

Abstract

Theory and empirical evidence indicate that male secondary sex traits can evolve by co-option of pre-existing biases in females. However, relatively few studies have explored whether male pre-existing biases could drive the evolution of traits important in male contests. Male spiny lizards (Sceloporus) are characterized by the expression of sexually dimorphic blue throat and abdominal patches. These features are revealed to conspecifics during social interactions, and variation in ventral color can predict the outcome of male contests in some species of spiny lizards. In Sceloporus minor, males in some populations also express bright blue color on the dorsal surfaces. Given the significance of blue color in intrasexual signaling in other species of Sceloporus, blue dorsal color may have evolved in S. minor by co-option of a male sensory bias for the color blue. We tested this hypothesis in a population that exhibits an ancestral phenotype for male dorsal color (brown/orange), and lacks males with bright blue dorsal coloration. Resident territorial males were presented with one of three types of intruder males manipulated in dorsal color by painting. Orange males mimicked the ancestral dorsal phenotype found at the study site; blue males resembled those from a population with the derived (blue) form of this trait; and green males represented a novel stimulus control. If blue dorsal color evolved in S. minor in part due to co-option of a male sensory bias, we predicted that resident males would exhibit either increased or decreased levels of aggression to blue intruders relative to controls. We found no difference in resident aggressive behavior across all treatments, thus failing to support the predictions of a pre-existing bias. We discuss these findings in the context of social behavior in Sceloporus, and propose directions for further study in this species.