Sexual selection driven by direct benefits leads to the erosion of direct benefits
Most sexual selection models assume that the evolution of female choosiness is driven by indirect (genetic) benefits, such as the production of more viable offspring or more attractive sons. There is ample empirical evidence that female choosiness can also provide direct (non-genetic) benefits, including access to good territories or paternal care. Yet, theoretical models of direct-benefits sexual selection are scarce. Here, we use individual-based evolutionary simulations to explore the joint evolution of female mating preferences and male ornamentation under sexual selection driven by direct benefits. We find that, within each generation, male ornamentation reliably indicates paternal quality (the direct benefit); females thus benefit from being choosy and readily evolve preferences for male ornamentation. Yet this in turn selects for males to invest resources in ornamentation at the expense of paternal care. Across generations, female selection for ornamented males (driven by the pursuit of direct benefits) thus erodes the very benefit it seeks, leading to population decline and potentially extinction. Our results highlight the complexity and non-intuitive nature of direct-benefits sexual selection.