Measuring Pre- and Post-Copulatory Sexual Selection and Their Interaction in Socially Monogamous Species with Extra-Pair Paternity.

The biased analytical approaches also showed spurious non-significant results, whereby selection on the sperm trait was less likely to be significant when the pre-copulatory episode was also active, and when the male trait vs. sperm correlation was weakly negative (−0.1; % fewer significant results ± SE: univariate selection gradient, 22.53 ± 3.2; paired t-test, 16.1 ± 2.4; unpaired t-tests, regression on detected extra-pair copulation partners, and regression on the proportion of within-pair young sired, approximately 6%; all F > 8.8, p < 0.01).

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