Male biases in parasitism of mammals: Effects of study type, host age, and parasite taxon

Sex biases in parasitism may be expected in mammals because estrogens stimulate immunity, whereas androgens depress immunity. Parasites should, therefore, become more readily established in male hosts, leading to higher levels of parasitism for males than for females. We tested this general hypothesis using 145 tests based on mammal hosts taken from 38 published studies. Male biases in parasitism existed overall and for tests restricted to arthropod (but not helminth) parasites. We then controlled for potential dependence by choosing single parasite species based on their likelihood of eliciting immune responses. For these 48 tests, male biases still existed for arthropod, but not for helminth, parasites. As predicted, the average degree of male bias was much higher for studies involving an experimental infection than for field studies. In experimental tests, researchers controlled for differences between the sexes in exposure to parasites. Any biases in parasitism, therefore, should have been due primarily to gender differences in immune response. There also were no male biases in parasitism among juveniles, presumably because juvenile males and females differ less in their hormone profiles or stress levels than do adult males and females. In summary, our results suggested that hormonally mediated sex differences in susceptibility to parasitism exist for mammals, even though our tests were extremely conservative. Our results also indicate that, on average, differences in parasitism between the sexes are small, and that statistically significant male biases in parasitism are not a general rule.