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.