Cuckoldry and the stability of biparental care

In socially monogamous species, females map engage in extra-pair fertilizat ions to gain direct or indirect benefits not provided by the social mate, w ith the potential risk of a reduction in the social mate's paternal effort. I present an ESS model of cuckoldry frequencies, which considers both facu ltative and nonfacultative male responses to losses in paternity. Two possi ble equilibria exist: stable social monogamy with varying degrees of extra- pair paternity, and polygamy with Little or no male care. Monogamy with lim ited cuckoldry can be stable only if the initial cuckoldry frequency is low , intrinsic cuckoldry benefits are not high, males can reasonably accuratel y detect cuckoldry, and female compensation for losses in male care is inco mplete. Deviations from these assumptions lead to stronger mate acquisition in males at the expense of paternal care, and eventually to runaway evolut ion towards polygamy. Average female fitness is reduced in the runaway, alt hough it is initiated by females maximizing the survival of offspring - a p otential "tragedy of the commons" in breeding systems.