Positive correlations between pre- and post-copulatory sexual traits in warblers

Theoretical models predict that investment in pre-copulatory and post-copulatory sexually selected traits should trade-off. At the macroevolutionary scale, the majority of studies to date have focused on male weaponry as the target of pre-copulatory sexual selection, but the trade-off should equally apply to traits used to attract females, such as bird song and plumage. We studied the Old World leaf warblers (Phylloscopidae), a group of socially monogamous songbirds that experience relatively high levels of sperm competition. We examined the relationships between song duration and number of elements in the song with sperm length across 21 species, and between the same song variables and combined testes mass in a subset of these species (n=10). Across species, these song variables and testes mass/sperm length were generally positively correlated, albeit not statistically significantly so or with borderline significance. In contrast to theory, we found no evidence for negative associations between pre- and post-copulatory traits. We argue that this is a consequence of males of some species investing more into overall fertilization success (i.e. the sum of pre- and post-copulatory sexual selection) than males of other species, and high fertilization success is achieved through investment into both mate attraction and sperm competition.