Sex biases in parasitism may be expected in mammals because estrogens
stimulate immunity, whereas androgens depress immunity. Parasites shou
ld, therefore, become more readily established in male hosts, leading
to higher levels of parasitism for males than for females. We tested t
his general hypothesis using 145 tests based on mammal hosts taken fro
m 38 published studies. Male biases in parasitism existed overall and
for tests restricted to arthropod (but not helminth) parasites. We the
n controlled for potential dependence by choosing single parasite spec
ies based on their likelihood of eliciting immune responses. For these
48 tests, male biases still existed for arthropod, but not for helmin
th, 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 im
mune response. There also were no male biases in parasitism among juve
niles, presumably because juvenile males and females differ less in th
eir hormone profiles or stress levels than do adult males and females.
In summary, our results suggested that hormonally mediated sex differ
ences in susceptibility to parasitism exist for mammals, even though o
ur 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 ge
neral rule.