Fig. 3.

Fitness as a function of size achieved at first reproduction. (Left) estimated relative fitness (r) as a function of age at first birth for Aeta (solid line). The histogram shows observed frequencies of first birth in Aeta women; the dashed vertical line shows the predicted fitness peak (age at first birth = 15 years). (Right) Models (adapted from refs. 18 and 25) of relative fitness as a function of age and size at first birth for Ache, !Kung (18), and Gambian rural women (25).