Extrapair Copulations in the Mating System of White Ibis (Eudocimus Albus)

Abstract

Mating behavior of white ibis was intensively observed in a large breeding colony in coastal South Carolina, U.S.A. Extrapair copulations (EPCs) were found to be a frequent and regular feature of the mating system. EPCs appeared to be fully capable of fertilizing eggs, on the basis of their form, timing, and male and female behavior in response to them. Females appeared to cooperate fully in some EPCs; this cooperation included cloacal flaring, and sperm was clearly transferred during some EPCs. Females were able to reject EPCs, and EPCs were not forced by males. Based on the relative frequency of apparently successful extrapair and withinpair copulations, EPCs were estimated to fertilize 6.05% of all eggs. EPC involvement among males was based on abilities to remain on the colony, to displace other males, and to take only short foraging trips. Female EPC involvement was based on the amount of time they were left unguarded. A male's EPC involvement was inversely correlated with that of his mate. Extrapair males were of distinctly different quality than the female's nestmate. Among study groups, EPC rate was not correlated with degree of local nesting density, sex ratio, clutch size. Female cooperation and the efficiency of male mate guarding were concluded to be important to the evolution of this mating strategy in white ibis. It is concluded that EPC is an important strategy in this otherwise monogamous species. The exact costs and benefits of this strategy to both sexes are discussed.