Intra-specific correlations between ejaculate traits and competitive fertilization success: a meta-analysis across species and fertilization modes

Understanding of how selection can act on traits that improve competitiveness and subsequent paternity has advanced, including the idea that internal and external fertilization presents different environments that may select differentially on ejaculate traits. However, no studies have quantitatively synthesized the intra-specific relationships between these traits and paternity. Therefore, we conducted a meta-analysis across 52 papers to determine which ejaculate traits positively correlate with paternity share and how these correlations vary with fertilization mode. Overall, most ejaculate traits were positively associated with paternity, with the notable exception of sperm length. Sub-analyses on sperm number, sperm length, and sperm velocity revealed no statistical differences between fertilization modes in the relationship between traits and paternity when all effect sizes across species were combined. However, in a sub-analysis on fish species only, we found evidence that sperm velocity may be more important in external fertilizers. We also observed differences in the importance of phylogenetic relatedness and some species-specific differences. Our results suggest that while most ejaculate traits should be under positive directional selection in both internal and external fertilizers, sperm length may be subject to more nuanced selection pressures. Overall, we highlight important patterns of intra-specific relationships between ejaculate traits and competitive fertilization success.