Evolutionary origins and persistence of infidelity in Malurus: the least faithful birds

Fairy-wrens (genus Malurus) maintain territories year round, and breed cooperatively, with all members of the social group provisioning young. Despite living with several adult males, the breeding female typically cuckolds all of them, seeking fertilisations from extra-group males that provide no care to her offspring, instead caring for the young reared on their own territory. We trace the evolutionary origins and persistence of this extraordinary combination of traits. We argue that the high rate of infidelity in some fairy-wrens facilitates social pairing among nuclear family relatives, rather than being an evolutionary response to avoid inbreeding. It seems likely that females mate with extra-group males to improve the genetic quality of their offspring. The ability of males to maintain breeding plumage for long periods is the primary criterion for female choice; only older males can do so. Several features of the mating system undermine the accuracy of female choice, and low-quality males exploit this uncertainty. Extra-group matings by low-quality males can help stabilise the mating system but may leave it vulnerable to collapse under certain circumstances. Nonetheless, sexual selection in most species is very strong, confirming the utility of fairy-wrens as model organisms for the study of mate choice and intersexual selection.