Strong sexual selection despite spatial constraints on extra-pair paternity

Extra-pair paternity should contribute to sexual selection by increasing the number of potential mates available to each individual. Potential copulation partners are, however, limited by their proximity. Spatial constraints may therefore reduce the impact of extra-pair paternity on sexual selection. We tested the effect of spatial constraints on sexual selection by simulating extra-pair copulations for 15 species of socially monogamous songbirds with varying rates of extra-pair paternity. We compared four metrics of sexual selection between simulated populations without spatial constraints and populations where extra-pair copulations were restricted to first- and second-order neighbors. Counter to predictions, sexual selection as measured by the Bateman gradient (the association between number of copulation partners and offspring produced) increased under spatial constraints. In these conditions, repeated extra-pair copulations between the same individuals led to more offspring per cop...