Coevolution of Parental Care, Parasitic, and Resistance Efforts in Facultative Parasitism.

Here we develop a tug-of-war game theory model of mixed-strategy facultative parasitism. In this framework, individuals decide how to strategically invest in parental care effort, parasitism, and resistance to being parasitized, choosing their investments to respond optimally to their opponent's behavior and vice versa. We have implemented the model in the well-studied case of mixed-strategy conspecific brood parasitism, which occurs when a female raises her own clutch of eggs and flexibly lays some eggs in a conspecific's nest. However, the nest of a parasitic female in this case may also be parasitized. We model this as a tug-of-war, allowing parasitic and resistance efforts to strategically coevolve with each other. We then derive expressions for parasitism outcomes commonly observed in the field. The model also captures the trade-offs between parental care and parasitism when both are possible. We make a number of novel predictions about the rate of successful versus attempted parasitism as well as how parental care effort, resistance to parasitism, and fitness vary as functions of group size and individual differences between players in energy budget and production efficiency. Although we focus primarily on conspecific brood parasitism, the model is general enough to be extended to other systems in which facultative parasitism is possible, including the parasitism of food, fertilizations, nest sites, or other resources from conspecifics.