Generalised linear least square tests of: a) the effect of the relative testis weight (proxy measure for post-copulatory sexual selection) on sperm length traits, b) the effect of hook length on its variation c) the effect of sperm tail length on its variation.

CVwm is the average coefficient of within-male variation in hook length and tail length. CVbm is the coefficient of between-male variation in sperm traits, adjusted for sample size (see Methods). The statistical analyses were performed in R and based on transformed variables to approach normality (arcsine square-root for the proportion of relative testis weight, log for CVwm and CVbm). Slopes were tested against the prediction of 0 using the t-test. Lambda (λ) indicates the level of phylogenetic dependence in the data, with superscripts giving p-values for test of λ = 0 and λ = 1 respectively. R2 values indicate the proportion of total variance explained.