Reproductive success.

<p>(a) Lifetime reproductive success (LRS: number of children raised to age 15) as a function of intra-sex birth-order category (among sons): firstborn sons had the highest LRS, as compared to their younger brothers (χ²₂ = 5.26, <i>p</i> = 0.07). No such effect was found among sisters (χ²₂ = 1.60, <i>p</i> = 0.45, not shown in the figure). (b) Lifetime fecundity (number of children produced) as a function of intra-sex birth-order category (among sons): firstborn sons had the highest fecundity, as compared to their younger brothers (χ²₂ = 12.91, <i>p</i> = 0.002). No such effect was found among sisters (χ²₂ = 1.33, <i>p</i> = 0.5, not shown in the figure). Figures show predicted values of the minimal model, error bars represent standard errors of the means and sample sizes are indicated at the bottom of the bars.