Why Do Aptenodytes Penguins Have High Divorce Rates?

In long-lived birds, monogamy is thought to enable breeders to retain the same mate from year to year, but exceptions occur. For example, King Penguins (Aptenodytes patagonicus) and Emperor Penguins (A. forsteri) have much lower mate fidelity than do small- er species of penguins, despite their high rates of survival. Recently, Olsson (1998) suggested that divorce in King Penguins could be adaptive. Although Olsson was the first to propose an adaptive function for divorce in this species, he was unable to assess the relationships among individual quality, date of arrival, mate choice, and breeding success. Accordingly, we studied King Penguins and Emperor Penguins to further examine the determinants and consequences of divorce. Mate retention was not affected by breeding performance in the previous year or by experience, and neither mate retention nor divorce had significant con- sequences on chick production the following year. King Penguins were more likely to divorce as arrival asynchrony of previous partners increased. In Emperor Penguins, this tendency to divorce occurred only when females returned earlier than their previous mates. Most Em- peror Penguin pairs formed within 24 hours after the arrival of males, which were outnum- bered by females. King Penguins that had nested successfully in their next to last attempt were favored as mates for ones that had been unsuccessful, and individuals of both species probably chose the best mates available. Most of our results for King Penguins and Emperor Penguins supported Olsson's (1998) conclusions in that divorce appears to be adaptive. Mate retention in the absence of a true nesting site (neither species builds a nest) would be mal- adaptive for these species, which face strong time constraints for breeding. Therefore, di- vorce occurs because costs of mate retention are high. Aptenodytes penguins appear to have