Phylogenetic relationships of immune function and oxidative physiology with sexual selection and parental effort in male and female birds

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
2,048 individuals of 116 and 106 European 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
or 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.