Sexual differences in physiology are widely regarded as potential proximate mechanisms
that underlie sex differences in mortality, life history and disease risk of vertebrates.
However, little is known about the causes of sex‐specific variation in physiology.
Sexual selection and parental workload are two key components suggested to play a
role. Theory predicts that, within males, species with stronger male sexual selection
(greater sexual dichromatism and more frequent social polygyny) and higher male parental
effort should have lower immune capacity and stronger oxidative imbalance. Within
females, a weak or no direct effect of male sexual selection on physiology is expected,
but species where females invest more in parental care should have lower immune capacity
and higher oxidative imbalance. We tested these predictions by phylogenetic comparative
analyses conducted separately for the two sexes and based on 11,586 physiological
measurements of samples collected in the field from 2048 individuals of 116 and 106
European bird species for males and females, respectively. For males, we found that
the degree of dichromatism, polygyny and male parental effort correlated negatively
with multiple immune indices, and the level of antioxidant glutathione correlated
positively with polygyny score. In contrast, female immune and oxidative variables
were unrelated or weakly related to both male sexual selection and female parental
effort. We conclude that sex roles can drive inter‐specific variation in immune function
(primarily in male birds), but less so in oxidative physiology. These findings support
earlier claims that males pay higher physiological costs of sexual selection than
females, but apparently also of caregiving. We discuss how females might avoid such
costs.
