The evolution of parental sex roles

In many animals, parents provide care to their offspring, such as building nests, or feeding the young. Species differ considerably in how parental care is distributed between the male and the female parent. In my PhD thesis, I shed new light on the question how the baffling diversity of parental care patterns 1,101 bird species, in order to find out whether ecological factors (such as nest type) or life-history characteristics (such as body size) explain when species exhibit uniparental or biparental care. Although I could identify some significant factors (e.g., breeding in a colony), my main conclusion is that many hypotheses proposed in the literature are not confirmed by the data. Second, I constructed theoretical models in order to gain a better understanding of how and why diverse parental care patterns emerge in the course of evolution. I analysed these models by means of individual-based simulations, which are more versatile and based on fewer simplifying assumptions than standard mathematical approaches. My simulations provide surprising new insights. During evolution, regularly ‘care polymorphisms’ emerge where very different care strategies coexist in males and/or females. Although these polymorphisms may disappear again after a brief period of time, I could show that they are often decisive for the course and outcome of evolution. This finding has important implications for the evolution of parental sex roles, and it necessitates a reconsideration of many seemingly well-established predictions of sex role theory.