The Effect of Female Ornamentation on Aggressive Male-Male Interactions in the Striped Plateau Lizard (Sceloporus virgatus)

Theoretical and empirical studies have shown that the intensity of male-male competition is greatly influenced by the quality of the contested resource. To date few studies have focused on whether the quality of a contested female will have the same effect. Within a number of species, the quality of the female may be signaled with sexually selected female ornamentation. In the striped plateau lizard (Sceloporus virgatus), females develop ornamental orange throat patches before ovulation, and previous research has shown that females with larger ornaments are of higher phenotypic quality and also produce offspring with higher phenotypic quality, while females with darker patches have larger follicles and cause males to increase their courtship behavior. Therefore, I hypothesize that variation in the darkness and/or the size of the female ornament influences the intensity of male-male competition. Beginning approximately twenty days before ovulation, lizards were captured and held in groups (n = 51) of two males and one female for five days, such that all three lizards were in separate cages and the males could see the female but not each other. Males were then placed in the female’s cage and videotaped for twenty minutes. Male aggression was quantified using an ethogram and each aggressive behavior was weighted using Principle Component Analysis. As with previous studies, females with darker patches had larger ovarian follicles, indicting they were more sexually receptive. Males were found to increase their level of medium aggressive behaviors and high aggressive behaviors around females with darker patches, and to increase their level of low aggressive behaviors around females with high patch chroma. Both the hue and the size of the female’s patch had no effect on male aggression. I hypothesize that males are increasing their aggression around only females with darker patches, even though patch size had been found to indicate phenotypic quality, because of the risks that are associated with male-male aggression. Courting a female that is not sexually receptive will provide benefits to the male when the female does become receptive, but fighting over a female will only provide benefits when the female provides the male with an immediate mating opportunity