Precocial sexual selection in Crotaphytus collaris: A field study

Abstract

Color traits in animals can take on a variety of forms and serve many functions, including warnings to predators, advertisements to attract mates, and deterrents to rivals. Color is often sexually dimorphic and has variations in expression that can be used by adults to increase or decrease the fitness of the bearer because of sexual selection. Precocial sexual selection is proposed to occur when a sexually immature juvenile uses sexual dimorphism to increase its fitness as an adult. This study tests this idea by determining the development, costs, and benefits of the unique sexual dimorphism seen in juvenile collared lizards. Juvenile male collared lizards possess a sexual dimorphism in the form of dorsolateral orange bars that are lost upon reaching sexual maturity. To understand the development of the bars, we conducted feeding trials using carotenoids. We then conducted a four-year field study where we artificially modified the bars and aggression of male lizards to learn how the bars were used. Following this, we modeled the visual systems of predators and measured the survivorship of the lizards in the field study to show the costs of possessing the bars. Lastly, we calculated the fitness of lizards for the first year after reaching sexual maturity. We found that pteridines are likely the pigment used to express the orange bars and that higher egg incubation temperatures caused the color of the bars to be redder. We demonstrated that lizards use their bars and aggression as a linked signal and unmodified lizards retaliated against lizards that used an incongruent signal, meaning a lizard that had an increased amount of one signal without an increase in the other. We also found no difference in survival among our treatments despite the bars being conspicuous to the vision of predators. Lastly, we found that bars did not impact the fitness of the lizard, but instead found that more aggressive lizards were significantly more likely to produce offspring as yearlings. Overall, precocial sexual selection appears to favor more aggressive juveniles and the bars appear to operate as a socially maintained honest signal of the aggressiveness of the bearer.