Sexual conflict over parental care: a case study of shorebirds

Shorebirds provide excellent model organisms to study breeding system evolution. We argue that sexual conflict theory is a useful approach to understand breeding system evolution in general, and specifically in shorebirds. Here, we focus on two major questions: (1) why do species shift from biparental care to uniparental care, and (2) why do some species shift toward female-biased care whereas others shift toward male-biased care? We overview recent phylogenetic and experimental studies that address these two questions. Firstly, current evidence suggests that the demand of chicks has a major influence on whether a species exhibits biparental or uniparental care. The demand of chicks has further implications for phenotypic rates of evolution. Secondly, experimental manipulations in the field using a small shorebird, the Kentish Plover Charadrius alexandrinus, are consistent with the idea that the shift from biparental care toward male-biased care relates to female-biased mating opportunities, and thus to higher remating probability for deserting females than for deserting males. Finally, we call for further studies of shorebirds, since the breeding system of most species has not been studied in detail. Long-term monitoring of population ecology, breeding systems and behaviour, and experimental manipulations and genetic analyses, are all needed to test predictions of sexual conflict theory.