Extra-pair paternity, sperm competition and their evolutionary consequences in the Maluridae

Abstract

Fairy-wrens (Malurus) are socially monogamous and yet exhibit among the highest rates of extra-pair paternity (EPP) known in passerine birds. Here, we review patterns of EPP in fairy-wrens and their allies (Maluridae), and discuss the causes and evolutionary consequences of multiple mating in this group. Rates of EPP vary widely across species, but correlate strongly with relative testes mass suggesting that testes mass can be used as a proxy for EPP in comparative studies. With respect to variation in EPP, there appear to be consistent habitat differences between the subfamilies of Maluridae that may contribute to the observed inter-specific variation in rates of EPP, and specific habitat requirements of some species may influence patterns of paternity between species of Malurus. Within species, however, there is equivocal evidence for a role of group size influencing rates of EPP. Regardless of the causes of variation in EPP, available evidence suggests that female fairy-wrens may benefit from extra-pair matings through indirect genetic benefits, such as inbreeding avoidance and choice of high-quality males (i.e. good genes). Using Bayesian modelling, we show that the ancestral state in Australian malurids is one of low sperm competition, and that, in general, increased levels of sperm competition (and hence EPP) are a derived condition in this family. Given the broad range of levels of sperm competition in this family and the relationship between relative testes mass and EPP, we suggest that the Maluridae are a model system for studies of EPP and sperm competition.