SEXUAL DIMORPHISM IN IMMUNE DEFENSE

Sex differences in parasitism are common and may depend on sex differe nces in intensity of sexual competition, immunosuppression, or exposur e to parasites. We used a large data set on the mass of two immune def ense organs in birds (the bursa of Fabricius and the spleen) to test f or consistent sex differences in immune defense. Males had a relative spleen mass that was consistently smaller, but more variable, than tha t of females across species of birds. A sex difference in the size of the spleen was not present among juveniles but was large and statistic ally significant among adults. The suppression of spleen mass in adult males increased with a measure of intensity of sexual selection: the frequency of extra-pair paternity. These findings suggest that sex dif ferences in parasitism may arise as a consequence of sex-differences i n immune function, as mediated by sexual selection.