The effect of exogenous testosterone on ectoparasite loads in free-ranging western fence lizards.

Numerous factors impact the dynamics of host-parasite relationships, such as host sex, hormonal state, reproductive condition, host health, and behavior. In particular, males from a variety of taxa frequently carry heavier parasite burdens than females, particularly during breeding season when testosterone concentrations are elevated. Using western fence lizards (Sceloporus occidentalis), we tested the hypothesis that high circulating testosterone concentrations in male lizards induce high tick and mite loads. We implanted male lizards with either testosterone or control implants in the field during the spring, when tick and mite loads are highest. One month later, testosterone-implanted males had significantly higher tick loads, but lower mite loads, than control males. These results suggest that testosterone differentially impacts ectoparasitic acarine burdens. Testosterone may modulate aspects of lizard physiology and behavior that enhance or diminish parasitism by certain acarines during periods of peak reproductive effort.