Essential Site Maintenance: Authorea-powered sites will be updated circa 15:00-17:00 Eastern on Tuesday 5 November.
There should be no interruption to normal services, but please contact us at [email protected] in case you face any issues.

Peter Laszlo Pap

and 7 more

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.

Nóra Ágh

and 4 more

Sex-biased mortality can occur in birds during development, for example due to sexual differences in energy requirement and/or environmental sensitivity, or the effects of sex hormones or sex differences in expression of mutations linked to sex chromosomes. Urban habitats often provide poorer conditions for nestling development resulting in higher offspring mortality compared to natural habitats, which may accelerate sex differences in offspring mortality in cities. To test this hypothesis, we examined sex-specific offspring mortality in great tits (Parus major), using 660 samples of dead offspring collected in two urban and two forest sites between 2013-2019. Overall, the sex ratio of dead offspring was significantly male-biased (56.80%). When habitats and age groups were analysed separately, the sex ratio of dead offspring was significantly male-biased in urban habitat (57.60%) and in young nestlings (58.62%), and non-significantly in the forest habitat (56.58%) and in unhatched embryos (54.11%) or in old nestlings (54.55%). However, these estimates were associated with wide confidence intervals, thus sex ratios of dead offspring did not differ significantly among study sites and between age groups when these were analysed together. 70.30% of unhatched eggs was not fertilized, and their proportion did not differ between urban and forest habitats. These results suggest male-biased offspring mortality in great tits, and highlight the need of large datasets to detect subtle differences between habitats and developmental stages.