TESTOSTERONE EFFECTS ON THE IMMUNE-SYSTEM AND PARASITE INFESTATIONS IN THE BARN SWALLOW (HIRUNDO-RUSTICA) - AN EXPERIMENTAL TEST OF THE IMMUNOCOMPETENCE HYPOTHESIS

The immunocompetence hypothesis predicts that testosterone (T) enhance s the expression of male secondary sexual characters while exerting a suppressive effect on the immune system thereby exposing hosts to high er intensities of parasite infestations. In a natural population of ba rn swallow (Hirundo rustical males, the intensity of infestation by so me ectoparasites was negatively correlated with tail length and was po sitively correlated with immunoglobulin levels, but no clear relations hip was observed between immune: responses (leukocyte counts, immunogl obulins) and tail length. Males implanted with T had higher intensitie s of parasite infestations at the time of recapture than control males , and T-implanted males experienced an increase in counts of eosinophi ls. In T-implanted males, immunoglobulin levels initially decreased an d then increased as time from implantation elapsed. Among T-implanted males, those with longer tails had a smaller increase in eosinophil co unts, tended to experience. a smaller increase of parasite infestation s, and were more likely to survive until the following breeding season than those with shorter tails. The relationships between parasite bur den, immune system, and exaggeration of tail length in the natural pop ulation of males are consistent with some aspects of the immunocompete nce handicap hypothesis. The results from the manipulation of T plasma levels are also partly consistent with the hypothesis, since T-implan tation resulted in higher levels of parasite infestations, but contrad ict the assumption of an obligatory immunosuppressive effect of T High er activation of the immune system of T-implanted males indicate that high T plasma levels imposed a two-fold cost because of the effects on parasites and the immune response to parasites, and this suggests tha t the effect of T on parasites might not be mediated by the immune sys tem of the host. The results of the manipulation of T plasma levels su pport the handicap version of the immunocompetence hypothesis since hi gh quality, long-tailed males paid less in terms of activation of the immune system, change in parasite infestations, and chances of surviva l than low-quality, short-tailed males.