Males and females generally differ in resource investment strategies to maximise reproductive output, sometimes at the expense of important systemic processes such as self-maintenance and immune activity. Here, we used wild Lethrus apterus, a sexually dimorphic beetle with parental care, to investigate the influence of sex roles (e.g., offspring provisioning) on their sex-specific immune gene expression across a pseudo-longitudinal sampling framework. By determining immune gene activation of males and females at five successive moments within the active period, we showed that their sex-specific immune gene expression varies substantially, alternating between male bias and female bias across the active period. However, when pooling all sampling dates together, there was no overall difference in the number of up-regulated immune genes between the sexes. We discuss the potential influence of sex roles on our results and highlight the importance of sequential sampling schemes to understand ecological dynamics in the wild.

Males and females generally differ in resource investment strategies to maximise reproductive output, sometimes at the expense of important systemic processes such as self-maintenance and immune activity. Here, we used wild Lethrus apterus, a sexually dimorphic beetle with parental care, to investigate the influence of sex roles (e.g., offspring provisioning) on their sex-specific immune gene expression across a pseudo-longitudinal sampling framework. By determining immune gene activation of males and females at five successive moments within the active period, we showed that their sex-specific immune gene expression varies substantially, alternating between male bias and female bias across the active period. However, when pooling all sampling dates together, there was no overall difference in the number of up-regulated immune genes between the sexes. We discuss the potential influence of sex roles on our results and highlight the importance of sequential sampling schemes to understand ecological dynamics in the wild.

Males and females often differ in ecology and behaviour, and these differences are expected to lead to sex differences in parasite susceptibility. However, sex differences in parasite prevalence have not yet been established across a broad range of taxa, and the ecological and evolutionary drivers of such differences have remained contested. Here we meta-analyse 942 sex-specific effect sizes in blood and gastrointestinal parasites of 179 wild bird species, representing 17 avian orders. Females exhibit higher infections in both Haemoproteus and Protozoan parasites than males and females also have higher prevalence in both Leucocytozoon and Haemoproteus blood parasites during the breeding season. Importantly, sex differences in parasite prevalence are not associated with sexual selection nor parental care, although male-biased prevalence in Leucocytozoon parasites were predicted by male-biased size dimorphism. Taken together, breeding systems and sexual selection have modest influence on sex difference in parasite prevalence across the avian tree of life.