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 copulation partner. In contrast, spatial constraints did somewhat reduce sexual selection – as measured by the opportunity for selection, s'max, and the selection gradient on male quality – when the association between simulated male quality scores and copulation success (e.g., female preferences or male-male competition) was strong. Sexual selection remained strong overall in those populations, even under spatial constraints. Spatial constraints did not substantially reduce sexual selection when the association between male quality and copulation success was moderate or weak. Thus, spatial constraints on extra-pair copulations are insufficient to explain the absence of strong selection on male traits in many species.Please see readme file.One data file shows the distribution of extra-pair offspring among clutches, with extra-pair paternity identified using genetic tools. These data were compiled from published studies in some cases and for other species, they represent unpublished additional information connected to publications. Sources are provided in the associated manuscript. Permission to provide the unpublished data were provided by the authors of that information. The second data file shows the estimated values of m and s, two parameters used in the simulation code. Finally, the simulation code is included