Fig 4.

(A) Probability of survival as a function of hatchling mass and condition for sons. (B) Probability of survival as a function of hatchling mass and condition for daughters. Fitness surfaces for hatchling mass and condition of male and female progeny were fitted with a multivariate cubic-spline regression (22). The difference in directional selection gradients on sons versus daughters (see Discussion) favored females that could simultaneously produce large sons and small daughters. That is, the ridge of high fitness for sons occurs at larger values of hatchling mass than does the ridge for daughters. The complex disruptive surface for daughters is not an artifact of our plotting technique but reflects correlational selection on two alternative female morphs, which are discussed elsewhere (27).