Extra-pair paternity does not result in differential sexual selection in the mutually ornamented black swan (Cygnus atratus)

We studied patterns of parentage in 85 broods (332 cygnets) of black swans during three breeding seasons, using a set of eight polymorphic microsatellite markers. We detected both intraspecific brood parasitism (IBP; < 5% of cygnets per year) and extra-pair paternity (EPP). In these years, 10-17% (mean = 15.1%) of cygnets resulted from EPP, and 27-40% (mean 37.6%) of broods contained at least one extra-pair cygnet. Compared with levels of EPP in closely related species with similar life histories, these values are unexpectedly high. EPP in black swans appears unrelated to ecological factors (breeding density and synchrony) or genetic factors (genetic similarity between pair members or genetic quality of the offspring). We found no evidence that a mutual sexual feather ornament known to play a role in social mate choice in black swans (curled wing feathers) is involved in extra-pair mate choice. EPP does not lead to greater variance in reproductive success in males, relative to females in this species. We therefore suggest that EPP does not result in differential sexual selection on males and females, explaining why they are ornamented to the same degree.