Why do aptenodytes penguins have high divorce rates?

Auk, The, Apr 1999 by Jiguet, Joel Bried Frederic, Jouventin, Pierre

ABSTRACT.-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 smaller 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 consequences 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 Emperor Penguin pairs formed within 24 hours after the arrival of males, which were outnumbered 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 maladaptive for these species, which face strong time constraints for breeding. Therefore, divorce occurs because costs of mate retention are high. Aptenodytes penguins appear to have adopted an optimal divorce strategy in order to adapt to their long breeding cycle in a demanding environment. Received 27 June 1998, accepted 30 October 1998.

SOCIAL MONOGAMY iS the predominant mating system among birds (Lack 1968), and many long-lived species have a long reproductive life span (Stearns 1992). High survival may enable monogamous species to retain the same mate between successive breeding attempts more frequently than do species with higher mortality (Rowley 1983). In addition, breeding success often is enhanced by mate retention (e.g. Rowley 1983, Ens et al. 1996).

High survival rates and long reproductive life spans are common in seabirds (Ricklefs 1990), but mate fidelity varies among groups. For example, albatrosses are very faithful to their partners (Rowley 1983, Warham 1990), whereas frigatebirds regularly divorce (see Black 1996) between successive breeding attempts (Nelson 1976). This variability also occurs within the same family, as is the case for penguins (Williams 1996). The genus Aptenodytes is unique in that adults do not build a nest, incubating their single egg and brooding their chick on top of their feet. Only two species occur in this genus, the King Penguin (Aptenodytes patagonicus) and the Emperor Penguin (A. forsteri). Being the largest species of penguins, they have a long breeding cycle, part of which takes place during the austral winter (Stonehouse 1953, 1960). Consequently, they must optimize their reproductive output during severe conditions that involve a decline in food availability and a four-month winter fast for chicks of the subantarctic King Penguin (Stonehouse 1960, Weimerskirch et al. 1992), and low temperatures and ice for the antarctic Emperor Penguin (Stonehouse 1953). In addition, both species face strong time constraints for breeding. Thus, laying after the end of January at Iles Crozet or after 1 January on South Georgia Island invariably results in breeding failure in King Penguins (Weimerskirch et al. 1992, Jouventin and Lagarde 1995, Olsson 1996) because late-hatched chicks have not been able to store sufficient fat reserves before the austral winter (Stonehouse 1960, Van Heezik et al. 1994, Jouventin and Lagarde 1995). In the more synchronously breeding Emperor Penguin (Stonehouse 1953, 1960; Isenmann 1971; Weimerskirch et al. 1992), only the earliest-hatched chicks (i.e. before 10 August; Isenmann 1971) are able to depart to sea when the ice breaks in December, having completed their molt but attaining only 50% of adult body mass (Isenmann 1971).

Aptenodytes penguins are long lived, with a mean annual survival rate of 0.91 to 0.95 (Jouventin and Weimerskirch 1991, Weimerskirch et al. 1992). Yet, they show low mate fidelity between years (15% in Emperor Penguin [Isenmann 1971, Jouventin 1971]; 19 to 29% in King Penguin [Barrat 1976, Olsson 1998]) compared with other penguins (x = 84%; Williams 1996). For divorce to be adaptive, individuals that divorce should obtain some benefit; i.e. it would be "optimal" for an individual to divorce if its reproductive success from breeding with a previous partner in the next year is lower than its average future success (i.e. the average for all future breeding attempts until the individual dies; McNamara and Forslund 1996).