Males and females often exhibit different behaviour in reproduction (termed sex roles), which is believed to have driven sex differences in many aspects of the animals’ biology, such as morphology and physiology. A central consequence of this divergence in behaviour are sex differences in mortality that, despite being widespread across animals, often their causes are unknown or ignored. Birds are charismatic vertebrates with remarkable features that make them ideal candidates for the study of breeding system evolution, such as diverse parental care, and flexible mating systems. Importantly, in most birds, females have higher mortality rates than males. Several life-history variables have been associated with female-biased mortality, but the proximal causes of this sex bias are widely unknown. The main objective of this dissertation is to explore how pathogens and the immune system are involved in sex-specific mortality in birds. My PhD used both single- and multi-species approaches to evaluate different aspects of macro- and micro-parasite infection and of the avian immune system. By using a small shorebird, the Kentish plover (Charadrius alexandrinus), as an ecological model organism, I show that bacterial infection on island and continental plover populations are not different, although females have higher infection rate than males. In a follow-up comparative study, I show that the prevalence of gastrointestinal and blood parasites does not differ between the sexes and parasite prevalence is not associated with sex-specific mortality across 138 species of birds. Then, taking a immunological approach, I used samples from wild populations of Black-winged stilts (Himantopus Himantopus) and Kentish plovers, to show that these two species lack sexual differences in agglutination and lysis titres, which partly follows predictions from demographic variables, although transcriptomic data of two different populations of Kentish plovers do show a male bias in expression of immune genes in the brain. Finally, in a comparative study across 41 wild bird species I showed that sex-specific immune response depends on the breeding status of the birds, where males tend to be more immunocompetent than females during the breeding season. Taken together, my PhD work suggests that the relationships between parasite prevalence, immune-responses and sex-different mortalities are not as simple as one would hypothesise. Finally, I discuss the contribution of my studies to the understanding of sex-specific mortality in birds, considering limitations and potential future studies.
|Date of Award||17 Feb 2021|
|Supervisor||Tamas Szekely (Supervisor), Samuel Sheppard (Supervisor) & Jordi Figuerola (Supervisor)|