a, Rate-sensitive estimates of individual female fitness (wind) over a gradient in population growth rates (r). Female fitness was estimated to be higher under high within-group male relatedness for values of r ranging from −0.1 to 0 (dark shaded area), a similar non-significant (0.05 < P < 0.08) pattern was extended for r = −0.2 and r = 0.1 (light shaded area). b, The effect of within-group male relatedness on population fitness. The relative fitness cost of reducing within-group male relatedness at different population growth rates (r). The dashed line identifies relative fitness of 1, where reduction in within-group male relatedness has no fitness cost. Reducing within-group male relatedness is always costly over the range of population growth rates explored, but particularly so with smaller growth rates.