Testosterone supplementation in subordinate, small male lizards: Consequences for aggressiveness, color development, and parasite load

According to the immunocompetence hypothesis, testosterone stimulates the expression of male sexually selected traits while decreasing immunocompetence. This proposed trade-off was studied by experimental supplementation of testosterone to small, subordinate, dull-colored male lizards, Psammodromus algirus. Experimental males showed a tendency to overlap their home range with fewer small males than did control males and tended to be more aggressive. However, control males were observed more frequently attending females than experimental males. The area of patches of breeding coloration, the number of ticks, and the frequency of recoveries of testosterone-supplemented and control males did not differ significantly. The results suggest that small adult males with high levels of testosterone behave more aggressively, which may be advantageous to securing a breeding territory in the next season. However, the hormone did not apparently affect ornamentation or parasite load. We argue that, whatever the mechanisms involved, blocking effects of testosterone may be adaptive because being cryptic facilitates a sneaking strategy, and low ectoparasite load may improve survival.