Social monogamy vs. polyandry: ecological factors associated with sex roles in two closely related birds within the same habitat.

Why mainly males compete and females take a larger share in parental care remains an exciting question in evolutionary biology. Role-reversed species are of particular interest, because such 'exceptions' help to test the rule. Using mating systems theory as a framework, we compared the reproductive ecology of the two most contrasting coucals with regard to sexual dimorphism and parental care: the black coucal with male-only care and the biparental white-browed coucal. Both species occur in the same lush habitat and face similar ecological conditions, but drastically differ in mating system and sexual dimorphism. Black coucals were migratory and occurred at high breeding densities. With females being obligatory polyandrous and almost twice as heavy as males, black coucals belong to the most extreme vertebrates with reversed sexual dimorphism. Higher variance in reproductive success in fiercely competing females suggests that sexual selection is stronger in females than in males. In contrast, resident white-browed coucals bred at low densities and invariably in pairs. They were almost monomorphic and the variance in reproductive success was similar between the sexes. Black coucals were more likely to lose nests than white-browed coucals, probably facilitating female emancipation of parental care in black coucals. We propose that a combination of high food abundance, high population density, high degree of nest loss and male bias in the adult sex ratio represent ecological conditions that facilitate role reversal and polyandry in coucals and terrestrial vertebrates in general.