Divorce Rates in Birds: Predictions from an Optimization Model

Abstract

We present a theoretical investigation of the dependence of divorce rate on longev- ity, variation in quality between mates, and the costs of divorce or mate retention. We model the decision of a female on whether to divorce her mate after breeding. This decision is based on his quality. We show that it is optimal for the female to divorce if the reproductive success in breeding with the mate next year is less than the average annual future success if she divorces. Under this strategy the female's mean annual divorce rate declines with age, while her mean annual reproduction success rises. We investigated the annual divorce rate, averaged over the lifetime of the female. Costs caused this divorce rate to increase with variance of male quality. When divorce resulted in a loss in reproductive success in the following year, this divorce cost reduced divorce in short-lived species far more than in long-lived species. The reverse was true for mortality costs. A mate retention cost of overwinter mate death increased divorce in short- lived species dramatically, while long-lived species were little affected. Female choosiness in- creased with longevity, but this did not necessarily increase divorce rates averaged over the female's life. The selection pressure against the suboptimal strategies of obligate divorce and obligate mate retention increased strongly with longevity.