Reproductive effort affects immune response and parasite infection in a lizard: a phenotypic manipulation using testosterone

Life-history theory predicts that there is a trade-off between reproductive effort and several traits that determine fitness. Infectious disease has gained acceptance as a crucial Factor linking both variables. In most instances phenotypic manipulation is necessary to convincingly demonstrate a causal relationship of reproductive effort on parasitism. However, experimental studies that manipulate reproductive effort or parasite load have been rarely conducted in reptiles. In this study, we manipulated reproductive effort of male lizards (Psammodromus algirus) through testosterone implants, and measured the associated response in some haematological variables and parasite load. Testosterone-supplemented males had lower scores than control males in factor 1 of a PCA for different blood parameters. This factor is correlated with the number of white blood cells, especially lymphocytes, and with plasma glucose levels. Experimental males also had higher scores in factor 3 that is mainly related to protein catabolism. Scores of males in component 1 tended to be correlated negatively with tick load, while scores in component 3 were correlated positively with the number of haemogregarines in the blood. These results suggest that higher investment in reproduction decreases the immune defences, and conduces to the use of structural resources, which may render individuals more susceptible to some haemoparasites. This is consistent with the idea that an increase in reproductive effort mediated by testosterone has a negative effect on the ability to counteract parasite infections.