Ectoparasites of house sparrows (Passer domesticus): an experimental test of the immunocompetence handicap hypothesis and a new model

Conspicuous secondary sexual traits may have evolved as handicap-revealing signals or as badges of status. We present results of an experiment using m ales of the sexually dimorphic house sparrow (Passer domesticus), that supp ort the idea that the male-specific bib can be both a handicap-revealing si gnal and a reliable badge indicating the physical condition of the bird. In a test of the immunocompetence handicap hypothesis, wild-caught adult male house sparrows were studied in captivity. Birds implanted with elevated do ses of testosterone were more dominant, had higher circulating levels of bo th testosterone and corticosterone and they also harboured relatively large r ectoparasite loads. Higher parasite loads were also associated with indiv iduals showing lower immunocompetence and larger changes in bib size. A new model for immunocompetence effects in sexual selection is introduced, inte grating actions that the hypothalamopituitary axis exerts on gonads, adrena ls and the thyroid gland. The "integrated immunocompetence model" synthesiz es both the "handicap" (i.e. survival-decreasing) and "badge of status" (i. e. survival-enhancing) models for evolution of secondary sexual traits.