Facultative investment in offspring sex is related to maternal condition in many organisms. In mammals, empirical support for condition-dependent sex allocation is equivocal, and there is some doubt as to theoretical expectations. Much theory has been developed to make predictions for condition-dependent sex ratios in populations with discrete generations. However, the extension of these predictions to populations with overlapping generations (OLGs; e.g., mammals) has been limited, leaving doubt as to the specific prediction for maternal-condition-dependent sex ratios in mammals. We develop a population genetics model that incorporates maternal effects on multiple offspring fitness components in a population with OLGs. Using a rare-gene and evolutionarily stable strategy approach, we demonstrate that sex ratio predictions of this model are identical to those for equivalent discrete generations models. We show that the predicted sex ratios depend on the sex-specific ratio of R0 (offspring lifetime fitness) for offspring of good and poor mothers. This offspring lifetime fitness rule indicates that empirical research on conditional sex ratios should consider all three components of offspring R0 (juvenile survival, adult life span, and fertility).
Trivers-Willard model, Charnov-Bull model, mammal sex ratios, overlapping generations, population genetics model
American Naturalist 168(4):521-530