Natural parasites, rival’s color, and rival’s behavior explain aggression in a lizard

Male competition conforms to a cost-benefit model because aggression may increase reproductive prospects while increasing risks of injuries. We hypothesize that these agonistic interactions can also incur in energetic trade-offs in parasitized hosts. We staged dyadic male contests in the lab to investigate the relationships of multiple parasites with the agonistic behavior of lizard hosts, Sceloporus occidentalis. We also included in the analyses both color and behavioral traits from rivals because (i) color patches of lizards may serve as intraspecific signals used by conspecifics to assess the quality of opponents, and (ii) contests between male lizards fit classical models of escalated aggression, where focal lizards increase aggression displays in response to a rival’s behavior. Based on multimodel inference applied to mixed models, lizards displayed more pushups when they had more ticks and also when rivals performed more lateral compressions. The higher aggression in infested lizards might be explained as a correlate of host reproductive effort. Moreover, lizards infected by hematic coccidians performed fewer pushups, conforming to our initial hypothesis. Interestingly, focal lizards also displayed fewer pushups when the chroma and size of rival’s blue patch were greater. The results support the role of the blue patch of S. occidentalis as a sexual armament, because it contributes to the deterrence of aggression from rivals. They also revealed that natural infections in lizard hosts can contribute to explain their agonistic behavior. We encourage to account for natural infections in behavioral tests with lizards.