Abstract
One of the primary assumptions of the immunocompetence handicap hypothesis is that testosterone has an immunosuppressive effect, but conflicting results have been reported in a variety of bird species concerning the effect of testosterone on the humoral and the T cell‐mediated components of the immune system. The T cell‐mediated component of the immune system is particularly important during the breeding season, because the likelihood of injury during sexual competition is high and T cell‐mediated immunity is essential for healing wounds and resisting infection. In this study we examined the effect of experimentally increased levels of testosterone during breeding season on T cell‐mediated immunity in male lizards of two Mediterranean lacertid species, Psammodromus algirus and Acanthodactylus erythrurus. The hormonal treatment significantly increased testosterone of the experimental individuals. T cell‐mediated responses to phytohemagglutinin stimulation were significantly suppressed in testosterone‐treated males of both species. Furthermore, there was a significant negative relationship between individual variability in T cell‐mediated responsiveness and plasma testosterone concentration. The present study is the first to demonstrate testosterone‐induced suppression of T cell‐mediated immunity in lizards. J. Exp. Zool. 301A:411–418, 2004. © 2004 Wiley‐Liss, Inc.