Figure 2. Positive associations between within-pair reproductive success and sperm morphology.

(A) flagellum length and (B) relative length of flagellum to head section. Within-pair reproductive success is measured as the proportion of young at a social dominant male's nest(s). Analyses were conducted using male identity as a random factor (i.e. mixed models). Plots refer to mean paternity for all broods sampled. Males that are potential active breeders past the date of the current paternity assessment (2008/9 season) are represented by the open circles (n = 17 of the total n = 47 males). Fitted curves were calculated using the regression estimates from non-mixed effects fitted models.