Sexual armaments are usually structures or other traits used during
agonistic displays that evolve by intrasexual competition. The role of
parasites in their evolution remains obscure. We investigated the
relation of infection by three parasites with the agonistic behavior of
males Sceloporus occidentalis in male contests staged in the lab. The
rivals' both behavior and blue patch chroma best predicted the
aggression intensity of focal males. Infections by haemococcidians and
ticks also contributed to explain the intensity of aggression in focal
males. Lizards with fewer ticks engaged in more intense fights, while
lizards with more ticks or haemococcidians were less aggressive.
Interestingly, males with lower blue chroma were infected by intestinal
coccidians and received more aggression. This study reveals direct roles
for ticks and haemococcidians in male competition, and an indirect role
of intestinal coccidians mediating sexual selection of a color armament
in lizards.