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