SEX-BIASED PARASITISM OF AVIAN HOSTS - RELATIONS TO BLOOD PARASITE TAXON AND MATING SYSTEM

Immunosuppressive effects of testosterone lead to a prediction of male -biased parasitism. To test this prediction, prevalences of blood para sites were compared between male and female birds using statistically correct vote counts of data From 33 studies. We found no overall diffe rence in prevalence between males and females, in either breeding or n on-breeding birds. However, infections by Haemoproteus (the most commo n genus of blood parasite found) were significantly more common among breeding females than breeding males. Restricting the analysis to bree ding birds of polygynous species, females again were more likely than males to be infected by blood parasites: this result held for an intra -family comparison that controlled for phylogenetic effects. In compar ison, measures of sexual size dimorphism did not relate to sex biases in parasitism as predicted, after controlling for phylogeny using inde pendent comparisons. Because testosterone is often implicated in suppr essing the immune system, female biases in parasitism are unexpected. Female biases in parasitism by blood parasites could result from diffe rential exposure of the sexes to vectors, or from oestrogen-based effe cts on immunity.