Divorce in king penguins: asynchrony, expensive fat storing and ideal free mate choice
In long-lived and monogamous species, with partners separated between breeding seasons, the previous mate usually returns and waiting is therefore rewarded, thus contributing to mate retention as an adaptive strategy. However, in the long-lived monogamous king penguin, Aptenodytes patagonicus, studied at South Georgia for six years, the overall average divorce rate was as high as 81% (range 76-85%). Outcome of previous breeding attempt had no significant effect on the probability of divorce when all years were pooled, but there was an effect when two years with total breeding failure were excluded. It was obvious that asynchrony in arrival of former mates had strong effect on the probability of divorce. I suggest that the explanation for the high divorce rate in king penguins consists of two parts. (1) Asynchrony in combination with "the expensive fat storing hypothesis". At arrival, birds have only about half their maximum body-reserves, and are close to exhaustion of reserves at the end of the fast ashore. Thus, even birds that switched mates, rather than waited, barely endured the period of fasting. This suggests a trade-off between the costs of divorce and the cost of storing the extra fuel required to endure waiting and thus ensure mate retention. (2) "The better option hypothesis" in combination with "ideal free mate choice". Even among pairs arriving synchronously, ≥ 50% divorced. To explain this, I suggest that many birds, due to the time constraint created by the near-inadequacy of the fat store, had previously bred with a partner of lower quality than it could have obtained and therefore may benefit from divorcing if a higher-quality partner was available. Readiness to switch mate would be enhanced by low search cost and easy access to potential new mates in this non-territorial species. Hence, mate choice would be potentially ideal free in all aspects but arrival time asynchrony and sometimes competition from the previous mate