Causes of sex ratio bias in Malagasy plovers: experimental tests of the role of parasites and immuno-competence

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The adult sex ratio (ASR, proportion of males in the adult population) is a central concept in population demography and a key factor in evolution under sexual selection. As part of a recent DFG-funded research project (KR 2089/9-1), we developed a population demographic model to estimate ASR in a well-studied shorebird, the snowy plover Charadrius nivosus (Eberhart-Phillips et al. 2017 in PNAS). Importantly, we also used this stage-specific matrix model to estimate ASR in six closely related plover populations (Eberhart-Phillips et al. 2018 in Nature Communications). This is currently the most comprehensive comparative study of ASR to date using demographic models based on unusually large sample sizes of individuals (totalling 3862 chicks and 3107 adults). The latter work has revealed large variation among species and populations in ASR, ranging from strongly male-biased to strongly female-biased. We have also shown that sex-biases in survival are generally strongest during the first year of life. Consequently, as yet unknown factors that influence early life-stages appear to be the main drivers of ASR variation across multiple species. Importantly, ASR variation among plover populations is associated with the extent of parental cooperation. Here, we seek to test the causes of sex-specific mortalities in chicks and adults by focusing on bacterial and parasite infection and the resulting immune response. We will compare patterns of sex-specific immunity and parasite pressures among three sympatric populations of closely-related plover species with different mating systems in Madagascar. Sex differences in immune defence components and mortality could be due to testosterone immunosuppression and can be expected mostly during gonadal maturation in adults, while other factors may cause sex differences in juvenile mortality. Therefore, we will study both juveniles and adults in parallel. We then aim to incorporate our experimental findings into refined demographic models of ASR and mating system variation, using both individual-based models and integrative population models. These models will explore the consequences of sex-specific survival for the evolution of ASR across populations and associated population growth trajectories.