Social role specialization promotes cooperation between parents.

Biparental care of offspring is a widespread social behavior, and various ecological, life-history, and demographic factors have been proposed to explain its evolution and maintenance. Raising offspring generally requires several types of care (e.g., feeding, brooding, and defense), and males and females often specialize in providing different types of care. However, theoretical models of care often assume that care is a single variable and hence that a unit of care by the mother is interchangeable with a unit of care by the father. We hypothesize that the ability of one parent to provide all types of care may be limited by nonadditive costs or by sex-based asymmetries in the costs of particular care types. Using an individual-based simulation, we show that synergistic costs of investing in two tasks or negligible sex-based cost asymmetries select for task specialization and biparental care. Biparental care persists despite intense sexual selection and sex-biased mortality, suggesting that previous models make overly restrictive predictions of the conditions under which cooperation can be maintained. Our model provides a mechanistic underpinning for published models that show that the synergistic benefits of individuals cooperating can stabilize cooperation, both in the context of parental care and in other social scenarios.